Applications

Iron-Copper Alloys. Liquid-phase sintering is extensively used for Fe-Cu alloys in structural components for automobiles, household appliances, farming equipment, office machines, and electrical motors. An especially large market is for drivetrain components for the automotive industry. These parts require the high production rates, excellent mechanical properties, and dimensional control to within ±0.025 mm available through uniaxial die compaction and LPS (Ref 27). The Fe-Cu system is characterized by a high solubility of copper in iron and a low solubility of iron in copper. These characteristics result in swelling with the initial copper melt formation, as shown in Fig. 6. Swelling occurs due to the penetration of copper into the iron grain boundaries. Pores form at the prior-copper particle sites. The amount of swelling is dependent on the green density, amount of copper addition, particle sizes, internal powder porosity, copper distribution, amount of carbon, and atmosphere. Normally, swelling would be undesirable, but under controlled conditions, it enables the sintered dimensions to match the die dimensions to greatly simplify tooling design and achieve better dimensional tolerances. Zero dimensional change is accomplished by using the swelling to offset the die-to-green shrinkage and any other shrinkage that may occur during heating. Carbon and atmosphere control are critical because of their role on the surface energies and consequently the degree of penetration of the bonds between the iron particles. The solid solution-strengthening of copper in iron provides adequate mechanical properties, even with the presence of 10 to 15% porosity. Properties improve with increasing density or by heat treating, as shown in Table 3. Further increases in strength and ductility are possible by infiltrating the remaining pore space with copper.

Table 3 Typical properties of an Fe-10Cu-0.3C alloy after liquid-phase sintering

Property

6.4 g/cm3,

6.4 g/cm3,

7.1 g/cm3,

7.1 g/cm3,

sintered

heat treated

sintered

heat treated

Hardness

50 HRB

25 HRC

80 HRB

40 HRC

Yield strength, MPa

280

395

655

Tensile strength, MPa

310

380

550

690

Elongation, %

0.5

0.5

1.5

0.5

Fatigue strength, MPa

115

145

210

260

Impact energy, J

4

11

Elastic modulus, GPa

90

90

130

Source: Ref 27

Initial state Swollen structure after Cu melts

Fig. 6 Swelling in the Fe-Cu system associated with liquid copper penetration of the iron grain-boundaries, leading to a separation of the solid grains and formation of a pore at the prior copper site

Cemented Carbides. Another large commercial application of LPS is in the fabrication of cemented carbides for machining inserts, punches, dies, cutting tools, milling inserts, and saw blades. They are also used in rock drilling equipment, electrical contacts, armor-piercing projectiles, and in applications that require resistance to wear, erosion, cavitation, abrasion, and/or penetration. Cemented carbides consist of transition metal compounds of carbon in matrix phases, which are typically composed of cobalt, iron, or nickel alloys. Typical properties of industrial cemented carbides are given in Table 4.

Table 4 Typical properties of industrial cemented carbides after liquid-phase sintering

Type

Binder, wt%

Density, g/cm3

Elastic modulus, GPa

Transverse strength, GPa

Hardness, HRA

TiC-Fe

55

6.6

300

1.9

87

WC-Co

14

14.1

520

2.8

87

WC-Co

11

14.4

550

2.7

90

WC-Co

6

15.0

630

1.9

91

WC-Co

3

15.2

680

1.5

93

WC-TaC-TiC-Co

5

12.0

500

1.5

Practically all commercial cemented carbides, including the carbides of titanium, tantalum, vanadium, and niobium, are liquid phase sintered. WC-Co is a classic example. Densification occurs during heating prior to liquid formation, due to a high stored strain energy and defect structure introduced by milling applied to the fabrication of these powders.

The anisotropic surface energy of the hexagonal close-packed tungsten carbide results in grain polygonization on heating. Additions of TiC or TaC are effective in controlling the grain shape and size during sintering. The peak sintering temperature is about 1400 °C under a vacuum of approximately 100 Pa. Carbon control is key in attaining the desired attributes, including hardness, strength, wear resistance, and fracture strength.

0 0

Post a comment