Sintering Atmospheres

Sintering in Nitrogen. Nitrogen at high purity and low dew point is particularly well suited for sintering aluminum P/M parts because of ready availability and moderate cost. Special handling is not required, nor are a generator and adsorbent dryer required to convert nitrogen to a dry, gaseous form. The highest sintered strength in both 601AB and 201AB parts is achieved in nitrogen.

Dissociated ammonia is used in many P/M applications for sintering brass and bronze parts. It can be used for aluminum as well (Ref 39).

Dissociated ammonia contains high concentrations of flammable hydrogen; consequently, care must be taken in handling, particularly for aluminum processing during which sintering temperatures are not high enough to ensure self-ignition upon contact with air. One precautionary method involves purging the furnace with an inert gas such as nitrogen prior to introducing dissociated ammonia.

Conditions and production rates for aluminum P/M parts sintered in dissociated ammonia were similar to those sintered in nitrogen; however, tensile properties of 601AB and 201AB alloys sintered in dissociated ammonia (Table 19) tend to be lower than for nitrogen sintered parts. The lower properties of parts sintered in dissociated ammonia appear related to the presence of hydrogen and/or undissociated ammonia in the sintering atmosphere. The tendency of hydrogen to cause gassing of aluminum has been well documented (Ref 40). Also, ammonia reacts with aluminum to liberate hydrogen. When 100% ammonia vapor was used as a heat treating atmosphere for aluminum, the tensile strength of 2024 alloy sheet was reduced 29%, and the elongation was reduced 82% (Ref 40).

Table 19 Properties of P/M aluminum alloys sintered in dissociated ammonia

Alloy

Green density

Temper

Tensile

strength

Yield

strength

Elongation

%

g/cm3

MPa

psi

MPa

psi

in 25 mm (1 in.), %

601AB(a)

90

2.42

T1

93

13,500

76

11,000

2.5

T4

108

15,700

88

12,700

3.5

T6

159

23,100

1.0

95

2.55

T1

121

17,600

87

12,600

3.5

T4

146

21,200

99

14,300

5.0

T6

207

30,100

205

29,700

1.5

201AB(b)

90

2.50

T1

161

23,300

141

20,500

2.0

T4

198

28,800

163

23,700

2.5

T6

247

35,800

0.5

95

2.64

T1

174

25,200

152

22,000

2.0

T4

221

32,000

180

26,100

3.0

T6

288

41,800

287

41,600

1.0

(a) Sinter 10 to 30 mm at 620 °C (1150 °F) at a dew point of -

(b) Sinter 10 to 30 min at 595 °C (1100 °F) at a dew point of -

(a) Sinter 10 to 30 mm at 620 °C (1150 °F) at a dew point of -

(b) Sinter 10 to 30 min at 595 °C (1100 °F) at a dew point of -

Dimensional Change During Sintering. Dimensions of sintered aluminum P/M parts are affected by compact density, sintering atmosphere, temperature, and dew point. The effects of green density and atmosphere on sintered dimensions of 601AB and 201AB alloys are illustrated in Fig. 41(a) and 41(b). Dimensions increased with increased green density in all atmospheres. Shrinkage or a lack of growth was exhibited by 85% green density parts, whereas high-density compacts exhibited growth when sintered in dissociated ammonia and vacuum. Nitrogen-sintered parts experienced shrinkage over the full range of densities—except for 95% density 601AB, in which no change was noted.

Fig. 41 Effect of green density and atmosphere on sintered dimensions. (a) 601 AB. (b) 201 AB

These dimensional changes were consistent as long as the sintering temperature was constant and the dew point in the furnace was at least -40 °C (-40 °F). Higher than normal temperatures caused excessive shrinkage and distortion or even melting in extreme cases. Lower than normal temperatures produced parts with increased dimensions and reduced properties.

Thus, for example, lowering the sintering temperature of 201AB alloy from 595 to 570 °C (1100 to 1060 °F) caused 95% density specimens sintered in dissociated ammonia to change from 0.25% shrinkage to 1.0% growth. At 580 °C (1080 °F), 0.25% growth was observed. Mechanical properties were less affected, although a reduction in tensile strength of 2 to 10% was observed, depending on thermal treatment. High dew points in the furnace resulted in excessive part expansion and a significant reduction in properties.

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