Studies On Novel Al-Si-Cu-Mn Heat-resistant Alloy Used For Automobile Engine Block

To meet the more rigorous requirement of the high power density engine for aluminum alloy block, Mn as a main alloying element is designed to add into the near eutectic Al-Si-Cu alloy, creatively introducing a new manganese-rich phase Al15Mn3Si2 as the key heat-resistant phase. In this paper, effects of the amount of Mn-rich phase, solution treatment process and addition of rare earth elements on microstructure and mechanical properties of the novel Al-Si-Cu-Mn alloy were investigated by high temperature tensile test and microstructure observation using OM, SEM-EDS, TEM-SAD and XRD.Experimental microstructure observation combined with thermodynamic phase diagram calculation by Pandat demonstrates that the as-cast microstructure of Al-Si-Cu-Mn alloy is composed of the primary manganese-rich phase Al15Mn3Si2, ternary phase eutectic of Al15Mn3Si2+a-Al+Si and quaternary phase eutectic of Al15Mn3Si2+a-Al+Si+θ(CuAl2). During solutionizing, the eutectic copper-rich phase (i.e.θ (CuAl2)) is partially redissolved, whereas the survived CuAl2 phase becomes granulated and coarsened, and some survived CuAl2 particles with a few hundred nanometers are observed mainly at grain boundaries. But, both the primary and eutectic manganese-rich phases do not change at all during the solution treatment process, confirming that this manganese-rich intermetallic compound has more excellent thermal stability at high temperature. Also, fine and dispersed Al15Mn3Si2 and Al2oCu2Mn3 particles are precipitated in Al matrix during solutionizing by the TEM observation, which can increase the high temperature deformation resistance of the matrix and hence improve the heat-resistance of the alloy. The most appropriate solution treatment process for the Al-Si-Cu-Mn alloy is determined by vickers micro-hardness test and high temperature tensile test, as A3 B1(510 ℃ ×5h).Among the five designed alloys(S1-S5), S3(Al-12Si-4Cu-1.2Mn) alloy, with solutionizing at A3B1(510℃×5h) and aging at 160℃×6h, possess the optimum heat-resistance of which the ultimate tensile strength at 200℃,250℃,300℃ are 202.7MPa,191.9MPa,124.9MPa respectively. It is found that Mn-rich phase Al15Mn3Si2 possesses much better heat-resistance than CuAl2 phase, however, in high Mn content alloys, this Mn-rich phase would mostly grow as highly branched columnar dendrites or fine rods, of which the brittleness to matrix becomes severe and hence results in low high-temperature strength.Addition of rare earth element La or Ce completely alters the nucleation and growth of CuAl2 phase in eutectic reaction during solidification, which independently nucleates on the prior-formed needle-like RE-rich phase and grows with a divorced mode. But, RE addition has little influence on the morphology and size of the primary Mn-rich phase and hence results in only a little improvement in heat-resistance, not considerable, because of the brittleness from a great amount of fine needle-like RE-rich phase formed.