kyr

GABRIEL BELTRAME DERNER SILVA

METAL INJECTION MOLDING PROCESSING AND PROPERTIES CHARACTERISATION OF IRON AND STEEL ALLOYS SYNTACTIC FOAMS

Diploma Thesis presented to the Materials Science

Undergraduate

Course

of

Federal

University of Santa Catarina as a complement to obtain the degree of Materials Engineer. This work was written at Fraunhofer Institute for Manufacturing Technique and Applied Materials Research Bremen – Germany.

Orientation: Dr. Jörg Weise Orientation: Dr. Natalie Salk

FLORIANÓPOLIS, 2009

Institute of Manufacturing and Advanced Materials Wiener Straße 12 28359 Bremen – Deutschland Tel. +49(0)421/2246-0 www.ifam.fraunhofer.de

Diploma Thesis – Fraunhofer Institute for Manufacturing Technique and Applied Materials Research

Assessment Committee:

___________________________________ Dr. Jörg Weise

___________________________________ Dr. Natalie Salk

___________________________________ Dr. Joachim Baumeister

Diploma Thesis – Fraunhofer Institute for Manufacturing Technique and Applied Materials Research

Catalographic BELTRAME DERNER SILVA, Gabriel, 1987 – (METAL INJECTION MOLDING PROCESSING AND PROPERTIES CHARACTERISATION

OF

IRON

AND

STEEL

ALLOYS

SYNTACTIC FOAMS). – 2009. 83 p : il. Color. ; 30 cm Orientation: Dr. Jörg Weise Orientation: Dr. Natalie Salk Diploma Thesis presented to the Materials Science Undergraduate Course of Federal University of Santa Catarina as a complement to obtain the degree of Materials Engineer. This work was written at Fraunhofer Institute for Manufacturing Technique and Applied Materials Research Bremen – Germany.

1. Syntactic foam materials. 2. Micro glass bubbles. 3. Metal injection molding.

Diploma Thesis – Fraunhofer Institute for Manufacturing Technique and Applied Materials Research

ACKNOWLEDGEMENT

To Jörg Weise and Natalie Salk, my supervisors; To Aloisio Nelmo Klein, my professor; To Matthias Busse, the director of IFAM-Bremen; To Frank Petzoldt, the head of Powder Metallurgy Department; To every person of our department, Especially to Lutz Kramer, Axel Krebs, Jurgen Henke, Jörg Volkert, Gabriel Dutra, André Vieira Cardoso, Lukas Jiranek, Miroslav Palicka; To every person of others laboratories at IFAM, To the Federal University of Santa Catarina; To Berend Snoeijer and Antônio Pedro Novaes de Oliveira, the professors of the internship Coordination; To my family; To my Brazilian and European friends; For the whole work group and friends during the time here in Germany.

Diploma Thesis – Fraunhofer Institute for Manufacturing Technique and Applied Materials Research

ABSTRACT The study of metallic foams has become attractive to researchers interested in both scientific and industrial applications. Syntactic foams are used for absorbing the energy of impacts (in packaging and crash protection) and in lightweight structures (in the cores of sandwich panels, for instance). Their efficient use requires a detailed understanding of their properties. It is known that Zn and Al alloys (elements with a relatively low melting temperature) have interesting properties as matrices in syntactic foam materials produced by squeeze casting (SC). On the present work microporous metal foams were manufactured by means of integration of micro glass bubbles into Fe, Fe2Ni and Fe3Cu matrix alloys by metal injection molding (MIM), due to no resistance of micro glass bubbles (µGB) to the melting temperature of iron alloys. Mixed feedstocks with different contents of µGB were manufactured in tensile test specimens using at first a mini injection molding machine and later an industrial injection molding machine. The samples were submitted to chemical / thermal debinding and finally sintered at suitable temperatures and times in electric furnaces under H2. Using different µGB content, syntactic foams with different densities could be produced. The optimization of MIM technology process was evaluated in order to produce the different alloys containing two sorts of µGB. For every characterisation, sintered specimens with and without µGB were examined in order to comparison. The quality of injection and sintering experiments was examined by density and shrinkage evaluations as well as metallographic and EDX analysis. For the manufactured iron specimens, the tested mechanical properties indicated high compression strength and good compression energy absorption. Furthermore, corrosion behaviors in common and inchromized samples were investigated.

Diploma Thesis – Fraunhofer Institute for Manufacturing Technique and Applied Materials Research

RESUMO Nos últimos anos, o estudo de espumas metálicas tornou-se atrativo tanto para pesquisas científicas quanto para a indústria. Espumas sintáticas são usadas para absorver energia de impactos bem como em estruturas de baixa densidade. Seu uso eficiente requer um detalhado entendimento das suas propriedades. Através de estudos desenvolvidos no Instituto de Pesquisa Fraunhofer para Técnicas de Produção e Aplicação de Materiais (IFAM-Bremen), verificou-se que ligas de Zn e Al (elementos com relativamente baixos pontos de fusão) possuem propriedades interessantes como elementos de matriz para espumas sintáticas, produzidas por fundição sob alta pressão. No presente trabalho espumas metálicas foram produzidas integrando micro bolhas de vidro (µGB) em Fe, Fe2Ni e Fe3Cu através de moldagem de pós por injeção (MIM), já que as µGB não suportariam temperaturas de fusão destas ligas. Feedstocks (massa de injeção) com diferentes quantidades de µGB foram injetados em corpos de prova para tração utilizando inicialmente uma mini injetora e em seqüência uma injetora industrial. As amostras foram submetidas à extração química / térmica e finalmente sinterizadas a adequados tempos e temperaturas em H2. Misturando diferentes quantidades de µGB, amostras com níveis de porosidades distintos foram produzidas. Otimizações foram realizadas para MIM visando à injeção de diferentes tipos de ligas e µGB. Para todos os tipos de caracterização, amostras sinterizadas de liga / parâmetros de processo iguais com e sem µGB foram examinadas para comparação. A viabilidade da injeção e sinterização foi avaliada através da tomada de densidade, encolhimento bem como metalografia e análises de EDX. Para as amostras com matriz de Fe, propriedades mecânicas foram testadas indicando uma alta resistência e satisfatória absorção de energia sobre compressão. Ademais, testes de corrosão foram realizados em amostras comuns e em amostras com tentativa de passivação.

Diploma Thesis – Fraunhofer Institute for Manufacturing Technique and Applied Materials Research

LIST OF FIGURES Figure 1 – Stainless Steel Part – Metal Injection Molding Component .............................................14  Figure 2 – Aluminum syntactic foam. Right: general microstructure, left: Al matrix and µGB detailed [4] ...................................................................................................................................16  Figure 3 – Characteristics that define a foam material.......................................................................17  Figure 4 – Taxonomy of cellular metal manufacturing process. They exploit liquid, solid and vapor phase processing routes [7]..........................................................................................................18  Figure 5 - Cellular metals can be classified of stochastic or periodic. The periodic materials are characterized by a unit cell that is repeated in two directions (prismatic structures) or in three directions (truss or lattice materials) [7] ......................................................................................19  Figure 6 – Ultra sonic sensor equipment applied in an automobile ...................................................20  Figure 7 - Techniques for the production of porous metal structures [9] ..........................................21  Figure 8 - Classification of die casting process..................................................................................21  Figure 9 - Schematic of a HP die casting machine [13] .....................................................................22  Figure 10 – Flowchart of MIM process, adapted from [14]...............................................................23  - 

Figure 11 – Schematic drawing of debinding. Process is being showed by the arrows (red) from the binder (green) to the sample surface between the particles (blue circles) .............................24 

Figure 12 – (a) Loose powder. (b) Initial stage (the pore volume shrinks). (c) Intermediate stage (grain boundaries form at the necks). (d) Final stage (pores become smoother) [14]................25  Figure 13 – Tensile Test Scheme [15] ...............................................................................................26  Figure 14 – Vicker’s Hardness Test. Left: scheme of indentation, right: brass tested specimen with 3 indentations [16] ..........................................................................................................................28  Figure 15 – Compression test with Al-glass bubble composite .........................................................28  Figure 16 – Example of Cr coating layer microstructure on AISI H13 tool steel [21] ......................31  Figure 17 – Right: pre-form of sintered bubbles of S60HS sort, left: zinc composite after infiltration .....................................................................................................................................................31  Figure 18 – Fe50Ni + 5 wt% of S60HS cross section ........................................................................32  Figure 19 - Stainless steel matrix and integrated µGB ......................................................................33 

Diploma Thesis – Fraunhofer Institute for Manufacturing Technique and Applied Materials Research

Figure 20 – Tumbling Mixer Tarbula from WAB .............................................................................35  Figure 21 – SEM photo of micro glass bubbles .................................................................................36  Figure 22 – Feedstock mixer from Brabender ...................................................................................37  Figure 23 – Haake Mini Jet II and tensile specimen mold .................................................................38  Figure 24 – Mini tensile specimen .....................................................................................................38  Figure 25 – Left: injection machine Ferromatic from Klöckner. Right: tensile specimen mold with 4 cavities .........................................................................................................................................40  Figure 26 – Tensile samples dimensions............................................................................................40  Figure 27 – Thermal debinding and sintering programs ....................................................................42  Figure 28 - Fe3Cu + 10 wt% of S60HS before and after sintering ....................................................44  Figure 29 – Compression strength (arrow) ........................................................................................46  Figure 30 – Plateau stress (arrows) ....................................................................................................47  Figure 31 – Ideal and real absorber diagram ......................................................................................48  Figure 32 – Example of immersion test. Samples inside a 0.1 M NaCl solution ..............................49  Figure 33 – Example of Potentiodynamic Polarization Test’s Apparatus .........................................50  Figure 34 – Relative density...............................................................................................................54  Figure 35 – Glass reflection on alumina plates after Fe2Ni sintering at 1100 ºC. Right: no glass in the samples, no reflection. Center and left: 5 and 10 wt% of glass, respectively, and reflections shown ...........................................................................................................................................56  Figure 36 – Shrinkage evaluation. Variation: 11% to 22%. Decreasing shrinkage with increasing µGB content.................................................................................................................................57  Figure 37 – Ultimate strength versus relative density for Fe samples ...............................................58  Figure 38 – Backscattering images from rupture surface after tensile testing of Fe + 5 (left) and 10 (right) wt% of S60HS: 1 – fracture through bubble or matrix; 2 – fracture along interface bubble / matrix .............................................................................................................................60  Figure 39 – Vicker’s Hardness tests for Fe specimens. Left: matrix hardness (HV 0.025). Right: composite hardness (HV 0.2) ......................................................................................................61  Figure 40 – Fe specimens after compression tests. First line: Fe + 0, 5, 8 and 10 wt% of S60HS. Second line: Fe + 0, 5 and 10 wt% of IM30K. ............................................................................62 

Diploma Thesis – Fraunhofer Institute for Manufacturing Technique and Applied Materials Research

Figure 41 – Compression curves for Fe and Fe3Cu + µGB varying grade and content ....................62  Figure 42 – Hypothetic and real diagram showing the types of behavior for compression tests.......63  Figure 43 – Compression curves for comparing yield stress of samples without and with 10 wt% of µGB in Fe3Cu matrix ..................................................................................................................64  Figure 44 – Compression strength for all the Fe and Fe3Cu + µGB grades and contents .................64  Figure 45 – Plateau stress for all the Fe and Fe3Cu + µGB grade and content .................................65  Figure 46 – Strength max (50% compression) versus energy absorption for Fe and Fe3Cu + µGB grade and content .........................................................................................................................66  Figure 47 – Efficiency for Fe and Fe3Cu + µGB grades and content................................................67  Figure 48 – Specimens after 10 days inside the 0.1 M NaCl solution ...............................................68  Figure 49 – Cross section metallography after immersion test for pure Fe and Fe + 5% of S60HS grade, respectively .......................................................................................................................69  Figure 50 – Potentiodynamic polarization curves for Fe samples .....................................................70  Figure 51 – Cr line scan on inchromized samples. Left: pure Fe, right: Fe + µGB ...........................71  Figure 52 – Potentiodynamic polarization curves for Fe coated samples ..........................................71  Figure 53 – Fe ....................................................................................................................................72  Figure 54 – Fe + 5 wt% S60HS..........................................................................................................73  Figure 55 – Fe + 8 wt% S60HS..........................................................................................................73  Figure 56 – Fe + 10 wt% S60HS........................................................................................................73  Figure 57 – Fe + 5 wt% IM30K .........................................................................................................73  Figure 58 – Fe + 10 wt% IM30K .......................................................................................................73  Figure 59 – Fe + 13 wt% IM30K .......................................................................................................73  Figure 60 – Fe3Cu ..............................................................................................................................74  Figure 61 – Fe3Cu + 5 wt% S60HS ...................................................................................................74  Figure 62 – Fe3Cu + 10 wt% S60HS .................................................................................................74  Figure 63 – Fe2Ni ..............................................................................................................................75  Figure 64 – Fe2Ni + 5 wt% S60HS....................................................................................................75  Figure 65 – Fe2Ni + 10 wt% S60HS..................................................................................................75  Figure 66 – Fe7Cr ..............................................................................................................................76 

Diploma Thesis – Fraunhofer Institute for Manufacturing Technique and Applied Materials Research

Figure 67 – Fe7Cr + 5 wt% S60HS....................................................................................................76  Figure 68 – Fe7Cr + 10 wt% S60HS..................................................................................................76  Figure 69 – Fe15Cr ............................................................................................................................76  Figure 70 – Fe15Cr + 5 wt% S60HS..................................................................................................76  Figure 71 – EDX point quantification showing 100% of Fe..............................................................77  Figure 72 – EDX evaluations. Left: line scan on Fe + 5 wt% S60HS, right: point quantification on µGB edges ...................................................................................................................................78 

Diploma Thesis – Fraunhofer Institute for Manufacturing Technique and Applied Materials Research

LIST OF TABLES Table I – Owerview of commom MIM alloys ...................................................................................34  Table II – Utilized metal / alloys ........................................................................................................35  Table III – Utilized metal powders ....................................................................................................35  Table IV – S60SH Typical Physical Properties [23]..........................................................................36  Table V – Mixing settings ..................................................................................................................37  Table VI – Volume % of binder, µGB and feedstock density ...........................................................38  Table VII – Injection molding parameters for each group of samples...............................................41  Table VIII – Thermal debinding and sintering programs ..................................................................43  Table IX – Results of first optimized chemical debinding.................................................................52  Table X – Relative density .................................................................................................................53  Table XI – Comparison between real and theoretical density of the foams ......................................54  Table XII – Shrinkage evaluation ......................................................................................................57  Table XIII – Tensile properties for Fe samples..................................................................................59  Table XIV – Vicker’s Hardness for Fe specimens .............................................................................61  Table XV – Result of immersion tests for Fe samples .......................................................................68 

Diploma Thesis – Fraunhofer Institute for Manufacturing Technique and Applied Materials Research

TABLE OF CONTENTS  1 

INTRODUCTION .................................................................................................................... 13  1.1 

MOTIVATION..................................................................................................................... 13 

1.2 

GENERAL AIM ................................................................................................................... 14 

1.2.1 

1.3 

2 

Specific Objectives .................................................................................................................... 14 

WORK TO BE DONE.......................................................................................................... 15 

BIBLIOGRAPHIC REVIEW .................................................................................................... 16  2.1 

SYNTACTIC FOAM MATERIALS ....................................................................................... 16 

2.1.1 

Metal Foam Materials .............................................................................................................. 17 

2.1.1.1  Metal Foam Production ........................................................................................................ 17  2.1.1.2  Metal Foam Properties and Applications.............................................................................. 19  2.1.2 

Some Manufacturing Methods for Processing Metal Foams Applied at IFAM .................. 20 

2.2 

METAL INJECTION MOLDING .......................................................................................... 23 

2.1 

CHARACTERISATION ........................................................................................................ 26 

2.1.1 

Mechanical Tests ....................................................................................................................... 26 

2.1.1.1  Tensile Test ............................................................................................................................ 26  2.1.1.2  Hardness Test ........................................................................................................................ 27  2.1.1.3  Compression Test .................................................................................................................. 28  2.1.2 

Corrosion Tests ......................................................................................................................... 29 

2.1.2.1  Immersion Test ...................................................................................................................... 29  2.1.2.2  Potential Polarization Test (PPT) ......................................................................................... 30  2.1.3 

Inchromizing ............................................................................................................................. 30 

3 

PREVIOUS SYNTACTIC FOAM`S WORK BY MIM ................................................................ 31 

4 

EXPERIMENTAL APPROACH ................................................................................................ 33  4.1 

MATERIALS ....................................................................................................................... 33 

4.2 

METAL INJECTION MOLDING PROCESS ......................................................................... 36 

4.2.1 

Feedstock Optimization ............................................................................................................ 36 

4.2.2 

Injection Process ....................................................................................................................... 39 

4.2.3 

Chemical Debinding.................................................................................................................. 41 

4.2.4 

Thermal Debinding and Sintering ........................................................................................... 42 

4.3 

CHARACTERISATION OF THE SINTERED PARTS ............................................................. 43 

Diploma Thesis – Fraunhofer Institute for Manufacturing Technique and Applied Materials Research

4.3.1 

Density Evaluation .................................................................................................................... 43 

4.3.2 

Shrinkage Evaluation ............................................................................................................... 44 

4.3.3 

Mechanical Tests ....................................................................................................................... 45 

4.3.3.1  Tensile Test ............................................................................................................................ 45  4.3.3.2  Hardness Test ........................................................................................................................ 45  4.3.3.3  Compression Test .................................................................................................................. 46  4.3.4 

Corrosion Tests ......................................................................................................................... 48 

4.3.4.1  Immersion Test ...................................................................................................................... 48  4.3.4.2  Potentiodynamic Polarization Tests (PPT) ........................................................................... 49  4.3.5 

5 

Inchromizing ............................................................................................................................. 50 

RESULTS AND DISCUSSION .................................................................................................. 51  5.1 

PROCESS OPTIMIZATION ................................................................................................. 51 

5.1.1 

Feedstock Optimization ............................................................................................................ 51 

5.1.2 

Chemical Debinding.................................................................................................................. 51 

5.1.3 

Density ....................................................................................................................................... 52 

5.1.4 

Shrinkage ................................................................................................................................... 56 

5.2 

CHARACTERISATION OF THE SINTERED PARTS ............................................................. 58 

5.2.1 

Mechanical Tests ....................................................................................................................... 58 

5.2.1.1  Tensile Test ............................................................................................................................ 58  5.2.1.2  Hardness Test ........................................................................................................................ 60  5.2.1.3  Compression Test .................................................................................................................. 61  5.2.2 

Corrosion Tests ......................................................................................................................... 67 

5.2.2.1  Immersion Test ...................................................................................................................... 68  5.2.2.2  Potentiodynamic Polarization Tests (PPT) ........................................................................... 69  5.2.3 

Inchromizing Samples .............................................................................................................. 70 

5.2.4 

Metallography and EDX Tests................................................................................................. 72 

5.2.4.1  Optic Microscopy .................................................................................................................. 72  5.2.4.2  SEM + EDX ........................................................................................................................... 77 

6 

SUMMARY AND OUTLOOK ................................................................................................... 79 

7 

REFERENCES ....................................................................................................................... 81 

APPENDIX A – SCHEDULE OF DEVELOPED ACTIVITIES ............................................................... 83  APPENDIX B – PUBLICATION IN ADVANCED ENGINEERING MATERIALS ................................... 84 

Diploma Thesis – Fraunhofer Institute for Manufacturing Technique and Applied Materials Research

1

INTRODUCTION

1.1 MOTIVATION The present work describes the Diploma Thesis developed during the internship at Fraunhofer - Institute of Manufacturing and Advanced Materials (IFAM). The internship comprised the weeks from February 16th to August 31th of 2009. Production of syntactic foam by metal injection molding process was addressed. Syntactic foam is a composite material synthesized by filling a metal, polymer or ceramic matrix with hollow particles. When the matrix is metallic, these materials are called syntactic metal foams. Besides the basic metallic properties, syntactic foams have specific characteristics like low and controllable density, high impact energy absorption capacity. There are many methods available to produce metallic foams [1][2]. Nowadays, studies about aluminum and zinc syntactic foams produced by squeeze casting are known. Some projects in this area were carried out at Fraunhofer IFAM. Nevertheless for alloys with higher melting temperatures like iron or steel, liquid infiltration of micro-bubble structures is very difficult because the µGB do not resist when submitted at iron or steel melt temperatures. The application of these kinds of alloys is quite interesting concerning mechanical properties, like hardness, tensile and compression for instance. The actual work exhibits tests and evaluations about iron and steel alloys syntactic foams produced by MIM (figure 1 – MIM component).

Diploma Thesis – Fraunhofer Institute for Manufacturing Technique and Applied Materials Research 13

Figure 1 – Stainless Steel Part – Metal Injection Molding Component

1.2 GENERAL AIM The main focus of the work is to perform an optimization of the processing parameters and composition of sintered iron and steel-matrix glass-bubble composites, according to previous studies carried out at IFAM in this particular area with a Fe-Ni MIM-alloy [3]. To achieve this aim, new systematic investigation and process optimization with simple system Fe and extended to some typical MIM-alloys were carried out. For understanding the interaction of bubbles and matrix during the processing and their dependence upon the composition, mechanical and corrosion properties were evaluated. Excepting inchromizing process all the experiments were performed at Fraunhofer-IFAM-Bremen. Finally, Fe and FeCu production of foam by MIM was never examined before and it will be shown in this project.

1.2.1 Specific Objectives The following topics were the specific objectives of this project: a) determination of the suitable powder and binder system;

Diploma Thesis – Fraunhofer Institute for Manufacturing Technique and Applied Materials Research 14

b) determination of the µGB limit to mix into the feedstock (mixture with metal powder and binder system); c) screening of different MIM alloys; d) production of larger MIM components; e) characterisation of the mechanical and corrosion behavior of the syntactic foams;

1.3 WORK TO BE DONE The project can be divided in three main steps: 1. Metal injection molding process: o

feedstock preparation with different powder alloys, glass bubble content and grade;

o

injection molding of the feedstocks into standard tension bars;

o

debinding and sintering of the resulting parts.

2. Characterisation: o

determination of the density and shrinkage of the sintered parts;

o

examination of the mechanical properties performing tensile, hardness and compression

tests; o

corrosion tests;

o

metallographic investigation of the foam structure;

o

EDX (Energy Dispersive X-ray) analyses.

3. Documentation of the results.

Diploma Thesis – Fraunhofer Institute for Manufacturing Technique and Applied Materials Research 15

2

BIBLIOGRAPHIC REVIEW

2.1 SYNTACTIC FOAM MATERIALS At first, syntactic foams are composite materials produced by filling a ceramic, polymer or metal matrix with hollow elements. The size of the porosity is controlled by the size of the hollow particles. These hollow elements can be made from glass, ceramic or even metal. Their shape is varied from spherical to totally irregular. Figure 2 shows, for instance, aluminum syntactic foam with integrated micro glass bubbles produced by squeeze casting [4].

Figure 2 – Aluminum syntactic foam. Right: general microstructure, left: Al matrix and µGB detailed [4] The resulting closed-cell matrix offers a unique spectrum of properties, determined by the combination of the properties of the hollow elements and the matrix. The increasing use of syntactic foams is explained by their excellent properties: •

high specific stiffness;

•

outstanding energy absorption properties;

•

low density;

•

reduced thermal and electrical conductivity;

•

improved acoustical properties (mech. dampening, sound absorption).

Diploma Thesis – Fraunhofer Institute for Manufacturing Technique and Applied Materials Research 16

2.1.1 Metal Foam Materials Metal foams can be made from diverse materials [5]. They include examples drawn from all known classes of solid: polymers, metals and ceramics. Sometimes the final foam metal is a mixture of these classes, in this case one hybrid material. In modeling their properties it is important to know the characteristics of the material from which the foam is made, the regime of stress and temperature to which it is subjected, and of course to use one adequate foaming approach. The most important structural characteristics of foam solids are relative density and the fraction of pore space in the foam, or porosity [4]. According Degischer et al. [6] the main characteristics whose define a foam material are described in figure 3. When all these attributes can be finding in a unique material, then it is called a metal foam material.

solid state cell structure

M etal Foam

metal matrix porosity > 50 % open/ closed porosity

Figure 3 – Characteristics that define a foam material

2.1.1.1 Metal Foam Production As the engineering applications of cellular metals (like syntactic metal foams) grow, many methods for their manufacture are being developed. They result in materials that can be classified by the size of their cells, variability in cell size (stochastic or periodic), the pore type (open or closed) and the relative density of the structure [7]. For the present work metal foams will be

Diploma Thesis – Fraunhofer Institute for Manufacturing Technique and Applied Materials Research 17

manufactured with closed stochastic porosity. Manufacturing routes concerning solid, liquid and vapor phase for metal foams are organized in figure 4 and the names of the products associated with each are shown [7].

Figure 4 – Taxonomy of cellular metal manufacturing process. They exploit liquid, solid and vapor phase processing routes [7] The metal foams can be further classified by their metal topology, see Figure 5. Those created by foaming or from foamed templates, or from random size / dispersed spherical powders or from sacrificial random templates have a statistical variation of cell size and shapes. These materials cannot be characterized by a single unit cell and are referred to as stochastic foams. Materials made using templates characterized by a unit cell that can be translated through the structure are referred to as periodic materials [7].

Diploma Thesis – Fraunhofer Institute for Manufacturing Technique and Applied Materials Research 18

Figure 5 - Cellular metals can be classified of stochastic or periodic. The periodic materials are characterized by a unit cell that is repeated in two directions (prismatic structures) or in three directions (truss or lattice materials) [7]

2.1.1.2 Metal Foam Properties and Applications Metallic foams are applied in several areas. Having high specific stiffness, these materials are used in structural applications. Due to its low density, it is interesting in lightweight structures, like air and space industry. Furthermore, the metal foam is applied in the following areas: • noise reducing; • foamed sandwiches; • absorption of impact energy (in packing and crash protection); • spark catchers (diesel engines); The main application field for this work will be to build the structure of ultra-sonic sensor equipments for applications in high-temperature and/or corrosive environments and crash energy absorption for high loads (ballistic as well as non-ballistic crash). The foam shall be used for impedance matching of the sensor. Impedance is an acoustic property which depends on the frequency f. Mathematically, it is the sound pressure p divided by the particle velocity v and the

Diploma Thesis – Fraunhofer Institute for Manufacturing Technique and Applied Materials Research 19

surface area S, through which an acoustic wave of frequency f propagates. The impedance can be controlled by the porosity of the syntactic foam. For instance, ultra sonic sensor equipments are applied in automobiles rear parking, see figure 6.

Figure 6 – Ultra sonic sensor equipment applied in an automobile

2.1.2 Some Manufacturing Methods for Processing Metal Foams Applied at IFAM There is a wide range of techniques for the production of porous metal structures [8][9], see figure 7. At Fraunhofer-IFAM Institute, casting techniques like foaming in metals and investment casting are already applied. In this study the metal injection molding process will be applied for producing iron and steel foams instead of the previous described techniques (bold route in figure 7). A variety of hollow elements has been applied for the production of syntactic metal foams, e.g. foam glass granules, metal hollow spheres, ceramic hollow spheres and micro glass bubbles. Most production techniques of syntactic foams are based on the infiltration of solid structures or loose bulks of the hollow elements by metal melts using different techniques [10].

Diploma Thesis – Fraunhofer Institute for Manufacturing Technique and Applied Materials Research 20

Figure 7 - Techniques for the production of porous metal structures [9] One of the methods to produce these materials is the pressure die casting (PDC). There are a number of die casting processes, as summarized in figure 8. PDC represents about 50% of all light alloys casting production. Low pressure die casting currently accounts for about 20% of production and its use is increasing. Gravity die casting accounts for the rest, with the exception of a small but growing contribution from the recently introduced vacuum die casting and squeeze casting process [11].

Gravity dieProcesses casting Die Casting

High pressure die casting

Low pressure die casting

~50%

~20%

Gravity die casting

Vacuum die casting

Squeeze die casting

Figure 8 - Classification of die casting process.

For instance, PDC is an important casting technique used for a big variety of products in the industry. It is the most common casting technique today since it is widely used for mass production

Diploma Thesis – Fraunhofer Institute for Manufacturing Technique and Applied Materials Research 21

of components based on aluminum, zinc or magnesium alloys. Therefore, it is the first choice to be tried out to produce syntactic foam materials. Pressure is needed for infiltration of liquid metal in a sintered pre-form. It means that it needs pressure casting technologies and then HP die casting might be a good choice. In high pressure die casting, molten alloy is injected into a metal mold, called the die, and the solidification of the alloy creates the desired shape. During a high pressure die casting cycle, molten metal is initially poured into the shot sleeve and then injected into the die cavity by the plunger under pressure. After die cavity is filled, pressure intensification occurs during the solidification to reduce the amount of gas porosity, feed shrinkage porosity and dimensional inaccuracies. Finally, the die is opened and casting is separated from the die [12]. As once can see in figure 9, HPDC equipment consist of two vertical platens that hold the die halves. One platen is fixed while the other can open and close the die.

Figure 9 - Schematic of a HP die casting machine [13]

In manufacturing syntactic foams, the melt alloy superheat, the pre-heating temperature of the pre-form, the pre-heating temperatures of the die cavity, injection speed and pressurization level as well as the time for solidification are parameters that must be controlled and optimized to obtain a satisfying microstructure and therefore good mechanicals properties. Sometimes the input pressure as defined does not equal the pressure which actually is inside of the cavity. This pressure is called “real pressure”.

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2.2 METAL INJECTION MOLDING In powder metallurgy, there is more than one type of manufacturing process. The most used and known around the world is compaction. However, there is another method that has increased quickly over the last years: metal injection molding. There are three main advantages of MIM that are the main reasons for its application nowadays: low cost, shape complexity and high volume production. MIM is a manufacturing process which combines the versatility of plastic injection molding with strength and integrity of machined, pressed or otherwise manufactured small, complex, metal parts. Competing processes include pressed powder sintering, investment casting and machining [14]. The process involves combining fine metal powders with plastic binders which allow the metal to be injected into a mold using conventional injection molding machines. After injection the part is called “green part”. The next step is to remove the binders with solvents and thermal treatments. The resultant metal part is sintered (densified) at temperatures around 2/3 to 4/5 of the melting temperature of the respective material. The products of metal injection molding are up to 99% of the theoretical density of the material. A broad range of applications fields exists, e.g. in the medical, dental, firearms, aerospace and automotive industry. The window of economic advantage in metal injection molded parts lies in the complexity and the economic series production [14]. The process is divided in 5 steps shown in the flowchart of figure 10.

Figure 10 – Flowchart of MIM process, adapted from [14]

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At selected temperatures and times, micron sized metal powder is mixed with a multiple component thermoplastic binder system that enables the powder to flow into the tool cavity during the injection molding process. When the mixture is cooled to room temperature it is granulated into small pea-sized particles which are called “feedstock” [14]. The parameters that can be adjusted while injecting the feedstock are listed below: •

number of injection stages;

•

plasticising stroke;

•

pressure, speed and time of injection;

•

switchover pressure and point (in case of two or more injection stages);

•

feedstock (scroll) and mold temperature;

After injecting, the parts must be debinded. Generally it occurs in two steps: chemical and thermal debinding. The main aim of chemical debinding is to remove the wax out of the binder system to facilitate the thermal debinding and sintering. Some considerations about this process must be done: -

The larger the powder size, the easier for the wax to leave the part. This is due to more space between the metal particles; see the schematic picture in figure 11.

-

Figure 11 – Schematic drawing of debinding. Process is being showed by the arrows (red) from the binder (green) to the sample surface between the particles (blue circles)

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-

If there is less space between the particles, cracks might occur in the end of the process, because it is more difficult for the binder to leave the sample, stressing it. Another stress generator occurs when the polymers swells, maybe causing cracks as well.

-

The utilized binder system consists mainly of mainly polymers and waxes. During the chemical debinding only the wax leaves the component [14]. Thermal debinding is performed at temperatures from 300 to 500 ºC. Polymers leave the

component by the voids generated in the previous solvent debinding step. The same problems previously mentioned in chemical debinding can occur here, like cracks and distortions due to the stress caused when the binder is being removed from the parts. Sintering is a process of heating the parts below the melting point of the metal / alloy in a controlled atmosphere to cause the whole part to shrink uniformly generating a high density. The original shape of the molded part is preserved during the process and final part dimensional tolerances are closely held. Still concerning sintering, there are three stages starting with the loose powder (figure 12-a): in the second step the “necks” (junction of particles) are made (figure 12-b). In the third stage, the necks increase (figure 12-c), which leads to a decreasing the porosity. The last step happens when the prior cylindrical pores collapse to spherical pores (figure 12-d). The driving force of the sintering is the reduction of the surface area and there are several mechanisms which run it. Summarizing, they are divided by surface transport and bulk transport. Examples of surface transport are surface diffusion and plastic flow.

Figure 12 – (a) Loose powder. (b) Initial stage (the pore volume shrinks). (c) Intermediate stage (grain boundaries form at the necks). (d) Final stage (pores become smoother) [14]

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2.1 CHARACTERISATION 2.1.1 Mechanical Tests 2.1.1.1 Tensile Test Tensile tests are simple, relatively inexpensive and fully standardized. By pulling on something, it very quickly determines how the material reacts to forces being applied in tension. As the material is being pulled, it finds its strength along with how much it will elongate, see the scheme in figure 13.

Figure 13 – Tensile Test Scheme [15] This test allows finding the amount of stretch or elongation the specimen undergoes during tensile testing. It can be expressed as an absolute measurement in the change in length or as a relative measurement called "strain" (ε) [15]. For most tensile testing of materials, it notices that in the initial portion of the test, the relationship between the applied force and the elongation the specimen exhibits is linear. It is called

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the "Modulus of Elasticity" or "Young's Modulus" [15]. For foam this property is very difficult to measure, because these materials do not have a linear deformation at the beginning of the test. A value called "yield strength" of a material is defined as the stress applied to the material at which plastic deformation starts to occur while the material is loaded [15]. One of the properties frequently determined about a material is its ultimate tensile strength (UTS). This is the maximum load the specimen sustains during the test. The UTS may or may not equate to the strength at break. This all depends on what type of tested material: brittle, ductile, or a substance that even exhibits both properties [15].

2.1.1.2 Hardness Test Hardness is the property of a material that enables it to resist a plastic deformation, usually by penetration. It is not an intrinsic material property dictated by precise definitions in term of fundamental units of mass, length and time. A hardness property value is a result of a defined measurement procedure. Vicker’s Hardness test was undertaken in this project, figure 14 shows a scheme and a brass specimen with 3 indentations. For this test, it was decided that the indenter shape should be capable of producing geometrically similar impressions, irrespective of size; the impression should have well-defined points of measurement; and the indenter should have high resistance to selfdeformation. A diamond in the form of a square-based pyramid satisfied these conditions. It had been established that the ideal size of a Brinell impression is 3/8 of the ball diameter. As two tangents to the circle at the ends of a chord 3d/8 long intersect at 136°, it is useful to use this as the included angle of the indenter. The angle is varied experimentally and it is finding that the hardness value obtained on a homogeneous piece of material remained constant, irrespective of load [16].

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Figure 14 – Vicker’s Hardness Test. Left: scheme of indentation, right: brass tested specimen with 3 indentations [16]

2.1.1.3 Compression Test The most important mechanical test for metal foam is the compression test, because it measures the foam capacity to absorb energy [17]. There are some properties those are measured with compression diagrams to compare foam materials. It is possible to evaluate some foam aspects with their diagrams, like plateau stress, compression strength, energy absorption and efficiency [18]. Figure 15 shows a compression test running.

Figure 15 – Compression test with Al-glass bubble composite

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2.1.2 Corrosion Tests The short-time corrosion behavior of pure Fe and Fe with the glass bubble composites were evaluated using immersion tests and potentiodynamic polarization tests (PPT). These two routines are utilized to check how the material behaves in use. In other words, they simulate how the corrosion goes on when the components are submitted in aggressive environments.

2.1.2.1 Immersion Test Historically, immersion tests have been extensively used to generate uniform corrosion data for alloys used in the process industries under immersion conditions. Immersion tests can take the following forms: •

Relatively simple tests involving constant immersion. The acceleration of corrosion damage can be achieved from the length of immersion in the corrosive medium and other accelerating factors.

•

Tests involving alternate immersions/drying in the form of cyclical tests.

•

Instrumentation of test specimens during immersion (such as connections to electrochemical instrumentation) to facilitate measurements other than simple weight loss measurements.

•

From simple beaker type tests to immersion of flush mounted coupons in flow loops to simulate service conditions more closely. The reason for these experiments is the possibility of existing different corrosion mechanism

of the foam in comparison to the compact matrix alloy, which can be caused by two reasons: contamination of the matrix by diffusion between matrix and bubbles during the sintering and/or increased corrosion along the interface. Metallographic sections and visual evaluations will be undertaken to check it. This test is described by ASTM G31 (Practice for Laboratory Immersion Corrosion Testing of Metals) [19]. The intention of the immersion test was to analyze the level of oxidation in the surface application, i.e. how deep the corrosion had advanced into the microstructure.

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2.1.2.2 Potential Polarization Test (PPT) Polarization methods such as potentiodynamic polarization are often used for laboratory corrosion testing. These techniques can provide significant useful information regarding the corrosion mechanisms, corrosion rate and susceptibility of specific materials to corrosion in designated environments. Polarization methods involve changing the potential of the working electrode and monitoring the current which is produced as a function of time or potential. Several methods may be used in polarization of specimens for corrosion testing. Potentiodynamic polarization is a technique where the potential of the electrode is varied at a selected rate by application of a current through the electrolyte. It is probably the most commonly used polarization testing method for measuring corrosion resistance and is used for a wide variety of functions [20].

2.1.3 Inchromizing In this process, the material is coated by a layer of chromium at controlled atmospheres and temperatures. Heat, erosion and corrosion resistant inchromizing is applied in industries producing heaters [21]. For over 35 years, the inchromizing process has proved its value in this. Instead of expensive, heat-resistant sorts of steel that are difficult to process, inchromizing can be a replacement at temperatures as high as 850°C. Even all unalloyed sorts of steel with < 0.1 wt% of C are suitable for soft inchromizing. Figure 16 shows an example of Cr coating layer microstructure on AISI H13 tool steel. Parameters, applications and advantages: •

Process temperature 850-980 °C;

•

Diffusion thickness: 35-200 µm;

•

Heat resistance up to 850 °C;

•

Increasing of corrosion and erosion resistance;

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•

Range of application: conveyor pipe lines, burner parts, several parts for the chemical and food industries.

Figure 16 – Example of Cr coating layer microstructure on AISI H13 tool steel [21]

3

PREVIOUS SYNTACTIC FOAM`S WORK BY MIM

For alloys with relatively low solidus-liquidus temperatures (like aluminum alloys), experiments have already been accomplished with metallic foams at IFAM [4]. The process utilized for it was the squeeze casting (SC) method, where sintered µGB pre-forms were infiltrated with a liquid metal, generating metal foams. See figure 17.

Figure 17 – Right: pre-form of sintered bubbles of S60HS sort, left: zinc composite after infiltration

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The combination of high pressures and temperatures during the filling stage can lead to the destruction of the µGB which have wall-thicknesses of few microns, especially when high process temperatures are needed e.g. for iron-based alloys. Therefore, tests have recently been carried out to examine the feasibility of the production of glass bubble syntactic foams with steel and iron matrices by MIM at IFAM. In this process the forming step is executed at moderate temperatures around 140 °C and pressures approximately 90 MPa. Hitherto, MIM experiments carried out at IFAM combine stainless steel / Fe50Ni with two different grades of µGB (S60HS and S38HS) [3][22]. Figure 18 shows a microstructure of Fe50Ni + 5 wt% of S60HS. Particle sizes: Fe d90 < 45 µm / Ni d50 ~ 2 µm. Sintering parameters: 2 hours at 1000 ºC.

Figure 18 – Fe50Ni + 5 wt% of S60HS cross section The conclusions from the previous work are summarized here: •

Scanning Electron Microscopy (SEM) images presented less percent of broken S60HS glass bubbles after sintering, than the grade S38HS. This can be explained by the mechanical strength of the respective µGB. The S60HS has 4 times more strength than S38HS;

•

Experiments with stainless steel matrix showed an increased amount of broken glass bubbles after sintering, see figure 19.

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Figure 19 - Stainless steel matrix and integrated µGB Based on the previous results, this project was carried out with the objective to develop a systematic approach to produce Fe and steel syntactic foams by integrating micro glass bubbles.

4

EXPERIMENTAL APPROACH

In this chapter the experimental approach will be shown, starting with the utilized materials, followed by the metal injection molding process and finally the characterisation of the sintered parts.

4.1 MATERIALS Here, metallic powders alloys were selected according to properties such as composition, mean particle size and particle shape. Table I shows common MIM alloys. After analyzing table I, which lists material properties such as maximum resistance and elongation in tensile, suitable sintering temperature and some remarks, it was decided to use powders which have alloying elements that generate higher strength, such as Ni and Cu [14]. Tables II and III show the metal / alloys and metal powders utilized. For Fe2Ni a pre-alloyed powder was used. But there was not Fe3Cu pre-alloyed powder available for experiments.

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Therefore, Fe + Cu powders were mixed in a tumbling mixer Tarbula from WAB along 45 minutes, see figure 20. The powders in table III are the finest available at IFAM for such compositions when the experiments were carried out. It would be better if a finer Fe2Ni powder was available to the project, d90 around 2 µm. Cu powder does not need be so fine, because only 3 wt% was used in Fe. Table I – Owerview of commom MIM alloys Alloy system

Composition

Fe

99,9% Fe

Fe-Cu