Materials and Applications

The availabilities of small powders that match the needed characteristics for molding determine the production materials used in PIM. Most of the classic engineering materials are available except aluminum, glass, lead, and tin. Typically ferrous alloys and alumina-base ceramics are used most often. For the stainless steels, the 316L composition is used frequently, because of its combined strength and corrosion resistance. Another favorite is 17-4 PH stainless steel, which is precipitation hardenable. Other compositions with more chromium are available, and for easier sintering, two-phase compositions that have high levels of molybdenum (up to 6%), chromium (up to 22%), or silicon (up to 3%) are preferred. These are sometimes called duplex stainless steels, because of the two-phase final microstructure. Stainless steels, iron-nickel compositions, and alumina are the materials most often used. Figures 10, 11, 12, 13, and 14 show a few examples of PIM components.

Fig. 10 Stainless steel gears for an electric toothbrush

Fig. 11 Paper tape cutter for a postage meter, formed out of tool steel

Fig. 12 An injection-molded iron-nickel housing used in personal computers

Fig. 13 Collection of parts and materials showing some of the possibilities with powder injection molding: a steel microelectronic package, tungsten bullet, tungsten-copper computer heat sink, and alumina split bolt

Fig. 14 Some wear components formed by powder injection molding out of cemented carbides

Properties. The properties attainable with PIM are generally equivalent to those possible via other production techniques. Although many PIM components are selected for thermal, optical, or wear applications, the bulk of the property evaluation has focused on mechanical properties. A few sample mechanical properties of PIM materials are given in Table 10. However, there is considerable variability. For example, there have been several reports on 316L stainless steel formed by PIM. Sintered densities ranged from 93 to 100% of theoretical, with a reported mean yield strength of 220 MPa, but a range from 170 to 345 MPa. The lowest strength occurred with the lowest density. Likewise, a range from 18 to 81% elongation has been reported, with a mean of 45% and a standard deviation of 17%. The scatter reflects variations between vendors, especially in controlling impurities such as carbon, oxygen, and nitrogen during debinding and sintering. These same impurities degrade corrosion resistance.

Table 10 Selected mechanical properties of powder injection molding metals and alloys

Material

Composition, wt%

MPa

Ultimate tensile strength, MPa

Elongation,

%

Hardness

4140 (HT)

Fe-1Cr-0.4C

93

1240

1380

2

40 HRC

4640 steel (HT)

Fe-2Ni-1Mo-0.4C

97

1400

2000

3

30 HRC

Iron-copper steel

Fe-2Cu-0.8C

95

700

10

92 HRB

Iron-chromium steel

Fe-1Cr-0.5C

94

600

10

90 HRB

Iron-nickel

Fe-50Ni

96

170

420

20

50 HRB

Iron-nickel steel (HT)

Fe-2Ni-0.5C

94

1230

1230

1

45 HRC

Iron-silicon

Fe-3Si

99

345

520

25

85 HRB

Kovar or F15

Fe-29Ni-17Co

98

350

520

42

60 HRB

Stainless 17-4 PH (HT)

Fe-16Cr-4Ni-4Cu

96

965

1140

12

35 HRC

Stainless 316L

Fe-17Cr-12Ni-2Mo-2Mn

96

220

510

45

75 HRB

Stainless 420 (HT)

Fe-13Cr-1Mn-1Si

92

690

1440

6

47 HRC

Stellite

Co-28Cr-4W-3Ni-1C

99

1020

3

40 HRC

Super Invar

Fe-32Ni-5Co

96

285

440

40

65 HRB

Ti-6-4

Ti-6Al-4V

98

800

880

12

35 HRC

Tool steel

Fe-6W-5Mo-4Cr-2V-1C

99

2000

0

66 HRC

Tungsten heavy alloy

W-4Ni-1Fe

99

650

1000

20

50 HRA

HT, heat treated

HT, heat treated

In the Fe-2Ni steels, when the carbon level is almost zero the sintered yield strength is about 190 MPa and the fracture elongation is 30%. When the carbon level is increased to 0.5%, the heat-treated material has a yield strength of 1230 MPa with 1% fracture elongation. These dramatic shifts in strength and ductility are due to the retained carbon. For ductile systems, the strength will typically have a standard deviation of 20 MPa and the elongation will have a standard deviation of approximately 1%.

The dynamic properties, such as fatigue and impact toughness, depend on the pore structure. In the case where the final pores are small and spherical, there is competitive resistance to crack propagation. Unfortunately, dynamic properties are not commonly tested. Early reports gave fatigue endurance strengths of 219 MPa for Fe-7Ni, 237 MPa for Fe-7Ni-0.5C, 575 MPa for 4640 steel, and 517 MPa for 17-4 PH stainless steel. Improved fatigue strength comes with case hardening. Hot tensile tests on PIM 316L stainless steel have shown a steady decline in strength as temperature increases, with yield strengths of 258, 177, 121, 71, and 62 MPa at temperatures of 180, 300, 500, 700, and 900 °C. The ductility remained fairly high, over 25% up to 900 °C.

Impact toughness tends to be low when measured with pre-notched samples, largely because of residual porosity effects on cracking. Fracture toughness has been evaluated for only a few PIM materials, but these are similar to more conventional processing routes.

Corrosion resistance of the PIM stainless steels is a concern. When properly processed without contamination, the PIM products are corrosion resistant, and they are often superior to wrought materials for thin sections. Corrosion is highly dependent on impurities, density, and final thermal cycles. However, as a simple generalization, often the sintered corrosion properties match or exceed the typical properties observed with alternative processing routes.

The soft magnetic characteristics are of interest for the ferrous systems such as iron, Fe-2Ni, Fe-3Si, Fe-0.45P, Fe-0.6P, and Fe-50Ni. Of these, the Fe-50Ni alloy has the most attractive combination of high magnetization and low coercive force. Experience shows considerable variability in the coercive force. Part of this variation is directly due to differences in sintered density, but impurity control is also a major factor. Like corrosion and strength, the magnetic characteristics are sensitive to contaminants in the compacts.

Economics. Cost is the critical parameter in determining the feasibility of PIM production. It depends on many factors, not the least being the initial tooling cost. Other factors include the number of cavities in the tool set, production quantity, powder cost, details of the fabrication steps, surface roughness, packaging requirements, labor rates, and required tolerances. The best applications for PIM involve high production quantities of complex parts formed from materials that are difficult to fabricate. A desirable production quantity is more than 50,000 per year, although at quantities over approximately 100,000 per year the tooling cost per compact is not significantly less, because of needed tooling refinishing. For production of 250,000 per year or more, the use of multiple cavity tooling is typical, and up to 16 cavities are typical. New products are planned to reach production rates of 300,000 to 500,000 per day. Current industry growth rates are near 22% per year, after reaching sustained rates of 32% per year through the 1990s.

Powder Injection Molding

Randall M. German, The Pennsylvania State University

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