Effect of mechanical vibration on the microstructure and the mechanical properties of 7075 aluminum alloy produced by semi-solid melting method

Poyraz, Cemre Metin
This study aims to understand the effect of the semi-solid melting process parameters such as casting temperature and vibration frequency on the mechanical, thermal and microstructural properties. For this purpose, rheocasting of 7075 aluminum alloy with and without B4C as 7075 matrix composite were carried out at various temperatures in between 620-635oC under different vibration frequencies in the range of 15-35 Hz. For this purpose, rheocasting experiments were carried out successfully Microstructure, thermal and mechanical properties of both rheocast 7075 alloys and 7075 matrix composites were investigated. Optimum microstructure was achieved as globular grain structure in the plain matrix of rheocast 7075 alloy with average grain size below 40 μm and the same rheocast alloy with B4C addition yielded finer average grain size of 20 μm. Examination of microstructures of sand and rheocast specimen produced by using squeeze casting unit revealed that the amount of porosity decreased below 1% after conditioning by applying mechanical vibration frequency of 25 at delivery temperature at 635oC SSM temperature then solidification under pressure of 208 MPa. The highest UTS and flexural strength values were obtained as 483 MPa and 1020 MPa, respectively for the plain rheocast 7075 alloy. The highest mechanical property value of flexural strength was in the range of 960-1030 MPa for 10wt. % 7075/B4C metal matrix composite prepared and hot rolled at 475oC. The highest hardness values of plain 7075 rheocast alloy was measured as 145 HB and the hardness of the same alloy with 10wt.% B4C addition was measured as 190 HB. This study revealed that the optimum processing condition for rheocasting of 7075 alloy without and with B4C addition stands for 635oC SSM temperature and 25 Hz mechanical vibration frequency.