High Strength and Lead-Free Machinable Brass by Powder Metallurgy Process

Copper-40mass%zinc (Cu-40Zn) brass alloy powder containing 1.0 mass% Cr was prepared by the water atomization. Graphite particles, having a mean particle size of 5 μm, were added to the as-atomized powders by the ball milling equipment for 4h under 120 rpm. Spark plasma sintering process was used to consolidate the above elemental mixed powders (sintered material). Sintered materials were heat-treated for the precipitation of much Cr (HT material). The machinability of Cu-40Zn brass alloys was evaluated by a drilling test using a drill tool under dry conditions. The matrix hardness of sintered material was higher than that of HT material. On the other hand, the machinability of sintering material was higher than that of HT material. There is no trade-off relationship between the matrix hardness and machinability of the brass alloys. SEM-EDS observation indicated that Cr content dissolved in the brass matrix of sintered material and HT one was 0.42 mass% and 0.19 mass%, respectively. As the reason why machinability of HT material lowered, the precipitation of the hard Cr particle or generation of Cr-C compound caused to inhibit the machinability.