Process Parameters

Figure 4 outlines a simple flowchart for the CIP process. Critical elements of the overall process for producing uniform and reproducible parts include: part design, mold design, reproducibility of metal powder density in filled mold, mold support, and CIP pressure. Operations that can lead to process failure if not properly executed include mold closure, mold sealing, evacuation, sealing of mold evacuation port, depressurization rate, and handling.

Fig. 4 Process flowchart for production of cold isostatically pressed parts

Dwell pressure for producing "green" strength that allows handling after CIP and maximum density if full density (as opposed to intentionally porous) parts are desired is determined by metal and powder type. Typically, pressures in excess of 200 MPa (30 ksi) are required for the process. Applicable ranges of dwell pressures for a variety of powder metals are:

Powder

Pressure

range

MPa

ksi

Aluminum

55-140

8-20

Copper

140-275

20-40

Iron

310-415

45-60

High-speed tool steel

240-345

35-50

Stainless steel

310-415

45-60

Titanium

310-415

45-60

Tungsten

240-415

35-60

For isostatic compaction, lower pressures are required than for die compaction due to the absence of die-wall friction effects. Nominal CIP pressure for a metal powder is also substantially independent of part size, again because of the isostatic nature of the process. Figure 5 shows density as a function of pressure for select cases of isostatically and die-compacted powders.

Fig. 5 Density as a function of pressure for cold isostatically pressed and die-compacted parts

Dwell time is generally only a few minutes for the process. At room temperature (the usual process temperature), there is negligible time-dependent particle deformation for the materials listed above. Thus, density follows pressure very closely in time, and no further increase occurs at constant pressure.

Depressurization rate is a unique process parameter for CIP. Under full process pressure, the densified metal part stores elastic strain energy in the solid and the P • V (pressure • volume) energy due to compression of air and gases entering into the mold in the void space of the low-density metal powder. Rapid release of process pressure can result in cracked parts. Entrapped gas can be eliminated by evacuation and sealing of the filled mold, but this process option, which adds complexity to the process and mold design, is frequently bypassed. Because of the relatively low compressibility of water (most frequently used pressurization medium), a very small volume release at high pressure causes a large pressure drop in the CIP system. Therefore, controlled depressurization is achieved by use of a high-pressure flow-control valve in the overall system. A depressurization schedule that avoids part cracking should be determined experimentally.

Evaluation of green strength and density of a cold isostatically pressed part may be carried out for a given metal powder and process cycle in the development or qualification phase of a powder and/or part by molding rectangular bars and carrying out three-point bend tests according to the protocol of ASTM B 312 (1997). These same bars may be measured and weighed before the bend test to determine approximate green density. Because the mold is elastomeric and not a hard die as specified in the ASTM procedure, oversize parts can be produced and machined green to standard dimensions for the development and control tests. Uniformity of density and green strength for larger parts can be determined using cutups taken along various directions of interest.

Thermal processing of cold isostatically pressed parts usually starts with sintering. Sintering cycles may be designed to maximize density or produce a specified percentage of porosity. Sintering atmospheres are based on metal chemistry requirements and include vacuum, reducing, and chemically active atmospheres such as carburizing and nitriding. Full density of cold isostatically pressed and sintered P/M parts can be achieved by hot isostatic pressing following sintering if a closed-pore condition can be established. This is common for titanium alloy parts and metal compositions that can be liquid-phase sintered such as tungsten carbide/cobalt, iron-bonded TiC, and tungsten/copper. Low-pressure liquid-phase sintering can be used as a complete alternative to HIP for selected materials. More complicated thermal processing sequences can be invoked such as sinter plus HIP plus forge to produce a fully dense forging with desired grain flow structure. This sequence is material efficient and eliminates the requirement for multiple forging steps, dies, and reheats.

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