The effect of Ni addition on microstructure and mechanical properties of cast A356 alloy modified with Sr

Abstract

In this study, casting A356 alloy was pre-alloyed with Ni, and the modification process with Sr was applied by the casting method. The role of Ni on the microstructural and mechanical properties of the produced A356-Ni alloys was investigated. The produced alloys were naturally aged at room temperature for 24 h and artificially aged at 170 • C for 10 h. Optical microscopy (OM), scanning electron microscopy (SEM), and X-ray diffractometer were used in microstructure studies of the A356-Ni alloys. Tensile tests and hardness measurements were carried out to determine mechanical properties. Statistical analysis of Weibull distribution on the tensile test results revealed that the characteristic ultimate tensile strength increased due to increasing the amount of alloying elements, while the characteristic percentage elongation is decreased. This was because Al-Si-Fe intermetallic decomposition and Al-Si-Fe-Ni intermetallics were formed, primarily, 0.5 and 1.0 % Ni addition. It was also determined that the Al-Si-Fe-Ni intermetallics with a morphology non-sharp corners formed more homogeneous in the microstructure. But in the A356 alloys with the addition of 1.5 % Ni, Al-Si-Fe-Ni intermetallics are formed in coarser morphology. In A356 alloys, aluminium dendrites, Al-Si eutectic between dendrites, Mg2Si precipitates are formed in the structure by ageing. Depending on the amount of Ni added to the A356 alloy, the tensile strength of the alloys containing 0.5 and 1.0 % Ni is increased.

Surface Quality of Fe, Ni and Cr added Hyper-eutectic Al-Si Automotive Alloys under Up-milling and Down-milling Operation

“…An earlier investigator has discovered a similar result in Cr added Al-Si alloy with 0.52 wt% Fe [24]. The intermetallic compound is broken down into tiny bits and disseminated throughout the matrix [23].…”

“…As a result, the bond strength with the matrix is lower, resulting in a rougher surface. When Cr is added, the morphology of the platelet iron intermetallic complex changes to innocuous forms, improving the mechanical capabilities but lowering the surface quality [11], [23]. An earlier investigator has discovered a similar result in Cr added Al-Si alloy with 0.52 wt% Fe [24].…”

“…It can be seen that when Ni is added, AlSiFeNi and Chinese script intermetallics develop. According to the Ni content, these intermetallic structures are more common in Al dendrites [23]. These intermetallics are long needle-like, making the alloy more prone to blow-holes and porosity.…”

“…On the other hand, from Figure 6(d) for up-milling and Figure 6(h) for down-milling, it is noticeable that the chips are rather longer relatively. When Cr is added, however, the morphology of the intermetallic compound changes to a more favorable state, improving mechanical properties [11], [23].…”

“…But Fe added Alloy 2 shows more cracks due to presence of higher level of Fe-rich intermetallic followed by Fe and Ni added Alloy 3 which is due to both Fe and Ni rich needle-like intermetallics. However, Cr Added Alloy 4 can reduce this phenomenon by changing the morphology of Fe-rich intermetallic phases [11], [23]. In case of down-milling operation, lower cracks and pinholes are observed due to compressive stress, which occurs like cold rolling.…”