High Pressure Die Casting of Aluminium and Magnesium Alloys- Microstructure Heat Treatment, Energy Absorption Characteristics and Microstructure

Abstract

The high-pressure die-casting process (HPDC) is extensively used as a commercial process for the production of complex near net shape automotive castings. Owing to the fast and turbulent filling of the liquid metal into the die, air and gas in the die cavity get trapped in the solidifying metal, thus affecting the mechanical properties during the life cycle of the product. Due to stringent demands of safety and environment, stronger and lighter automotive castings are to be developed.

A low temperature heat treatment process has been developed in this work for the new generation die cast alloys that can improve the energy absorption capacity as well as the strength of the product without affecting the surface features of the finished product.

The tensile and Charpy tests of the magnesium alloys (AM40, AM50 and AM60) and aluminium (AlSi4MgMn, AlSi7MgMn and AlSi9MgMn) alloys have been performed at room temperature and higher temperature, as well as after various heat treatments. The aluminium alloys responded well to the heat treatment and significant improvement of the energy absorption has been observed.

The magnesium alloys did not show significant improvement in terms of the energy absorption of the material after heat treatment, but showed improvement in the energy absorption when tested at higher temperature. This difference has been explained in terms of the activation of more slip systems in magnesium alloys at higher temperature as compared to the room temperature.

On the basis of a higher response of aluminium alloys to heat treatment, further work was carried out with AlSiMgMn aluminium alloys. After a few preliminary trials and discussions, a set of parameters were decided to work upon. The mechanical properties of AlSi4MgMn and AlSi9MgMn were investigated for various period of ageing. Optical-microscopy and electron-microscopy (scanning electron microscope, electron probe microscope, electron back scattered diffraction, insitu tensile test in SEM) investigations were carried out at different stages of heat treatment to study the microstructural features influencing the properties of the material. HPDC U-profiles of aluminium alloys were cast with the inlet, bottom and outlet sections, the profile geometry is described in paper 2. The tests showed that the inlet and bottom sections tested had superior quality compared to that of the outlet section of the selected Uprofile tested.

U-profile of aluminium alloys were also studied in the shear mode of fracture by shearbolt test using a cylindrical bolt, to ascertain that the results found in the uniaxial testing are also valid in other mode of fracture. It was found that after the heat treatments, the material has improved in terms of the energy absorption in the shear mode also.

Surface observations on the flat tensile deformed specimens of AlSi9MgMn alloys were carried out using interference contrast. Transmission electron microscopy (TEM) studies have also been conducted to study the type of precipitates formed in the fully aged (T6-12 hr) AlSi9MgMn material. The observation of coarse slip lines on the surface is assumed to be caused due to the shearing of the needle shaped precipitates observed during TEM studies in the fully aged material. This phenomenon is supposed to be the reason for the low work hardening of the fully aged material observed during the tensile test as compared to the as-cast and stored (T1) aluminium alloys.