Experimental Techniques of Cold Sintering

Most elemental powders and rapidly solidified alloys can be consolidated at ambient temperature to densities close to the theoretical (>95% TD) at the pressure of 3 GPa. With careful design, tool steel dies and punches can be used at this pressure for hundreds of cycles. Dies and punches made from CPM T15 high-speed steel have been used by the authors of the present article over the last 10 years. The T15 high-speed steel (12W-5V-5Cr-5Co-1.6C (wt%)), fabricated from the inert-gas-atomized powder by HIP, has a fine homogeneous structure and, as a result, a considerably higher strength than that of the ingot tool steel of the same composition (Ref 19). The high concentration of fine homogeneously dispersed carbides in the steel results in very good wear resistance. Some of the punches used for high pressure consolidation were prepared from the water-atomized T15 powder by cold sintering. Schematic of a pressure cell for cold sintering with punches, die walls, and punch supports made from high-speed tool steel is shown in Fig. 1. The inner and outer supporting rings are made of the high strength 4340 steel. Provided the alignment of the tooling is maintained, it is possible to use dies and punches made from CPM T15 steel up to ~3.5 GPa. The outer diameter of the die is usually only three times larger than the diameter of the part being consolidated (die opening). Typically, the compact is loaded to 2 GPa in a few seconds, further loading proceeding more slowly over the 10 to 15 s interval to achieve the pressure of 3 GPa. Pressures as high as 4.5 GPa are possible utilizing cemented carbide dies. These dies are more brittle, and high pressure consolidation of composite materials containing hard or super-hard particles often results in a failure of a cemented carbide die after several cycles.

Fig. 1 Schematic of a pressure die for cold sintering

Cylindrical dies with diameters ranging from 3 to 50 mm and rectangular dies with cross sections ranging from 2 by 18 to 40 by 42 mm have been used, the maximum size being limited mainly by the press capacity (Ref 3). Examples of dies and punches are shown in Fig. 2. A laboratory press with 5 MN capacity provides applied pressure (P) equal to 2.5 to 3 GPa for the largest dies. A large number of smaller size samples can be produced by placing each of them in a separate rigid cell (see Fig. 2b). Upscaling of the pressure die to an 88 by 88 mm opening has been done, and densities close to the theoretical have been obtained for aluminum, iron, and T15 high-speed steel employing a press with >25 MN capacity (Ref 4).

Fig. 2 Examples of dies and punches for cold sintering

For some prealloyed, rapidly solidified, or mechanically alloyed powders, consolidation at ambient temperature and at P = 3 GPa results in densities 96 to 97% TD. Even lower densities have been obtained for some amorphous and microcrystalline rapidly solidified powders. For such powders, densities close to the theoretical can only be obtained by high pressure consolidation at temperatures above the ambient (warm consolidation). Thus, densities >99% TD were achieved at T = 450 °C and P = 3 GPa for amorphous powders (Allied Signal) and microcrystalline powders (Marco) with compositions corresponding to iron- and nickel-base wear-resistant alloys and high-speed steels (Ref 3). For some highly alloyed, rapidly solidified alloys, such as high-speed steels, stainless steels, and superalloys, consolidation at T = 400 °C and P = 3 GPa resulted in densities close to the theoretical and good bonding integrity of the powder particles. In many cases, annealing followed by repressing at P = 3 GPa provides consolidation to full density. At the pressure of 3 GPa, high-speed T15 steel pressure dies can be used at temperatures S450 to 500 °C.

Densities >96% TD were obtained by warm consolidation of amorphous ribbons of Fe-B-Si and Fe-B-Si-C alloys at 0.78 GPa and 2 GPa, respectively, at a temperature of 350 to 380 °C, which is below the glass transition temperature (Ref 20).

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