Research On High Strength Hypoeutectic Al-Si Alloy

With the rapid development of automobile industy, cars are tending to develop in thedirection of lightweight, low energy consumption. Flywheel shell is one of the important basicparts, and its lightweight has a great significance on the development of auto industy.Aluminium and aluminium alloys as a light metal material, have the more incomparableproperties than other materials, widely used in the automotive, aerospace and mechanicalindustry, especially Al-Si alloys. Although Al-Si alloy has many advantages, the strength andhardness are low, and generally speaking it is not a high strength alloy. So, development ofnew and high performance aluminum alloy has become the current hotspot.In order to obtain a high strength hypoeutectic Al-Si alloy under the condition of lowpressure casting, meeting the requirements of the use of flywheel shell, the subject adoptscomposition optimization, refining modification, alloying and heat treatment to improve thestrength, hardness and rigidity. Through a lot of data and preliminary study, the basic alloyAl-9Si-0.5Mg-xCu is decided. By experiment, the mechanical properties of the abovementioned alloy are the best when the content of Cu is1.5%, especially under the state of heattreatment T6. So the hypoeutectic Al-Si alloy is decided completely, which is Al-9Si-0.5Mg-1.5Cu, called ZL118alloy.Al-5Ti-1B master alloy used in this study mainly contains TiAl3and TiB2two particles,and TiAl3has three morphologies of block, long rod-like and petaliform, and the best effecton refining aluminium alloys is the block. The addition of the Al-5Ti-1B master alloy caneffectively refine the alloy ZL118, enhancing the mechanical property. When the addition ofAl-5Ti-1B is1.0%, the tensile strength, yield strength, hardness, elongation and elasticmodulus of the tested alloy reach a peak value totally under the state of T6, which arerespectively343.7MPa,281.4MPa,116HB,3.1%and73GPa, improving11.3%,13.5%,5.9%,55%and12.6%correspondingly comparing with the alloy without Al-5Ti-1B master alloy. Atthis moment, the size, shape and distribution of primary α-Al and eutectic Si achieve the beststate, in conformity with the highest mechanical properties. The elastic modulus amongmechanical properties is insensitive to the microstructure, and refinment and heat treatment have a certain influence on the elastic modulus. With the amount of Al-5Ti-1B exceeding1.0%, Ti trends to blocky morphology, making TiAl3and TiB2lose the role of heterogeneousnucleation, which leads to grain coarse, letting the mechanical properties declining. Becauseof Cu and Mg in the alloy, CuA12, W（Al4CuMg5Si4）and little Mg2Si phases can produce,and the solid solution phases are distributed dispersively after T6heat treatment to strengthenthe alloy, increasing the mechanical properties obviously. Adding0.1%rich La rare earth tothe refined alloy, the mechanical properties of the tested alloy do not improve obviously, andwith the further increase of Re addition, the mechanical properties get down instead.The ZL118A alloy is treated by different solution time, and the study result shows that astime increases, the strength and hardness of the alloy rise gradually, and elongation decreases,but the increased range of the strength from10to14h is small, varying quite moderately.Elastic modulus is about70GPa, and the solution time has a slight influence on it. With thesolution temperature rising, the strength and elongation of the tsted alloy decline slightly, andelastic modulus is in a state of small fluctuation.In order to increase the rigidity of the alloy further, adding0.3%Sc to the ZL118A alloy,the sthength, hardness and elastic modulus of this alloy increase, but the elongation decreases.The elastic modulus is increased by7.8%in as-cast, and2.7%in the state of heat treatment.The particle phase of Al3Sc in this alloy is coherent with the Al matrix, with smaller mismatchdegree and gap, and denser microstructure, which has a big ability to resist deformation,improving the elastic modulus. Different amount of Li added into the ZL118A alloy, thetensile strength, yield strength, hardness and elastic modulus of the alloys begin to rise thendecrease along with addition of Li, but the elongation decreases straightly. Under thecondition of the heat treatment, when the amount of Li is1.0%, the tensile strength, yieldstrength, hardness and elastic modulus of the alloy are separately370.1MPa,313.8MPa,122HB and78GPa, which increase by7.7%,11.5%,5.2%and6.8%than that without Liaddition correspondingly. Addition of1.25%Li, the elastic modulus reaches the peak value of79.1GPa, increasing8.4%compare with the alloy no added Li. Addition of Li not only refinesα-Al and eutectic Si but also produces AlLiSi phase and δ’（Al3Li） particle phase coherentwith the Al matrix, improving the strength and rigidity of the alloy further.Through the above experiments, the paper obtains a set of methods of hypoeutectic Al-Si alloy preparation, and the flywheel shell is produced practically, which completely conformsto the requirements of its usability.