Laser surface modification of copper alloy for enhancing cavitation erosion resistance and corrosion resistance

Abstract

Copper alloys are widely used as engineering parts in marine applications because of the combination of good corrosion resistance, strength, and machinability. Among these, bronze (CuSn) and brass (CuZn) have the lowest cost, but are less resistant to cavitation erosion than the more expensive copper alloys such as cupro-nickel, aluminium bronze and beryllium copper. Laser surface modification is a well-known technique to produce unique surface properties for enhancing wear and corrosion resistance. However, the high reflectivity and high thermal conductivity of copper alloys pose a problem for laser surfacing. It is thus the aim of the present project to investigate the feasibility of enhancing the cavitation erosion resistance Re of bronze and brass by laser surface modification. Laser surface modification of bronze (Cu8%Sn) and brass (Cu38%Zn) were achieved by a 2 kW continuous wave Nd-YAG laser. Two groups of alloying / cladding materials were employed in the present study: (i) nickel-based alloy powder (Ni-Cr-Si-B, Ni-Cr-Al-Mo) and (ii) nickel-based alloy-ceramic powder (Ni-Al203, Ni-Cr-Si-B-WC and Ni-Cr-Fe-WC). Varying degrees of enhancement in the cavitation erosion resistance were achieved in standard cavitation erosion test (ultrasonic vibration at 20 kHz, 60 um peak-to-peak amplitude). Under optimal processing conditions, improvement in the cavitation erosion resistance Re by a factor of 9 could be achieved by laser surface modification using Ni-Cr-Fe-WC or Ni-Cr-Si-B-WC or Ni-Cr-Si-B. On the other hand, the best improvement in Re that could be obtained in specimens Ni-Al2O3/bronze and Ni-Cr-Al-Mo/brass was 5 times and 4 times that of the substrate respectively. The improvement of Re was attributed to the formation of intermetallic phase, metal carbides and borides, or to the increase in hardness of the modified surface layer. In general, the improvement of Re increased as the laser scanning speed increased, or as the preplaced coating thickness increased or as the power density decreased, with two exceptions. In Ni-Al2O3/bronze specimens, Re increased as the power density increased since Al2O3 decomposed and reacted with Ni to form the Ni-rich intermetallic phase Ni3Al. In the case of Ni-Cr-Al-Mo/bronze, Re decreased as the coating thickness increased owing violent heating, leading to loss of alloying materials. The effect of laser surface modification on the corrosion resistance varied, and was less pronounced than on the cavitation erosion resistance even when the effect was positive. The corrosion current density was reduced in all the specimens except in those modified with Ni-Al2O3 owing to the formation of Ni3Al precipitates. In terms of passivation and pitting, only the specimens modified with Ni-Cr-Si-B exhibited passivity and a substantial noble shift of the pitting potential to -112 mV (Epit of brass: -173mV) in the best case, owing to the increase in Ni, Cr content, and to a more homogeneous microstructure and composition. [Part of the results of the present project was presented in 2000 MRS (Materials Research Society) Spring Meeting, April 2000 (Session J3.2) and the 45th International SAMPE (Society for the Advancement of Material and Processing Engineering) Symposium and Exhibition, May 2000 (Session 3B) and published in the corresponding proceedings]* Tam, K. F., Cheng, F. T. and Man, H. C. "Improvement of cavitation erosion resistance and corrosion resistance of brass by laser surface modification". Materials Research Society Symposium 2000, San Francisco, CA., USA., 24-28 April, 2000, Vol.617, pp. J3.2.1-J3.2.6 Tam, K. F., Cheng, F. T. and Man, H. C. "Laser surface alloying of brass with NiCrSiB for improving cavitation erosion resistance and corrosion resistance". Proceedings of the 45th International Symposium and Exhibition of the SAMPE, Long Beach, CA., USA., 21-25 May, 2000, pp. 635-644