PROCESSING OF ALUMIX 321 PM ALLOY AND ITS CORROSION BEHAVIOUR IN 3.5 WT% SALINE SOLUTION

Abstract

Aluminum powder metallurgy (PM) parts have found applications in automotive, aerospace and transportation. Sintered aluminum parts have been developed and compete with traditionally fabricated ingot metallurgy (IM) products for specific applications. To extend the range of application of (PM) alloys which offer the advantage of net and near net shape production, processing parameters and corrosion behaviour of the aluminum alloys need to be improved. In this research, processing parameters and corrosion behaviour of a commercial Al-Mg-Si aluminum alloy (Alumix 321) were investigated. This alloy is the PM equivalent of wrought AA6061. Four sintering temperatures (610 °C, 620 °C, 630 °C, 640 °C) and two pressing pressures (200 MPa, 400 MPa) were used and the optimum pressing and sintering procedure was selected. In addition to different processing routes of aluminum powder metallurgy alloys, a series of electrochemical experiments on both (IM) and (PM) aluminum alloy was performed with the aim of correlating corrosion behaviour with production techniques. As a modification step, post sintering treatments and surface alteration techniques were applied. Hot rolling, hot swaging, repressing, resin impregnation and shot peening were performed and their effect on corrosion behaviour was investigated; their effect on density, hardness, and microstructure was also studied. Hardness after hot swaging and hot rolling increases and near full density was achieved (? 99%), while for resin impregnation and shot peening surface nature and roughness were affected, respectively. Electrochemical techniques such as open circuit potential (OCP), Tafel extrapolation (TE), cyclic polarization (CP) and stair step polarization (SP) were performed on the ingot, wrought, and post sintered alloys immersed in a 3.5 wt% NaCl solution. Electrochemical experiments show that corrosion current decreases as a result of post sintering treatments. The electrochemical experiments also show different corrosion mechanisms that were later confirmed by the metallographic analysis. The corrosion product and corroded surfaces of the alloys were characterized by optical microscopy, scanning microscopy (SEM), energy dispersive spectroscopy (EDS), wavelength dispersive spectroscopy (WDS), and X-ray diffraction (XRD). Results show that pitting is the main corrosion mechanism of the wrought alloy. However, powder metallurgy alloys show pitting, crevice, and intergranular corrosion.