Effect of strontium modifier, spinning deformation processing and heat treatment on the wear-corrosion properties of A356 alloy

• Main Authors: Yin-Chun Cheng, 鄭穎駿
• Other Authors: Sheng-Long Lee
• Format: Others
• Language: en_US
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/92999259350676136583

博士 === 國立中央大學 === 機械工程研究所 === 100 === Hypoeutectic Al-Si alloy containing Mg (Al-7Si-0.3Mg, A356 alloy) is extensively used for aerospace and automobile industry due to its lightness, excellent abrasion and corrosion resistance, high mechanical strength and good castability. The aluminum castings have precise dimension and good quality that had gone through the melt treatment, pouring to the mold for designing shape as well as subsequent heat treatment and mechanical machining. Due to constantly increasing ecological concerns and demands for higher performance, lightweight construction is a key factor to success mainly in the transportation sector. The development of metal forming technology on the aluminum casting becomes attractive for the foundry manufacturer. As design, materials, and manufacturing processes have to be considered integratively, the modifier and heat treatment as well as spinning deformation processing (SDP) related to microstructure and various properties of A356 alloy are investigated accordingly. The results showed that the addition of Sr modifier changed the morphology of eutectic Si particles and further improved the wear and wear-corrosion properties. Compared to unmodified A356 alloy, the corrosion resistance decreased with addition of Sr. The deterioration in corrosion property triggered the reduction in improvement of wear-corrosion rate when the applied potential increased. The heat treatment of A356 alloy spheroidezed and coarsened eutectic Si particles after the solution treatment as well as strengthened the ?-Al matrix after the aging treatment. The enhanced microstructure reduced the wear rate and improved the corrosion susceptibility of modified A356 alloy, which further enhanced the wear-corrosion resistance. When the A356 alloy wear-corroded at open circuit potential, the improvement from heat treatment was higher than that caused by Sr modifier. T6 heat treatment significantly reduced wear-corrosion rate for 29 %. The cast structure of A356 alloy became elongated with increasing reduction of thickness caused by SDP. Compared to absence of SDP, the casting defects were reduced, the eutectic Si particles were refined and distributed throughout the Al matrix. The hardness reached a steady value due to uniformity of microstructure increasing aggressively with the reduction of thickness. The enhanced microstructure retarded crack nucleation and coalescence resulting in improvement of elongation, tensile strength and quality index as well as reduction of dry wear rate. In addition to improvement of mechanical properties, the corrosion susceptibility of interdendritc region was reduced by dispreading eutectic Si particles and eliminating casting defects due to SDP resulting in a much reduced pitting profile in comparison with A356 alloy absence of SDP. As wear-corrosion test, the strengthening of wear resistance and reduction of corrosion susceptibility caused by SDP diminished the synergistic attack of wear and corrosion, which led to improvement of wear-corrosion rate of A356 alloy. When the A356 alloy wear-corroded at open circuit potential, SDP significantly reduced wear-corrosion rate for 31 %.