Machinability Improvement

Machining P/M steels does present problems. Several different approaches to improve machinability are:

• Closure of porosity

• Green machining

• Presintering

• Microcleanliness improvement

• Free-machining additives

• Microstructure modification

• Tool materials

The effects of free-machining additives, microstructure modification, and tool materials are illustrated by controlled drilling tests conducted under laboratory conditions.

Closure of Porosity. Closing or sealing porosity improves the machinability of P/M steels significantly by changing the cutting process from intermittent to continuous. The reduction in vibration and chatter improves tool life and surface finish. Copper infiltration (Ref 8) and polymer impregnation (Ref 19) are efficient means to close porosity and can require an additional process step. Thus, they are most efficient when dictated by the end use, such as fluid power applications, that require a pore-free structure. However, the improvement in machinability can justify their use in severe machining operations or when a machining operation is the rate-limiting step in a process sequence.

Microcleanliness Improvement. The increase in the production and use of atomized rather than reduced iron powders has improved the microcleanliness of iron and low-alloy steel powders. Driven largely by the requirements of powder forging, the content of coarse nonmetallic inclusions in atomized powders has been reduced significantly (Ref 21). For an atomized FL-4600, the median frequency of inclusions greater than 100 /'m in size (F4) has been reduced from approximately 2.5 to 0.25 per 100 mm2. The maximum frequency of inclusions greater than 100 /'m was reduced from 9 to 1.3 inclusions per 100 mm2. These improvements suggest that the incidence of edge damage due to the presence of coarse inclusions should be reduced significantly. Because powder forging practices are now employed to produce all atomized steel powders, P/M users of these powders have benefited.

Green Machining. One way to reduce the machining problems of P/M parts is to machine them in the green (i.e., as-pressed) condition prior to sintering. The lack of bonding between particles in green compacts results in low cutting forces.

Such techniques are used in the processing of ceramic and hard metal powders. However, the green strength of metal powder compacts has been too low to withstand the cutting and clamping forces employed in machining operations.

The introduction of warm compaction technology (Ref 22) can change this perspective. The green strength of warm compacted parts is two to four times higher than that of conventional ferrous P/M parts (Table 5). This is sufficient to withstand both the cutting and clamping forces of modern machine tools. Research confirms (Ref 23) that green compacts produced with warm compaction can be machined with conventional cutting tools with low cutting forces. Drill testing (Table 6) shows that the cutting forces are relatively low. Both cutting forces and surface finish can be improved by changes to drill type and profile. These changes also alter the accuracy and surface finish of the drilled hole. Thus, the choice of tool will be a compromise between low cutting forces, surface finish, and tolerances.

Table 5 Green strength of ANCORDENSE premixes

Material

Green density, g/cm3

Green strength, psi

M-1

7.29

4221

M-2

7.33

4800

M-3

7.37

7703

M-4

7.31

9454

M-5

7.15

6286

Table 6 Mean drilling forces for warm compacted test pieces

Drill type

Mean force, lbf

118° parabolic geometry

100.4

135° split point

45.3

135° split point-wide land parabolic flute

49.3

135° split point-wide land parabolic flute, coated

55.5

Material: Ancorsteel 85HP, 2% Ni, 0.4% graphite; green density, 7.33 g/cm3. Machining: 0.375 in. HSS drill; speed, 3285 rpm; feed, 0.012in./revolution

Presintering the green compact at a lower temperature than the final sintering operation produces a compact of relatively low hardness and strength but with sufficient edge retention to be handled and machined. Thus, the machinability of a presintered compact can be substantially better than that of the sintered part. However, presintering introduces an additional step to the manufacturing process and increases cost. It can be justified where the properties of the as-sintered part make its machining difficult or impossible. For example, high-carbon sinter hardened steels can require grinding rather than machining. In this case, providing that the part application and tolerances permit, it can be desirable to machine the part in the presintered condition rather than perform a grinding operation. Similarly, if part design calls for a through hole normal to the compaction axis, drilling the presintered preform followed by final sintering can be the only way to produce the hole economically in a high performance P/M steel.

Free-machining agents are added to P/M steel to improve machinability (Ref 8, 24). These agents are thought to perform several functions during the cutting process (Ref 9), including initiation of microcracks at the chip/workpiece interface, chip formation, lubrication of the tool/chip interface, and prevention of adhesion between the tool and chips (Fig. 5).

Chip Chio separation

Chip Chio separation

Mic roc racking-Built-up Crater Lubricant edge water

Fig. 5 Potential benefits of a machining agent

Several materials including sulfur, molybdenum disulfide (Ref 24), manganese sulfide (Ref 11), and boron nitride (Ref 25) are used as free-machining agents for P/M steels. They are most frequently introduced as fine powder to powder premixes, but sulfur and manganese sulfide are also available as prealloyed powders (Ref 26, 27, 28).

Sulfur and molybdenum disulfide can have strong effects upon the dimensional change and strength of P/M steels (Fig. 6, 7). Their use should be considered at the part design stage rather than as a "retrofit" when machining problems become apparent. Manganese sulfide has smaller effects upon dimensional change and strength (Ref 13) and can be used to improve the machinability of existing premixes. The effects of several potential machining agents upon the machinability of P/M steels are described below.

Fig. 6 Dimensional change of F-0008 atomized plus free-machining agents

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Fig. 7 Transverse rupture strength of F-0008 atomized plus free-machining steels

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