This Small Business Innovation Research Phase I project proposes to develop a 20-25% Si alloy based on the 4032 aluminum forging alloy composition that has low density and high wear resistance. The proposed Phase I research builds upon carbon dispersion in the melt that can increase machinability and fluidity of cast aluminum alloys. To achieve these advantages in hyper-eutectic alloys, it is proposed to adjust the carbon and copper contents in the aluminum alloy during melt processing, and to precipitate during solidification equiaxed particles of primary silicon, carbides and graphite flakes in the Al-Si eutectic. These micro- and nano-particles will provide for higher wear resistance, modulus and toughness. There are significant challenges in this research including the possibility of precipitation of undesired acicular silicon, unacceptable shrinkage, hydrogen embrittlement, segregation etc. during ingot solidification. The high silicon content can significantly reduce solidification and homogeniezation kinetics during processing making it hard to achieve uniform properties and microstructure. Since this research aims to create a new alloy composition using a novel casting process, it will generate considerable new fundamental knowledge in solidification and deformation processing.
The broader impact/commercial potential of this project is that a new market will open up to higher-integrity machinable forged hyper-eutectic parts that are heat treatable. Currently, most aluminum-silicon alloy components are limited to cast structures where their strength and wear capabilities override the additional costs of testing for physical defects, rejections, and high costs of machining. Applications in automotive engines include pistons, cylinder heads and connecting rods where wear resistance and light weight is important, and computer equipment manufacturing where thermal properties, weight and rigidity are critical.
Aluminum-silicon alloys have been widely used in automotive field for the manufacture of wear-resistant light-weight components such as pistons, connecting rods, cylinder blocks, cylinder heads etc., and in the computer applications for light-weight high-stiffness (high modulus) parts and heat exchangers (good thermal and electrical conductivity). They can be extruded (A4032: 12%Si, 1%Mg, 1%Ni etc.), cast (A336: 12%Si, 1%Mg, 1%Cu etc.) and diecast (A383: 11%Si, 2.5%Cu) in the hypoeutectic or eutectic condition. Alloys with higher silicon content are highly desired for wear resistance, increased stiffness and higher temperature applications. In this Phase I SBIR Research it was proposed to develop 19-22% Si alloys based on the 4032 (12% Si) aluminum forging alloy composition that have low density and high wear resistance. Increasing the Silicon content from 12% to 20% increases the Youngâ€™s Modulus from 86 Mpa to 99 Mpa (15%) and decreases the coefficient of thermal expansion from 20 x 106 to 17.5 x 106 (20%). The proposed Phase I research builds upon the discovery by Aluminastic that carbon dispersion in the melt using an electric field can increase machinability and fluidity of cast aluminum alloys. To achieve these advantages in hyper-eutectic alloys, it was proposed to adjust the carbon and copper contents in the aluminum alloy during melt processing, to use an electric field to disperse the carbon, and to precipitate in the Al-Si eutectic equiaxed particles of primary silicon, carbides and graphite flakes. Conclusions: Using the Aluminastic process, the team was successfully able to create hypereutectic AA4032 alloys with Silicon varying from 12% - 20%. The new alloys developed exhibited very little macro defects and have lower density and higher hardness than the conventional AA4032 alloy. The new alloys exhibited a homogenous structure as evidenced by the microstructures seen across the cross sections. An increase in the %Si helped in improving the surface finish during the machining process. The forces during the machining also reduced as Silicon increased. These will help in creating better products through the machining process using this alloy. An increase in the %Si helped in reducing the coefficient of friction and the wear rate for the new alloys. Thus, a piston developed during this alloy will potentially have better life (To be investigated in Phase II). As the %Si increased in the alloy, the forgeability of the alloy improved. This feature will help in creating more complicated geometries for products in automotive in computer industry.