Rheological behavior of wrought aluminium 7075 feedstock billet produced via semisolid metal processing technique / Arumugam Pillai Megalingam

Faculty of Mechanical and Automotive Engineering Technology

Thesis (Doctor of Philosophy) -- Universiti Malaysia Pahang – 2024

Includes bibliographical references

Semi-solid metal processing (SSMP) plays an important role in the foundry industry due to its cost-effectiveness, productivity, and ability to create high-quality near-net-shaped components in a single forming process. As an alternative to the conventional metal casting process, processing alloys between solid and liquid temperatures is attracting attention. SSMP offers an excellent opportunity for casting low-viscosity aluminium 7075 alloys, which have various advantages such as high mechanical properties and high formability. Experimental investigations, including microstructural analysis of aluminium 7075 alloys, should be thoroughly investigated to elucidate the influence of processing parameters such as rheological properties, temperature, strain rate, and fraction solid on the flow behaviour of the material in the semi-solid state. A globular microstructure is highly favoured for SSMP, and it is interesting to study rheological behaviours with microstructure effects in the semi-solid state. However, the SSMP of aluminium 7075 alloy appears to be lacking in terms of processing parameters optimization information for producing high-quality feedstock billets. Furthermore, an extensive investigation of the rheological behaviours in the semi-solid state is required. Therefore, this thesis aims to investigate in detail the rheological behaviour of aluminium 7075 alloys with globular microstructure. For this, globular microstructure aluminium 7075 feedstock billets were prepared using the direct thermal method (DTM) suitable for SSM (semi-solid metal) formation. All tests were performed using the design of experiments (DOE) strategy. Processing parameters were analyzed using the Response Surface Methodology (RSM). A new viscometer has been introduced, which overcomes the shortcomings of the viscometer in measuring viscosity at high fraction solids. The viscosity of globular microstructure aluminium 7075 alloy feedstock billets was analyzed in a viscometer and compared with the viscosity of dendrite microstructure. The RSM analyzed the influence of processing parameters. In addition, the creep deformation behaviour of aluminum7075 alloy feedstocks was synthesized. Using Central Composite Design (CCD), the effects of pouring temperatures (ranging from 645 to 685 °C) and holding times (ranging from 20 to 60 s) on billet production via the DTM method were explored. RSM analysis identified 665 °C and a 60 s holding time as the optimal conditions for finer and globular microstructures. Under these conditions, grain diameter, circumference, and aspect ratio values were measured at 61.12 µm, 0.707, and 1.44, respectively. Similarly, the viscosity test was designed using CCD, with heating temperatures from 600 to 616 °C and hydraulic pressures from 5 to 15 bar. For thixoforming, uniform deformation and low viscosity is crucial. RSM findings revealed that at 616 ºC (with fractional solids at 0.5) and 15 bar pressure, the minimum initial viscosity was 1.02 x 104 Pa.s. Optimization with RSM indicated that at 616 °C and 15 bar pressure, the compression rate, initial pressure, and maximum shear rate values were 11.28 mm/s, 88.723 kPa, and 0.112 s-1 , respectively. The globular microstructure billets showed reliable creep deformation at low-stress conditions of 30 MPa and 50 MPa, showing high resistance to creep at these conditions and longer creep rupture time than the higher stress. The strain exponent value of the globular microstructure billets was 4.6 at a constant temperature of 250 °C and a pressure range of 30 to 70 MPa. Although these studies provide valuable insight into the effect of microstructure and rheological properties of aluminium 7075 alloy billets produced by DTM, investigation on postprocessed and cold products and numerical analysis and simulation is recommended for future work.