Study On High Strength Heat-Resistant Al-Si Piston Alloys

The elevated temperature strengthening effects of several trace-alloying and alloying elements such as Mn,RE,Ni,Fe on Al-Si piston alloys and the influence of squeeze casting on the mechanical properties of piston alloys have been investigated for the purpose of increasing the elevated temperature strengths of industrial aluminium-silicon piston alloys.By means of SEM,OM,EPMA,XRD and DSC,the microstructures,phase compositions and phase types of the prepared alloys and the solidification of some piston alloys are analyzed in this thesis,which helps to discuss the strengthening mechanism of piston alloys at elevated temperatures.The high temperature strengthening effects of trace Mn and RE have been studied respectively.It has been found that there occurs an evolution of Mn-bearing intermetallic phases in ZL109 piston alloy as the Mn content increases in the alloy. When most of manganese exists in the dendritic Al₉FeNi phase,the elevated temperature strengthening effect is good.Further increase of Mn addition can lead to the formation of a kind of plate-like Mn-bearing phase,whose strengthening effect is poor.It is found that trace RE addition can also play an effective role in strengthening Al-Si piston alloy at elevated temperatures.The addition of trace RE into the alloy can increase the volume fraction of the heat-resistant AlNiCu compound in ZL109 alloy.In addition,a kind of nodular RE-rich intermetallic phase is formed in the alloy after addition of RE.The influences of squeeze casting on the mechanical properties and microstructures of Al-Si piston alloys were also evaluated.It is found by experiments that the squeeze casting can improve the mechanical properties of Al-Si piston alloys evidently at room temperature but reduce the ultimate tensile strength(UTS) at elevated temperatures.The eutectic silicon in the squeeze cast Al-Si piston alloy characterizes as short-rod shaped instead of needle-like in gravity die casting Al-Si piston alloys.This helps to weaken the harmful effect of needle-like eutectic silicon on the alloy.It is also found that the squeeze casting can promote the formation of Al₉FeNi phase with coarse sizes,which may be the reason for the decrease of tensile strength at elevated temperatures.The investigation of potential strengthening effects of Ni on a high-performance piston alloy was conducted.It has been found that with the increase of Ni content, there occurs an evolution of main heat-resistant compounds in alloys:from the net-like Al₃CuNi to the Al₉FeNi phase distributing inα-Al dendrites boundaries with the shape of closed or semi-closed vermicule,and then to the Al₉FeNi phase like coarse bones.Among them,the vermicular Al₉FeNi phase distributing inα-Al dendrites boundaries has the most evident elevated temperature strengthening effect on the piston alloy.For the purpose of decreasing the use of the expensive element Ni in high-performance piston alloys as possible as it can,the new heat-resistant Al-Si piston alloys strengthened synthetically by Fe,Cu and Ni were fabricated and investigated.It is found that the proper addition of iron into the alloy helps to form the blocky Al₉FeNi heat-resistant phase,which can increase the UTS of piston alloys at elevated temperatures especially when above 350℃;but the excessive amount of Fe addition will lead to the formation of very coarse Al₉FeNi with had morphologies. Based on the above research,the addition of element Cr into the alloy was also tried combining with Fe,Cu and Ni.The result has shown that it helps to further strengthen the alloy.It is found that Cr can form the Al₁₅(Fe,Cr,Mn)₃Si₂ phase,which is a kind of heat-resistant intermetallic compound with a very high thermal stability.Jointly with other heat-resistant phases such as Al₉FeNi,Al₃CuNi and Al₇Cu₄Ni in alloy,the Al₁₅(Fe,Cr,Mn)₃Si₂ compound can play important roles in strengtheningα-Al matrix at elevated temperatures,which results in the increase of ultimate tensile strength of the alloy:the value can reach up to 99.77MPa at 350℃.