Henry Hu

Experimental specimens prepared with stamped wrought Al 6451 alloy, which mainly contains 0.75 % Si and 0.58 % Mg and widely applied in automotive industries for body closure, were tested for tensile properties by comparing uncoated and plasma electrolytic oxidation coated specimens subjected to tensile tests. The properties including ultimate tensile strength (UTS), yield strength (YS), elongation (e _f ) and elastic modulus (E) were evaluated by tensile tests. Following the coating layer built up to 3 μm and 10 μm, the ultimate tensile strength reduced from 161.5 MPa to 143.6 MPa, 137.0 MPa, the yield strength reduced from 86.8 MPa to 38.6 MPa and 38.0 MPa, the e _f reduced from 16.2 % to 15.0 %, 15.3 %, the elastic modulus elevated from 62.9 GPa to 84.4 GPa, 79.0 GPa, respectively. The microstructure was analyzed by scanning electron microscope and energy dispersive spectroscopy, and the observations indicated that the coating layer provided initiation sites of cracking and consequently reduced the tensile strength and strength hardening rates of the alloy under tensile loading.

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The impact of cooling rates on the microstructural evolution of an Al-Sr eutectic alloy was investigated. Two distinct cooling rates, 0.02 and 57.12 °C/s, were employed during the solidification process. To elucidate the characteristics of phase transformations and microstructural evolution during solidification, thermal analyses were conducted on the recorded cooling curves. Both the first and second derivatives of these curves were examined. At the slower cooling rate, the microstructure predominantly consisted of the eutectic Al phase and the eutectic Al-Sr phase, as identified by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Conversely, at the higher cooling rate of 57.12 °C/s, primary Al phases were observed, indicating a significant departure from equilibrium solidification conditions. Additionally, a substantial quantity of nanosized eutectic Al-Sr particles was detected, resulting in a markedly refined microstructure.

Intermetallic formation in sludge during magnesium (Mg) melting, holding and high pressure die casting practices is a very important issue. But, very often it is overlooked by academia, original equipment manufacturers (OEM), metal ingot producers and even die casters. The aim of this study was to minimize the intermetallic formation in Mg sludge via the optimization of the chemistry and process parameters. The Al₈Mn₅ intermetallic particles were identified by the microstructure analysis based on the Al and Mn ratio. The design of experiment (DOE) technique, Taguchi method, was employed to minimize the intermetallic formation in the sludge of Mg alloys with various chemical compositions of Al, Mn, Fe, and different process parameters, holding temperature and holding time. The sludge yield (SY) and intermetallic size (IS) was selected as two responses. The optimum combination of the levels in terms of minimizing the intermetallic formation were 9 wt.% Al, 0.15 wt.%Mn, 0.001 wt.% (10 ppm) Fe, 690 °C for the holding temperature and holding at 30 mins for the holding time, respectively. The best combination for smallest intermetallic size were 9 wt.% Al, 0.15 wt.%Mn, 0.001 wt.% (10 ppm) Fe, 630 °C for the holding temperature and holding at 60 mins for the holding time, respectively. Three groups of sludge factors, Chemical Sludge (CSF), Physical Sludge (PSF) and Comprehensive Sludge Factors (and CPSF) were established for prediction of sludge yields and intermetallic sizes in Al-containing Mg alloys. The CPSF with five independent variables including both chemical elements and process parameters gave high accuracy in prediction, as the prediction of the PSF with only the two processing parameters of the melt holding temperature and time showed a relatively large deviation from the experimental data. The Chemical Sludge Factor was primarily designed for small ingot producers and die casters with a limited melting and holding capacity, of which process parameters could be fixed easily. The Physical Sludge Factor could be used for mass production with a single type of Mg alloy, in which the chemistry fluctuation might be negligible. In large Mg casting suppliers with multiple melting and holding furnaces and a number of Mg alloys in production, the Comprehensive Sludge Factor should be implemented to diminish the sludge formation.