Towards magnesium alloys for high-volume automotive applications

Al–12Si components were manufactured by Selective Laser Melting (SLM) using three different atmospheres: argon; nitrogen and helium. The atmosphere type did not affect the part's density or hardness and all components reached near full relative density (>97%). The mechanical properties of the components produced in Ar and N2 were superior to those in He, especially the ductility, which has been attributed to the formation of pore clusters in the microstructure. The mechanical properties in SLM-produced components are superior to those produced using conventional method.

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The effect of atmosphere on the structure and properties of a selective laser melted Al-12Si alloy

The effect of alloy composition on the microstructure and tensile properties of binary Mg-rare earth alloys

Effect of cooling rate on solidified microstructure and mechanical properties of aluminium-A356 alloy

The feasibility of incorporating fly ash cenospheres in die cast magnesium alloy has been demonstrated. The effects of fly ash cenosphere additions on the microstructure and some of the salient physical and mechanical properties of magnesium alloy (AZ91D) metal matrix composites were investigated. The control AZ91D alloy and associated composites, containing 5, 10, and 15 wt.% of fly ash cenospheres (added), were synthesized using a die casting technique. A microstructural comparison showed that microstructural refinement – occurred due to the fly ash additions and became more pronounced with an increase in the percentage of the fly ash added. The metal matrix areas nearer to the fly ash particles exhibited a greater degree of refinement than was observed in the areas further away from these particles. Both filled and unfilled fly ash cenospheres, and porosity were observed in the composite microstructures. The composite specimen densities decreased and the coefficient of thermal expansion did not change significantly as the volume percent of fly ash was increased within the range investigated. The hardness values of the composite specimens exhibited an increase in proportion to the increase in percentage of added fly ash. The tensile strength of the composites also increased as the concentration of fly ash cenospheres was increased. In contrast, the Young’s modulus of these composite samples, as measured by non-destructive pulse-echo method, decreased as the percentage of fly ash in the composite was increased. SEM micrographs of the tensile fracture surfaces showed broken cenospheres on the fracture surface and evidence of ‘pull outs’, where fly ash particles were previously embedded in the matrix. Compression testing results showed that the presence of 5 wt.% cenospheres decreased the compressive strength and compressive yield strength of the composite relative to that of the AZ91D matrix alloy. Surprisingly, a significant change in compression strength was not observed for the composites with 10 and 15 wt.% cenospheres in comparison to the AZ91D matrix alloy. In contrast to the tensile tests, no cenosphere remnants were observed on the compressive test fracture surface of the composites. This observation suggests that the fracture of the composite was initiated within the AZ91D matrix by normal void nucleation and growth, followed by crack propagation through the matrix, avoiding any of the cenospheres, leading to composite fracture of the matrix.

Microstructure and mechanical behavior of die casting AZ91D-Fly ash cenosphere composites

High-pressure die-casting is the preferred manufacturing process for cast Mg-alloy components used for numerous applications. High-pressure die-cast components usually contain micro-porosity that adversely affects their mechanical properties. In this contribution, the effects of three important process parameters, gate velocity, intensification pressure, and melt temperature on the micro-porosity distributions in high-pressure die-cast AM50 Mg-alloy are quantitatively characterized. The amounts of total porosity, gas porosity, shrinkage porosity, and pore size distributions are experimentally measured using novel digital image analysis techniques that permit quantification of both gas and shrinkage pores in an unbiased manner. The experimental data lead to the following conclusions: (1) Application of intensification pressure significantly reduces the total amount of porosity primarily via reduction in the gas porosity. The intensification pressure significantly reduces the number density and area fraction of the gas pores larger than 100 μm diameter. (2) A decrease in the gate velocity decreases the total amount of porosity predominantly via a decrease in the gas porosity and a small extent of decrease in the shrinkage porosity. The lower gate velocity uniformly decreases the number density and area fraction of gas pores of all sizes (small and large). (3) A decrease in the melt temperature also reduces the total amount of porosity primarily via reduction in the gas porosity. The lower melt temperature reduces the number density and area fraction of the gas pores larger than 30 μm.

Effect of process parameters on porosity distributions in high-pressure die-cast AM50 Mg-alloy

Origins of variability in the fracture-related mechanical properties of a tilt-pour-permanent-mold cast Al-alloy

Variability in the tensile ductility of high-pressure die-cast AM50 Mg-alloy

Cast magnesium alloys often exhibit large variability in fracture related mechanical properties such as ductility and strength. In this contribution, the variability in the tensile ductility of individually cast tensile test specimens of high-pressure die-cast AE44 Mg-alloy is examined at room temperature and at 394 K. Significant specimen-to-specimen variations in the ductility are observed at both temperatures. The variability in the ductility does not quantitatively correlate to the average volume fraction of porosity (or any other microstructural parameters) in the bulk three-dimensional microstructure. The area fraction of porosity measured in the fracture surfaces of the tensile test specimens is much larger than the average volume fraction of the porosity in the corresponding bulk microstructure. Therefore, the fracture path preferentially goes through the regions of highly localized clusters of gas and shrinkage pores. Interestingly, at both test temperatures, the percent tensile ductility e shows a quantitative correlation with the area fraction of the porosity f in the corresponding fracture surfaces , which can be represented by the following simple equation e  =  e 0 [1 −  f ] m , where e 0 and m are empirical constants.

Quantitative fractographic analysis of variability in the tensile ductility of high-pressure die-cast AE44 Mg-alloy

Variability in the ductility of tensile test specimens of semi solid process cast A356 Al-alloy (Al-7 wt.% Si-0.5 wt.% Mg-base alloy) is examined. The variability in the ductility does not correlate to the global average microstructural parameters such as dendrite cell size, Si particle size, and amount of porosity in the three-dimensional microstructure. Tensile fracture surfaces contain micro-defects that are essentially residues of modifiers, fluxes, grain refiners, and mold release agents: the energy dispersive analysis shows presence of elements such as O, Na, K, C, Cl, Ca, Fe, Ti, and S in these defects. The fracture path preferentially goes through the regions containing the defects. Scanning electron microscopy, quantitative fractography was used to estimate the total area fraction of such defects in the fracture surfaces of the tensile test specimens. The percent tensile ductility e shows strong quantitative correlation with the area fraction of the defects f in the corresponding fracture surfaces, which can be represented by the following simple equation. e = e 0 [ 1 − f ] n In this equation, e0 and n are empirical constants. For the present data set, e0 is equal to 11.5% and n is equal to 41.66. Thus, the variability in the ductility can be decreased through a reduction in the amount of processing defects via better process control.

G.R. Patel

Papers

Effect of process parameters on porosity distributions in high-pressure die-cast AM50 Mg-alloy

Origins of variability in the fracture-related mechanical properties of a tilt-pour-permanent-mold cast Al-alloy

Variability in the tensile ductility of high-pressure die-cast AM50 Mg-alloy

Quantitative fractographic analysis of variability in the tensile ductility of high-pressure die-cast AE44 Mg-alloy

Analysis of variability in tensile ductility of a semi-solid metal cast A356 Al-alloy