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PARAMETRIC OPTIMIZATION OF ULTRASONIC METAL WELDING ON COPPER JOINT P.Kumar, Dr.V.Karthik, Professor.P.G.Venkatakrishnan Metallurgy Department, Government College of Engineering, Salem-636011, India Abstract: - Ultrasonic metal welding (USMW) finds its largest applications in automotive industries, packaging, battery assemblies and aerospace industries owing to high quality weld, low power consumption, very homogeneous metallic structure between the base materials, free from pores and refined grains. In USMW the joint is produced by the simultaneous application of high frequency vibratory energy and moderate clamping pressure. It is used to weld sheet metals of similar or dissimilar nonferrous materials like copper, aluminium, magnesium and also to ferrous materials and polymers. Copper joints have been widely applicable in packaging, car panels and battery assembly products. In this study thin sheets of copper (0.5 mm thickness) has been joined by means of USMW. The various welding parameters like weld pressure, amplitude and weld time have been optimized through design of experiments technique using full factorial design. The weld strength was subsequently analysed through tensile-shear test and microstructural changes has been analysed through microhardness test and the optical microscopic examination. The results show how the weld quality is particularly sensitive to the combination of clamping force, vibration amplitude and weld time. Index Terms- Ultrasonic metal welding, Aluminium and copper joint, full factorial design, optimizing the parameters. I.

INTRODUCTION

Metal working is the process of working with metals and to create individual parts, assemblies, or large-scale structures and it is a science, art, hobby, industry and trade. Its historical roots span cultures, civilizations, and millennia. It has evolved from the discovery of smelting various ores, producing malleable and ductile metal useful

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for tools and adornments. Modern metalworking processes, though diverse and specialized, can be categorized as forming, cutting, and welding processes and each of these categories contain various processes [1]. The welding is a process that joins the materials, usually metals or thermoplastics by causing the coalescence. In some welding processes a filler material is added to facilitate coalescence. The assemblage of parts that are joined by welding is called a weldment. Many different energy sources can be used for welding including the gas flame, electric arc, laser beam, electron beam, frictional heat and ultrasound [2]. Many welding processes are accomplished by heat alone, with no pressure applied; others by a combination of heat and pressure; and still others by pressure alone, with no external heat supplied. There are some 50 different types of welding operations have been catalogued by the American Welding Society and they use various types or combinations of energy to provide the required power. We can divide the welding processes into two major groups as fusion welding and solid-state welding. Solid-state welding refers to joining processes in which coalescence results from application of pressure alone or a combination of heat and pressure. If heat is used, the temperature in the process is below the melting point of the metals being welded [3]. These processes are sometimes erroneously called solid state bonding processes and this group of welding processes includes cold welding, diffusion welding, explosion welding, forge welding, friction welding, hot pressure welding, roll welding, and ultrasonic welding. In all of welding processes time, temperature, and pressure individually or in combination produce coalescence of the base metal without significant melting of the base metals. Solid state welding includes some of the very oldest of the welding processes and some of the very newest.

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© 2015 IJIRT | Volume 1 Issue 12 | ISSN: 2349-6002 Some of the processes offer certain advantages since the base metal does not melt and form a nugget. The metals being joined retain their original properties without the heat-affected zone problems involved when there is base metal melting. When dissimilar metals are joined their thermal expansion and conductivity is of much less importance with solid state welding than with the arc welding processes. Time, temperature, and pressure are involved; however, in some processes the time element is extremely short, in the microsecond range or up to a few seconds. In other cases, the time is extended to several hours. As temperature increases time is usually reduced [4]. Nowadays the automotive industry mostly prefers the innovative solid state welding technologies that would enable to welding of lightweight and high performance materials. Ultrasonic welding is a solid state welding process and it would be a interesting method for joining dissimilar alloys that can potentially avoid many of the issues associated with fusion welding processes including rapid intermetallic formation [5]. Ultrasonic welding joins the metal parts by applying the high frequency vibratory energy and moderate clamping pressure onto the interface area between the parts to be welded. Since 1950s, it has been used in the manufacture of electronics, food packaging, electrical appliances, high-quality component sealants. Whereas in USMW the high frequency vibrations are introduced horizontally to increase the temperature and to plastify the material. The moderate clamping pressure applied vertically to disperse the oxides and contaminants and to bring in an increasing area of pure metal contact. The progressive shearing forces and plastic deformation of asperities result in the bonding of the faying surfaces [6]. Ultrasonic metal welding (USMW) was invented over 60 years ago and has been used to weld several types of metals and their alloys. USMW involves a solid-state joining process in which metals are fastened together through the application of pressure combined with localized high frequency shear vibrations at the welding zone. The action of high frequency relative motion between the metals locally softens the overlap zone of the specimens to be welded, forming a solid-state weld because of the progressive shearing and deformation between surface asperities which

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disperses oxides and contaminants through a high frequency, scrubbing motion. This increases the area of pure metal contact between the adjacent surfaces, in which the metal atoms are forced together to create a strong weld. In the present investigation a typical pure copper sheets have been ultrasonically welded in similar manner through a 2-KW lateral drive ultrasonic welding machine. The objective of this work is to obtain the optimized welding parameters through the full factorial design of experiments methodology and also the interactions of each welding variables on the weld strength has been studied.

II.

PREVIOUS STUDIES ULTRASONIC WELDING

ON

To study about the ultrasonic welding and its process characterstics and to obtain the suitable methodology for optimization process a lot of investigations were carried out and some of them are given below. Elangovan et al. [5] carried out a study to optimize the process parameters like weld time, weld pressure, and amplitude of vibration to maximize the weld strength in aluminium to alumina (Al- Al2O3) joint by using Taguchi’s design of experiments methodology. Also the Finite Element Analysis (FEA)-based studies were carried out for joints wherein the temperature and stress were obtained. It was found from this study that the stress levels increases with weld time and welded joint having better strength at increased weld time. FEA results for Al– Al2O3 joint indicate that higher interface temperatures were developed. This can be due to the reduced thermal conductivity and preheating of Al2O3. The work was carried out by Patel et al. [6] on Magnesium–Aluminium joint by USMW. It was observed that a layer of intermetallic compound (IMC) consisting of Al12M17 formed at the weld centre where the hardness became higher. The lap shear strength and failure energy of the welds first increased then decreased with increasing in welding energy. Failure predominantly occurred in between the aluminium alloy and the intermetallic layer. As a consequence, brittle

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© 2015 IJIRT | Volume 1 Issue 12 | ISSN: 2349-6002 fractures occur at the IMC side in the welded sample of 2500 J energy input and above. Devine et al. [9] conducted the experiments of Ultrasonic Sonic Metal Welding (USMW) on aluminium alloys and concluded that USMW is a good economical joining method for aluminium vehicle construction. Utilization of USMW joining technology is appropriate for a range of aluminium gauges, lubricant levels and aluminium alloys. Physical deformation at weld interface and tip and anvil interfaces were occurred concurrently. Mechanical mixing occurs at the interface and some deformation of grains obtained at the interfaces of the tip and anvil with the weldments. There is no evidence of melting. Also it required less energy and lower cost than for resistance spot welding than riveting. Zhao et al. [10] carried out the experimental investigation on Aluminium–Copper ultrasonic welded joint to obtain the effect of welding energy on joint strength, failure behaviour and microstructure. The results showed that joint strength increased with welding energy initially and reached its maximum at 1000 J, then dropped significantly instead. The various microstructures with different morphologies and properties were observed at the interfacial region. At lower energy, the joint was only partly bonded by number of dispersed micro bonds. However, cavity defects and intermetallic compound (IMC) were more likely to form under excessively high energies and a 0.5 mm thick IMC layer with dominant phase of Al4Cu9 was found in 2000 J welding energy. Also the swirl-like structure was observed at the weld interface, which would lead to a mechanical interlocking between the materials and enhance the joint strength. Sasaki et al. [11] analysed the dynamic vibration behaviours between the welding tip and aluminium sheet by using the digital image correlation method. The welding process consisted of the following three stages. First, the upper specimen in contact with the weld tip and the formation of partially welded regions was confirmed at this stage. Second, the vibration amplitude of the upper specimen decreased, while friction between the weld tip and the upper specimen increased. Growth of the partially bonded region was confirmed in the second stage. Third, the welding part began to plastically deform owing

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to the clamping force. The joint strength reached its maximum value at the third stage. The analysis demonstrated that the relative motion between the weld tip and the upper specimen predominantly affected the increase in joint strength. Kim et al. [12] suggested that by the thermo-mechanical analysis of an ultrasonic spot welding process that performed using developed finite element models. It was shown that the effect of frictional heating on the plastic deformation of aluminium during ultrasonic spot welding was significant and should be included in the analysis of the welding process in order to achieve good correlation with the physical. The experimental measurements as the temperatures and ultrasonic velocities at various points on the welded specimens has been observed from the coupled thermo-mechanical analysis using the developed model that the influence of heat. The aluminium does not melt during welding; it was recommended that frictional heating should be included in a model of ultrasonic spot welding since the heat generated during welding was a strong driver of the extremely large plastic flow observed in welded specimens. Choi et al. [14] investigated that on robotic ultrasonic welding to perform robotic welding on the battery module tab of general or specific configuration and to collect and analyse vibration data at various locations of interest on the robot and work piece and also to identify the technical challenges, demonstrate, or propose feasible solutions. Hence they were observed that the feature for vibration reduction should be small to the extent that the feature does not serve as a stressconcentrating region under the dynamic loading, which possibly caused a crack or failure on the structure during the welding. Some manufactures used a method of clamping that forces the traveling wave to stop. The vibration reduction performance was evaluated under the various clamping force. The clamping pressure ranging from about 15 psi to 35 psi not only reduce the vibration below the clamping position but also greatly reduce the amount of vibration on the tab when compared with no-block case. Michele carboni et al. [15] conducted the experimental characterization of the tensile-shear fatigue behaviour of both spot welded lap-joints obtained by USMW and hybrid lap-joints obtained by USMW plus adhesive bonding. All the

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© 2015 IJIRT | Volume 1 Issue 12 | ISSN: 2349-6002 considered lap-joints were realized joining thin sheets made of AA6022T4 aluminium alloy or AZ31B magnesium alloy. A careful failure analysis was carried out on all the tested specimens and a comparison of the fatigue performance to other spot welding techniques was also presented. In absolute terms, AA6022T4 joints seem to have a better fatigue performance than AZ31B ones in terms of both load entity and standard deviation; for high applied mean stresses (R=0.7), a different failure mechanism seem to take place with respect to R=0.1 and R=0.3; the performance of hybrid joints was significantly better than the one of USMW alone, while, in relative terms, AZ31B seem to have a better performance with, respect to AA6022T4; contrarily to traditional spot welding, it seem that the presence of the adhesive allowed just one failure mode (shear failure) for hybrid lap-joints. Ganesh and Praba Rajathi [17] were evaluated that on the relevance of various factors influencing the lap joining technique and allowing a deep understanding of the phenomena and the possibility to keep them under control. Ultrasonic welding was done on a specimen in three different parameters and also the weld time, amplitude were playing a vital role in weld strength. When amplitude was low with high weld time and moderate pressure, weld strength was poor. When pressure was low high amplitude and high weld time, the weld strength was good. When the amplitude was high, the weld strength also was high. Thus the ultrasonic welding process and its characterisations were studied through above mentioned papers and to carry out the welding on the materials as selected as copper sheets with 0.5 mm thickness. Also the weld parameters as selected as weld pressure, amplitude and weld time and each having the 3 levels. To optimize these parameters the full factorial design of experiment (FFD) methodology was adopted with the response variable of tensile shear strength. III.

EXPERIMENTAL STUDIES

The welding was carried out using a conventional ultrasonic metal welding machine as shown in Figure 1. The USMW system having the components of ultrasonic generator, converter, booster, horn, holding fixture, pneumatic assembly and start switches. Its specifications are shown below.

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Frequency

= 20 KHz.

Allowable pressure

= 0-10 bar.

Allowable amplitude = 0-80 µm. Allowable weld time = 0-10 s.

Figure 1 Ultrasonic metal welding machine. IV.

METHODOLOGY

In statistics, a full factorial design (FFD) of experiment is an experiment whose design consists of two or more factors, each with discrete possible values or "levels", and whose experimental units take on all possible combinations of these levels across all such factors. A full factorial design may also be called a fully crossed design. Such an experiment allows the investigator to study the effect of each factor on the response variable. In this experiment, the objective was to maximize the weld strength and hence it is the-higher-the-bettertype characteristic. Therefore, the optimal level of the process parameters has to be obtained with the combination of individual parameters. In this study the parameters as selected as the pressure, weld time and amplitude and each parameters having three levels through the full factorial design of

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© 2015 IJIRT | Volume 1 Issue 12 | ISSN: 2349-6002 experiment methodology. The FFD experimental design for the welding as copper to copper (Cu-Cu) joint and as shown in table 1 as following.

.

The full factorial design for ultrasonic welding on copper samples having the 3 factors and 3 levels type of design which having the totally 27 number of combination of parameters to obtain the maximized weld strength. The parameters range has been obtained through trial and error method. After the trial and error method the parameters range which is the weldability limits for copper samples through ultrasonic welding as identified as shown in table 1. Table 1 FFD design for Cu-Cu joint.

Figure 2 Joint configuration of USMW VI.

PARAMETERS

PARAMETER LEVEL 1

2

3

Weld pressure (bar)

6

6.5

7

Amplitude (µm)

56

60

64

Time (sec)

Weld time

2.6

3

3.4

Hold time

1

1.2

1.5

Delay time

1

1

1

V.

EXPERIMENTAL WORK

The specimens were prepared as per ASTM standard [5] and welding was carried out on 27 samples. The welded samples as per the combination of parameters of FFD as shown in figure 2 below.

SPECIMEN PREPARATION

The specimens were prepared as per the standard of ASTM D1002 [5]. Specimens were cut into the dimensions as 0.5 mm ×15 mm ×55 mm. with overlap distance of 15mm. The joint configuration to weld the specimen as shown in figure.2. A tensometer was used to determine the weld strengths which were coupled with the computerized data storage system. The specimens were folded by the hold and fixture to avoid slippage due high frequency shearing forces.

Figure 3 Copper welded samples.

Welded samples were tested using tensometer which is shown in figure 4 below. There were a 27 samples tested using this machine on copper joints.

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© 2015 IJIRT | Volume 1 Issue 12 | ISSN: 2349-6002 Figure 5 Micro hardness testing machine.

Figure 4 Tensometer machine set up.

The optimum weld strength was obtained after carried out of tensile shear test then to obtain the microhardness on welded joint which having maximized weld strength. The microhardness was carried out using Vickers hardness testing machine as shown in figure 5 below.

The welded specimen which having the optimized weld strength was taken to carried out the microscopical analysis. Microscopic test was carried out using the radical microscopic examination machine as shown in figure 6 below.

Figure 6 Radical Microscope

VII.

RESULTS AND DISCUSSION

The welded specimens were tested using the tensometer to obtain the tensile shear on each combination of parameters as totally 27. The optimized weld strength has been obtained and tabulated as shown in table 2 as following. For aluminium to copper joint the 16th combination of parameters given the maximum weld strength of 55.6MPa. Through table.10 the maximum weld strength obtained in ultrasonic welding on Cu-Cu joint at the welding parameters of Weld pressure = 6.5 bar, Amplitude = 64 µm, Weld time = 2.6 sec as =55.6 MPa. Welded samples were tested using tensometer which is shown above in n figure 4 to obtain the tensile shear strength and the broken samples which through tensile load as shown in figure 7 below. The samples were fractured mostly on weld metal section.

Table 2 USMW on Cu-Cu joint.

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S.NO

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

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WELD PRESSURE (bar)

6 6 6 6 6 6 6 6 6 6.5 6.5 6.5 6.5 6.5 6.5 6.5 6.5 6.5 7 7 7 7 7 7 7 7 7

AMPLITUDE (µm)

56 56 56 60 60 60 64 64 64 56 56 56 60 60 60 64 64 64 56 56 56 60 60 60 64 64 64

WELD TIME (sec)

TENSILE STRENGTH (MPa)

2.6

21

3

28.7

3.4

45.6

2.6

20.7

3

33

3.4

33.3

2.6

27.7

3

35.6

3.4

33.3

2.6

21.6

3

33.3

3.4

29.6

2.6

35.6

3

27.6

3.4

39.6

2.6

55.6

3

24

3.4

39.6

2.6

25.6

3

37.3

3.4

39.3

2.6

37.6

3

22.3

3.4

33.3

2.6

31

3

51

3.4

38

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© 2015 IJIRT | Volume 1 Issue 12 | ISSN: 2349-6002 Figure 8 Main effects plot for tensile strength on CuCu joint. Figure 9 shown that interaction of welding variables on weld strength. From this graph at 6 bar 6.5 bar the weld strength was dropped initially but raised after that of 30µm and when pressure of 6 bar the strength was increased gradually with weld time. At 7 bar pressure the strength was dropped initially then raised gradually. Due to amplitude of 28 µm the strength was raised with weld time and at 30 µm dropped initially then increased. when amplitude of 32 µm the strength was decreased slightly with weld Interface

Left side

Right side

HV-85.2

HV-88.7

HV-87.8

Figure 7 Copper tested samples. Through the minitab software the results of tensile shear test were analysed and the interaction of each welding parameters on weld strength has been investigated as following.

time. Interaction Plot for tensile strength Data Means

From figure 8 main effects plotted for tensile strength and it showed that weld strength was increased with increase in pressure and weld time. From this graph initially curve was flat but raised after 30µm.

28

30

32

2.6

3.0

3.4 40

32

pressure

24 40

Main Effects Plot for tensile strength Data Means

pressure

38

amplitude

pressure 6.0 6.5 7.0

32

amplitude

weld time

amplitude 28 30 32

24

37

weld time

36

Mean

35 34 33

Figure 9 Interaction plot for tensile strength on CuCu joint.

32 31 30 6.0

6.5

7.0

28

30

32

2.6

3.0

3.4

The parameters settings for Cu-Cu joint as the16th combination was have to be taken for microstructure and microhardness consideration as following in table 4. Load: 200kf,

Dwell time:

15sec

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© 2015 IJIRT | Volume 1 Issue 12 | ISSN: 2349-6002 Table 4.4 Cu-Cu joint hardness

Here the base metal section having the higher hardness value than intermetallic section. The results were analysed which arised during microhardness test through the minitab software as following. Here the hardness values are taken from intermediate section and at 2 points from intermediate to base material direction which has the average interval of 0.15mm. Through this analyse by minitab software the hardness values increased with increased in distance from intermediate section which means the hardness value was obtained higher in base metal section than intermediate section as show in figure 10 Also the hardness was higher in lower weldment than upper weldment which means lower weldment heat treated more than upper weldment. Figure 11 Microstructure of Cu-Cu joint.

Main Effects Plot of interface, upper 1, upper 2, lower 1, lower 2 Summary Report Group Means

• The figure 11 shows that there is intermetallic diffusion occurred from both sides which means strong bond obtained from copper joints.

Compare the means and intervals across the levels. 65

60

55.3

55 Y

• Bond width narrow in middle section and rise in sideward direction because of plastic flow can be occurred due to pressure.

50

45

40

VIII. interface

upper 1

upper 2 X

lower 1

Statistics

interface

upper 1

upper 2

lower 1

lower 2

N Mean 95% CI StDev

1 38.3 (*, *) *

1 57.3 (*, *) *

1 59.1 (*, *) *

1 56.6 (*, *) *

1 65.2 (*, *) *

Figure 10 Main effects plot for hardness.

In this paper the parametric optimization of ultrasonic metal welding on Cu-Cu joint has been studied. For this purpose full factorial design of experiments methodology was developed. The tensile shear strength as taken as response variable with consideration factors as pressure, amplitude and weld time. Moreover, main effect and interaction plots are used for detailed explanation of analysis of results. The obtained results are, 1.

The welded specimen which has the optimized weld strength was taken to the microstructural investigation. The investigated sample as shown in figure 11 below as following.

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CONCLUSION

lower 2

From Cu-Cu joint the maximized weld strength was obtained as 55.6MPa at weld time of 2.6 sec , amplitude of 64 µm and pressure of 6.5 bar then weld strength

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3.

4.

IX.

When pressure and weld time increased then the weld strength will also be increased. The weld strength is good in medium and lower level of weld amplitude. At higher amplitude level the crack will propagate with increase in pressure and welding time. The intermediate section as the weld metal section has lower value than the base metal section; hence hardness decreased. The hardness at upper specimen was higher than the lower specimen; hence the upper specimen obtained the higher temperature than lower specimen. The intermetallic diffusion occurred from both sides which means strong bond obtained from copper joints. REFERENCES

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7. Metals Handbook-Welding, Brazing and Soldering, American Society for Metals, 10th edition, Volume 6, USA, 1993. 8. Ziad Shakeeb, Al-Sarrafhttp, “a study of ultrasonic metal welding” ://theses.gla.ac.uk/4375/,2012. 9. Janet Devine and Joe Walsh, “ Ultrasonic Welding of Aluminium Sheet,” Sonobond Ultrasonics, Inc, 2012. 10. Y. Y. Zhao, D. Li and Y. S. Zhang, “Effect of welding energy on Al–Cu ultrasonic welded joint,” Science and Technology of Welding and Joining, VOL 18 NO 4 360, 2012, 2013. 11. T. Sasaki1, T. Watanabe1, Y. Hosokawa2 and A. Yanagisawa, “Analysis for relative motion in ultrasonic welding of aluminium sheet. Science and Technology of Welding and Joining,” VOL 18 NO 1 23, 2012. 12. T.H. Kim, J. Yuma, S.J. Hu, J.P. Spicer , J.A. Abell, “Process robustness of single lap ultrasonic welding of thin, dissimilar materials,” CIRP Annals - Manufacturing Technology 60 (2011) 17–20, 2011. 13. Zhengqiang zhu, haibo mi, xiaolong wang, chenyang xie, and zonghui wu, “Effect of Carbon Nanotube Addition on Strength of Aluminium Welds,” DOI: 10.1007/s11663-012-9744-1, 2012. 14. Sang Choi, George Zhang, and Thomas A. Fuhlbrigge “Vibration Analysis in Robotic Ultrasonic Welding,” 2011. 15. Michele Carbonia, Fabrizio Moronib, “Tensile-Shear Fatigue Behaviour of Aluminium and Magnesium Lap-Joints,” Procedia Engineering 10 3561–3566, 2011. 16. E. T. Hetrick, j. R. Baer, w. Zhu, l. V. Reatherford, a. J. Grima,d. J. Scholl, d. E. Wilkosz, s. Fatima, and s. M. Ward, “Ultrasonic Metal Welding Process Robustness in Aluminium”. 17. Ganesh and Praba Rajathi, “Experimental study on ultrasonic welding of aluminium sheet to

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© 2015 IJIRT | Volume 1 Issue 12 | ISSN: 2349-6002 copper sheet” eISSN: 2319-1163 | pISSN: 23217308, 2012. 18. ASTM International Codes Standard test method for apparent shear strength of single-lap-joint adhesively bonded metal specimens by tension loading (metal-to-metal). ASTM Int 01:52–55, 2005.

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