Net Shape Capability

A major advantage of the P/M approach is the net-shape capability, particularly for high-strength materials. This has led to continuing developments in P/M technologies for parts production.

Warm Compaction. One recent development in P/M production is warm compaction, which allows the production of higher-density ferrous P/M parts via a single compaction process. The process utilizes heated tooling and powder during the compaction step. The powder and tooling are typically heated between 130 and 150 °C (260 and 300 °F). In order for the powder premix to perform at these temperatures, a proprietary lubricant system has been developed that provides lower die ejection forces than conventional lubricants. This lubricant system also incorporates a polymeric binder system to limit segregation and provide enhanced flow characteristics of the powder premix. By utilizing warm compaction technology, the green density of the consolidated part can be increased from 0.10 to 0.25 g/cm3 over traditionally processed (single-pressed/single-sintered) materials. The green strength is typically increased between 50 and 100%.

Table 3 (Ref 7) compares the green properties of warm-compacted and cold-compacted P/M parts. This increase in green strength provides advantages such as a reduction in green chipping and cracking due to part handling prior to sintering and makes possible the crack-free compaction of complex multilevel parts. Additionally. the higher green strength provides an opportunity to machine the P/M part in the green state. This capability is critical in the use of highperformance alloy systems that achieve high hardness in the as-sintered state. Warm compaction also enables P/M fabricators to single press and single sinter ferrous P/M parts to densities as high as approximately 7.4 g/cm3, which is considerably higher than cold-compacted-and-sintered P/M parts.

Table 3 Effect of processing on the green properties of ferrous compacts

Base material

Processing technique

Compaction pressure

Green density, g/cm3

Green strength

Peak die ejection force

MPa

tsi

MPa

psi

MPa

tsi

Ancorsteel 85HP(a)

Warm compaction

415

30

7.14

23.2

3370

29.6

2.15

550

40

7.31

25.4

3685

33.5

2.43

700

50

7.37

24.7

3580

32.0

2.32

Cold compaction

415

30

7.00

9.9

1430

37.2

2.70

550

40

7.19

12.2

1770

50.7

3.68

700

50

7.29

13.4

1950

53.8

3.90

Distaloy 4800A(b) Warm compaction

415

30

7.07

28.3

4100

27.4

1.99

550

40

7.29

30.6

4445

31.7

2.30

700

50

7.36

31.1

4515

32.3

2.37

Cold compaction

415

30

6.93

12.2

1770

37.2

2.70

550

40

7.15

15.0

2170

48.5

3.52

700

50

7.26

16.9

2450

52.0

Source: Ref 7

(a) Ancorsteel 85HP is a prealloyed steel powder containing 2.0% Ni, 0.85% Mo, 0.4% graphite, and 0.6% lubricant.

(b) Distaloy 4800A is a diffusion-alloyed steel powder containing 4% Ni, 1.5% Cu, 0.50% Mo, 0.5% graphite, and 0.6% lubricant.

Warm compaction, although applicable to all ferrous material systems, produces the greatest benefits when coupled with high-performance compositions such as diffusion-alloyed steels or molybdenum-prealloyed steels. Achieving densities in excess of 7.25 g/cm3 using these compositions results in mechanical properties that are comparable to steel forgings and ductile iron castings.

Stainless Steels. Other developments in conventional P/M steels include new and improved stainless steels, with the goal of improving compressibility and corrosion resistance. Developing applications include automobile parts such as a solenoid spacer in an electronic fuel injector, sealing washers in the water pump, and brake components. They also include other applications such as bearing holders and pulleys in computers.

Metal injection molding (MIM), which is finding increased use in small parts manufacturing, uses fine powders (510 /' m) that exhibit good sintering densification in combination with binders that hold the particles in place for transportation (Ref 8). The basic five steps involved are feedstock characterization, mixing rheology, injection molding, debinding, and sintering (see Fig. 4 in the article "Powder Metallurgy Methods and Design" in this Volume). With tolerances as low as 0.3%, sizing is generally not required. Densification to a usable article is accomplished in the sintering furnace. However, surrounding these seemingly simple steps there is a great deal of "know-how" that has been developed.

Metal injection molding occupies a certain region in the part-size/production-run/part-complexity scenario (Fig. 3). Sizes are generally in the 1 to 200 g range, but parts as large as 1 kg have been made by MIM. The near-net-shape capabilities of the process can reduce machining costs to low levels, especially for long runs (> 10,000 parts). For small complex components, cost savings can be up to 80% compared to conventional approaches. A wide range of geometric options are possible including undercuts, tapered external surfaces, and crossholes. Short production runs can be cost effective, but because of die and powder costs, this only occurs with very complex shapes and/or hard materials.

Low Medium High

Part complexity

Fig. 3 Section of the part-size/production-run/part-complexity diagram where the MIM process is most effective. Source: Ref 8

Some MIM fabricators produce runs of 2000 to 5000 pieces, particularly on more expensive parts (Ref 9). They can do a short run such as these because there is no danger to the tooling at setup, while in conventional P/M there is more risk. The capital equipment (presses) for injection molding is more economical than that for large-scale P/M presses. Tool life is at least 300,000 pieces. These factors help offset the added short-term material cost.

Two of the first MIM production parts in automobiles were parts for an ignition lock (Fig. 4) and a single-part replacement (Fig. 5a) for a two-part turn signal lever assembly (Fig. 5b). Both have been in service since July 1988.

Fig. 4 MIM part (upper left) for an automobile ignition lock. The key forces the MIM part into contact with a security switch. Courtesy of SSI Technologies

Fig. 5 (a) Single-piece MIM part that replaced (b) a two-piece automobile turn signal lever assembly. The smaller MIM part in (a) was the first version, while the larger MIM part is the finished version that replaced the two-part assembly shown in (b). Courtesy Remington Arms Division of E.I. Du Pont de Nemours & Company, Inc.

Fig. 5 (a) Single-piece MIM part that replaced (b) a two-piece automobile turn signal lever assembly. The smaller MIM part in (a) was the first version, while the larger MIM part is the finished version that replaced the two-part assembly shown in (b). Courtesy Remington Arms Division of E.I. Du Pont de Nemours & Company, Inc.

Figure 4 shows the entire ignition lock and the MIM subcomponent. As the key is inserted in the lock, the cam-shaped MIM part moves away and depresses an electrical switch, which is part of the security system. The initial design of the part was too small and complicated for the model shop to make, and it was prototyped from the MIM tooling.

The turn signal indicator lever is an example of the replacement of a two-piece assembly (Fig. 5b) with a single MIM part. The lower portion of Fig. 5(a) shows the first version of the MIM part, and the upper view shows the final 19.0 g (0.670 oz) MIM part that replaced the assembly. The MIM material is iron with 2% Ni, sintered and then case hardened. It replaced AISI 4037 and SAE 1018 case hardened. The MIM part succeeded because of its superior strength compared to the two-piece assembly. The core properties of the materials are 415 MPa (60 ksi) tensile strength and 15% elongation at 60 HRB.

Hot powder forging (P/F) continues to be attractive for the production of fully dense P/M parts for demanding applications (Ref 10, 11, 12). In this process a loose powder is blended to the desired composition and pressed to a forging preform having the general shape of the final component. Powder forging has proven to be competitive when the overall economics are improved through some combination of enhanced machining characteristics, mechanical properties, and dimensional or weight tolerances.

It was initially believed that P/F products would displace a wide variety of conventionally processed P/M parts, as well as a significant number of conventional forgings. However, the number of high-volume applications has been limited to bearing races, connecting rods, and ring gears.

One of the more dramatic applications in the mid-1980s was P/F connecting rods. Compared with conventional forging, P/F technology improved weight and dimensional control (Fig. 6) and reduced machining requirements (Fig. 7). In addition, P/F connecting rods are always forged in one piece, where the rod and cap are separated by "fracture splitting." Use of this technique eliminates several machining operations. In addition, the irregular, mating fracture surfaces (ductile failure mode) provide an intimate interlock between rod and cap. This virtually eliminates both "cap shift"--rotation of the cap relative to the rod--and lateral movement of the cap relative to the rod. Cap shift can lead to accelerated wearing of surfaces and, in extreme cases, bearing seizure. Lateral movement can result in high shear stress on connecting rod bolts at high engine revolutions per minute.

Dimension

Powder forge variation(a), %

Conventional forge variation(a), %

A: Pin end outside diameter

0.19

2.09

B: Bolt boss width

0.11

0.28

C: Crank bore diameter

0.14

0.31

D: Pin bore diameter

0.15

(b)

E: Bore-to-bore center distance

0.10

0.17

F: I-beam width at pin end

0.42

3.16

G: I-beam width at crack end

0.35

0.95

H: Bolt head seat location

0.06

1.11

I: Crank end thickness(c)

0.58

1.13

J: Pin end thickness(c)

0.45

0.48

(a) Variation evaluated as the ratio of the range of measurements to the mean dimension.

(b) Not applicable. Pin end forged solid.

(c) Not shown in schematic

Fig. 6 Dimensional control of P/F versus conventional forging. Source: Ref 10

Fig. 6 Dimensional control of P/F versus conventional forging. Source: Ref 10

Fig. 7 Comparison of fabrication sequence for connecting rods. (a) Typical sequence for conventional forging of rods. The pin end is forged solid and requires machining steps that P/F eliminates. (b) Powder forged connecting rod for a Ford 4.6 L V-8 engine. Note its good surface finish, near-net shape, and small weight control pad at the crank end.

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