Effect of Chromium Precipitation on Machinability of Sintered Brass Alloys Dispersed with Graphite Particles

The machinability of high strength lead-free brass was investigated in this study. Copper-40 mass%zinc (Cu-40Zn) brass powder with chromium was prepared by the water atomization. Chromium was used for strengthening of brass matrix. Graphite particles as machinable element were added to the as-atomized powders by conventional mixing process. Spark plasma sintering process was used to consolidate the above elemental mixed powder (X K SPSed material) at 873 K and 1053 K. The sintered materials at 1053 K were heat-treated for the precipitation of much Cr (HT material) at 1053 K for 12 h in Ar atmosphere. The machinability 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. The trade-off relationship between the matrix hardness and machinability of the brass alloys was not established. The solution of chromium content dissolved in the brass matrix of sintered material at 873 K, 1053 K and HT material was 0.55 mass%, 0.42 mass% and 0.19 mass%, respectively in SEM-EDS observation. The chromium carbide increased with decreasing chromium solution in the brass matrix. The generation of hard chromium carbide and decrease in the relative graphite particle caused to inhibit the machinability of HT material. [doi:10.2320/matertrans.M2011051]