Conventional Powder Metal Is Still A Technology Leader

Conventional Powder Metal Is Still A Technology Leader Timothy R. Weilbaker, BorgWarner, Inc. Earl R. Lumpkins, Hoeganaes Corporation ABSTRACT Recent...
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Conventional Powder Metal Is Still A Technology Leader Timothy R. Weilbaker, BorgWarner, Inc. Earl R. Lumpkins, Hoeganaes Corporation

ABSTRACT Recent advancements in powder metal technology have made it possible to achieve physical properties rivaling many competitive technologies. Improvements in raw materials have made powder metal a viable replacement for several malleable and ductile cast irons. The combination of raw material and processing improvements continues to push powder metal technology performance into the wrought steel arena. Nevertheless, in the midst of all of these technological advancements, conventional powder metallurgy is still providing innovation in torque transfer systems. At BorgWarner’s TorqTransfer Systems division, conventional powder metallurgy has found application in six separate components of the newly created interactive torque management system dubbed ITM. This patented torque transfer device provides the all-wheel drive technology for MotorTrends SUV of the year – the Honda Acura MDX.[4] This paper describes how conventional powder metal technology provided the perfect solution for this highly innovative torque transfer technology. INTRODUCTION Technological advances are common in every industry today. Within the automotive industry, in particular, constant innovation is the only way for a company to survive and grow. Innovation was exactly what was required to revolutionize the traditional fourwheel drive systems used in rear-wheel drive vehicles. Once the electromechanical design was created, the components had to be manufactured. If the components of this technology were not equally innovative, some aspect of the new torque transfer system would be jeopardized. To meet these critical requirements, powder metallurgy (P/M) was researched, designed and application tested to demonstrate that indeed conventional P/M was both an effective and economical solution for this newly developed ITM torque transfer system. The goals of this paper are: 1) to briefly describe the various types of torque management systems, 2) to provide an overview of the Interactive Torque Management

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(ITM) system, 3) to describe the P/M components utilized in the ITM system, and 4) to show how conventional P/M technology, even today, can still be used to create innovation in automotive systems. TYPES OF TORQUE MANAGEMENT SYSTEMS In all types of torque management systems the primary axle is driven by a direct connection. The secondary axle is driven by a number of different methods including [1]: Viscous Clutch - A viscous clutch is a conceptually simple viscous device which resists relative motions. This device does not deliver torque to the secondary axle until a relative speed exists between the propshafts. The amount of torque delivered is a function of the relative speed until a hump event occurs. Gerotor Pumps - There are two types of gerotor pump systems. A single gerotor pump system pressurizes a hydraulic piston and applies a clutch pack when a relative speed occurs. This type of system has the two pump elements driven by the two output shafts. A twin gerotor pump develops a differential pressure when there is a speed difference between the shafts where the differential pressure applies a clutch pack. This type of system has one pump driven by the input member of the secondary driveline and the second pump driven by the output member. The clutch pack is applied by the difference in pressure between the two pumps. Visco-Lock - A viscous pump using silicon fluid develops pressure on a clutch pack when a difference in speed exists between the two output shafts. Hydraulic - An externally controlled hydraulic system can be used to apply a clutch pack using a piston. A control valve modulates the pressure delivered to a clutch pack. A hydraulic circuit or an electronic controller modulates the control valve. Roller Clutch - A roller clutch activated system allows the secondary shaft to rotate faster than the primary but will not allow the primary to rotate faster than the secondary. These devices are maintained in proper orientation by a friction plate to ground or an electromagnetic clutch. Electromagnetic Clutch - An electrically actuated clutch and an electronic controller causes a magnetic field to be developed resulting in friction in proportion to the field strength. This type of clutch may use a mechanical amplification device creating a clamp load on a secondary clutch pack. Mechanically Clutch Pack - A clutch pack applied by a cam mechanism. The cam is actuated by an electric or hydraulic mechanism. INTERACTIVE TORQUE MANAGEMENT An Interactive Torque Management device is an active torque management device that does not deliver torque to a secondary axle until acted upon by some outside force. The controller for this outside force is in constant communication with other vehicle systems. These systems include braking control (ABS), traction control (TCS) and

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stability control (ESP). Further, only those systems that maintain communication or systems that transfer torque at a variable rate qualify as ITM devices. Consequently, a device is an active one if it delivers torque as required and can engage/disengage to support the function of other handling systems. [1] BorgWarner ITM I TECHNOLOGY The device shown in Figure 1 is the latest generation of the BorgWarner ITM I technology. This device is an electromagnetic actuated multi-plate wet friction clutch mechanism, which is capable of maintaining constant communication with vehicle dynamic systems as well as adapting to road conditions. The ITM I is intended to control distribution of torque between primary and secondary axles. The ITM I utilizes an electromagnetic primary clutch consisting of an electronic coil, a rotor, friction plates and an armature ring. The rotor surrounds the stationary electronic coil and rotates at the same speed as the secondary axle pinion. The primary friction plates are alternately splined to the input housing and the base cam. The armature ring is adjacent to the friction rings and is splined to the input housing. The armature applies pressure to the rings when sufficient current is passed through the coil to attract the armature and friction rings to the rotor. This attraction leads to torque transfer across the ball cam mechanism. [1] [2] The cam mechanism amplifies the force from the primary clutch to the secondary clutch. The cam mechanism consists of an apply cam and base cam with corresponding ball ramp pockets and a multiple number of balls. The apply cam is splined to the output shaft and the base cam is acted upon by the input housing when the electronic coil is energized. When the primary clutch is engaged, torque is applied to the base cam, which is free to rotate about the output shaft. [1] [2]

Figure 1: ITM Cross Sectional View

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If a speed difference exists between the input housing and the output shaft the two cam pieces will rotate relative to each other and the balls will travel in the ball ramps and compress the secondary clutch pack. The amount of secondary clutch compression or torque output of the unit is dependent on the current through the coil and the amount of traction available at the wheels. The total torque output is a combination of the torque from the primary and secondary clutches. The multi-plate wet friction secondary clutch consists of a splined input housing, friction plates and a splined shaft. The wet friction secondary plates consist of intermittently spaced stamped steel and stamped steel with paper friction surface rings. These rings are alternately splined to the input housing and the output shaft. When the secondary clutch is compressed by the apply cam, torque is transferred to the secondary axle. [1] [2] The key to the interactive portion of this device is the Electronic Control Unit (ECU) which can be a separate, stand-alone device integrated into an existing controller or it can be integrated into the ITM I assembly. The ECU can receive important information from other systems such as wheel speeds, steering angle, throttle position, engine torque / speed, etc. This communication also allows the torque applied to the secondary wheels to be dynamically adjusted to improve traction, braking, and vehicle stability. [1] [2] ITM I APPLICATION OPTIONS Due to the ITM I’s relatively small size, less than 170 mm (6.7 in) ∅, the unit can be placed on the transmission power take-off (Figure 2), in the driveline (Figure 3) or on the Axle pinion (Figure 4). In all these applications the ITM I can either be supplied as a bolt-on device with its own housing or packaged into an existing transmission. [2] BorgWarner ITM II TECHNOLOGY The logical evolution of the ITM I technology is the ITM II. In the ITM II an ITM I is placed on either side of the ring gear on the secondary axle. This makes the torque independently variable across the axle, side to side as well as between the front and rear axles. Figure 6 shows the ITM II cross section. In this drawing, the ITM I modules can be seen mounted on either side of the hypoid gear set on the rear axle. Figure 5 shows the common mounting position for the ITM II. [2] ITM II APPLICATION OPTIONS Since the ITM II is an evolution of the ITM I technology, the ITM II will share most of the benefits of the ITM I performance. The ITM II components will share common construction with the ITM I, as well. Further, the ITM II system will deliver specific enhancements to handling in key areas. Use of the ITM II includes two optional axle ratios. The secondary axle ratio can be that of the primary drive wheels or set up to overdrive the primary drive wheels. Performance differences have been well detailed. [2]

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Figure 2: ITM I - PTO

Figure 3: ITM I - Driveline Mount

Figure 4: ITM I - Rear Axle Mount

ITM II

PTO

Figure 5: ITM II Mounting Position

Figure 6: ITM II Cutaway

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ITM ASSEMBLY COMPONENTS It is evident that the components used to construct the ITM I and ITM II systems require the use of high performance magnetic and structural materials. Further, the competitive pricing pressures within the automotive industry require a detailed analysis of all feasible manufacturing technologies. The culmination of this work resulted in the use of P/M technology for select components. The design work revealed that P/M components utilizing conventional materials and processing techniques met both cost and performance requirements. [3] The components designated for production by P/M techniques include the following [3]: Armature - Shown in Figure 7, the armature (see Figure 1: labeled E) is a binder treated MPIF material F-0000 Modified with a 0.80 w/o P (weight percent) content added as a master alloy of Fe3P. This material is compacted to a green density of 7.10 g/cm3. The green component is then sintered at 1120°C (2050°F) for 30 minutes in a 90/10 v/o (volume percent) mixture of N2/H2 and slow cooled. This combination of material and processing was chosen for its resultant excellent magnetic properties. Subsequent to sintering, the component undergoes a restrike operation to reduce dimensional variation and increase its density to 7.15 g/cm3. Secondary operations include a gas carburizing heat treatment for wear resistance and grinding to maintain strict flatness and overall length tolerances. A final machining operation creates grooves on the surface of the component to provide oil passageways. To meet the performance hurdles for this application, other techniques exist that could have been used to manufacture this component, such as: stamping, CNC machining and DPDS (double-pressed and double sintered) P/M to list but a few. However, conventional P/M processing, as described above, met the tough performance requirements of the ITM system at the lowest cost. [3]

Figure 7: Armature

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Clutch Backing Plate: - Shown in Figure 8, the clutch backing plate (see figure 1: labeled D) is an MPIF material SS-304-N1. This material is compacted to a green density of 6.80 g/cm3. The component is then pre-sintered at 950°C (1750°F) for 30 minutes in 100 v/o H2 and slow cooled. The component is then re-pressed for additional densification. A final sinter of 1120°C (2050°F) for 30 minutes again in a 100 v/o H2 with a slow cool yields a final density of 7.00 g/cm3. And lastly, two grinding operations are performed on the component. The first grinding operation is performed to control flatness and microfinish. The final grinding operation is performed to maintain the critical overall length requirement necessary to ensure proper stack-up in the ITM unit. This material and processing combination was chosen to satisfy the requirements for a component that would retain no magnetism. This characteristic is important to prevent any residual load (known as “drag”) while the ITM system is in the standby torque mode. The ability of P/M technology to compact stainless steel powder into a near-net shape component made P/M the most viable approach for producing this part. [3]

Figure 8: Backing Plate Clutch Hub: - Shown in Figure 9, the clutch hub (See Figure 1: labeled C) is a binder treated MPIF material FN-0205. This material is compacted at room temperature to a green density of 7.00 g/cm3. The component is then sintered at 1120°C (2050°F) for 30 minutes in 90/10 v/o N2/H2 and slow cooled. Next, the component has four small holes drilled through from the outer spline radially into the inner spline for oil passage. The component is then gas carburized followed with a quench and temper process to achieve an apparent hardness of 40HRC on the surface. This material and process combination was developed to solve the need for a high strength material with excellent surface wear resistance combined with a very tough core to satisfy the demands of the ITM application. [3]

Base Cam: - Shown in Figure 10, the base cam (See Figure 1: labeled B) is a binder treated MPIF material FL-4405 Modified to 0.4 w/o Graphite maximum addition. (This is a hybrid material composed of Ancorsteel 85HP base with graphite and lube added.) This material is compacted at room temperature to a green density of at least 7.10

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g/cm3. The component is then sintered at 1120°C (2050°F) for 30 minutes in 90/10 v/o N2/H2 and slow cooled. [3] This component is critical to the application of torque in the ITM system. Both the Base Cam and the Apply Cam must absorb axial loads during torque transfer in the ITM system. The “kidney-shaped” ball ramp areas must meet precise size specifications and resist wear. As a result, the component is gas carburized followed with a quench and temper process to achieve an apparent hardness of 40HRC on the surface. The material and process combination for each cam was developed to solve the need for a high strength material with excellent surface wear resistance that could be produced utilizing the lowest cost process route possible. [3]

Figure 9: Clutch Hub

Figure 10: Base Cam

Apply Cam: - Shown in Figure 11, the apply cam (See Figure 1: labeled A) is a binder treated MPIF material FL-4405. (This is a hybrid material composed of Ancorsteel 85HP base with graphite and lube added.) This material is compacted at room temperature to a green density of at least 7.10 g/cm3. The component is then sintered at 1120°C (2050°F) for 30 minutes in 90/10 v/o N2/H2 and slow cooled. Again, because this component is critical to the application of torque in the ITM system, the component is gas carburized followed with a quench and temper process to achieve an apparent hardness of 40HRC on the surface. The combinations of high wear resistance, toughness, and surface form complexity make both cams perfectly suited to the P/M process. [3]

Input Hub: - Shown in Figure 12, the input hub (not shown in Figure 1) is an MPIF material FL-4405. (This is a hybrid material composed of a binder treated 0.85 w/o Mo prealloyed iron base with additions of graphite and lube) This material is compacted at room temperature to a green density of at least 6.90 g/cm3. This component then receives a pre-sinter treatment at 850°C (1550°F) for 20 minutes in a 90/10 v/o N2/H2 atmosphere. After the pre-sinter, the hub is again compacted under approximately 150 tons to achieve final densification up to 7.35 g/cm3. Subsequent to the second compaction operation, the input hub is sintered at 1120°C (2050°F) for 30 minutes in 8

90/10 v/o N2/H2 and slow cooled. After this final sinter operation, the component undergoes a typical carbonitriding heat treatment. Like the Base and Apply Cams, this component is critical to the application of torque in the ITM system, the component is gas carburized followed by a quench and temper process to achieve an apparent hardness of 40HRC on the surface. At this point, the component is complete and ready for assembly into the vehicle. Again, the near-net characteristics of P/M technology make it difficult for alternate technologies to produce this component competitively.

Figure 12: Input Hub

Figure 11: Apply Cam

CONCLUSION This paper has identified the abilities of the ITM I and ITM II systems in real-world automotive applications. The ITM systems are ideal for packaging in all types of vehicle platforms. The ITM unit is particularly well suited for adding AWD capability to a FWDbased vehicle and giving the added advantage of tailoring performance to the needs of the vehicle. [3] This innovative technology will almost certainly change the face of torque transfer systems and vehicle stability systems. Without a doubt, ITM has already effectively raised the chinning bar of what is possible in powertrain systems. P/M provides attractive torque-transferring component solutions for several reasons. First, the near-net shape capabilities of P/M make this process advantageous from a cost standpoint. The P/M technology produces components where precision tolerances and high wear resistance are required. The many raw materials and processing options available through P/M satisfy even the toughest torque transfer requirements. Demanding physical properties such as axial loads, radial torque, and magnetic and non-magnetic properties are all possible utilizing conventional P/M technology. The ability of P/M to achieve assembly-ready components in a minimal number of process steps using countless material options ensures P/M’s structural advantage and cost competitiveness. The ITM system benefits by utilizing conventional P/M materials and processes as opposed to more complex P/M solutions or other manufacturing techniques. Conventional powder metal offers component solutions that weigh approximately 10%

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less than the same component would weigh at densities approaching wrought materials. Finally, using conventional P/M versus alternative solutions saves anywhere from 4060% in component costs. Obviously, the use of conventional P/M technology has made a difference in the world of torque management. By partnering the design of the ITM technology with the selection of the materials and processes used to manufacture the components, BorgWarner has created a torque management system that is light, quiet, and electronically sophisticated. [4] Consequently, the powder metal process is advancing all-wheel drive technology making lower cost, lighter weight, and higher performance systems achievable, available, and affordable. [3]

REFERENCES: 1) D. Showalter, et. al., “Torque on Demand 4x4 Systems”, 1997, SAE Technical Paper No. 974195, SAE, Detroit, MI. 2) C. Kowalsky and L. Pritchard, “Interactive Torque Management for Front Wheel Drive Based All Wheel Drive Systems” (BWA Torque Transfer Division Technical Review 2000) 3) T. Weilbaker and E. Lumpkins, “Creating Innovation in Torque Transfer Systems Through Optimization of Powder Metallurgy Components”, 2001, SAE Technical Paper No. 2001-01-0350, SAE 2001 World Congress, Detroit, MI. 4) C. Van Tune, et. al., (2001). “Motor Trend: 2001 Acura MDX” (online), 23 October 2000. http://www.motortrend.com/dec00/mdx/mdx_f.html [accessed 1 May 2001].

CONTACT: Mr. Timothy R. Weilbaker, CMfgE is Operations Manager for the Powdered Metals plant of the BorgWarner TorqTransfer Division. You may contact Mr. Weilbaker at: [email protected]

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