Divorced eutectic in a HPDC Magnesium-Aluminium alloy

The morphology of the eutectic in a thin-wall high pressure die cast (HPDC) U-shape AM60 magnesium box was investigated by light microscope, SEM, TEM and EPMA. The extremely fast cooling rate taking place in the solidification process produces a highly segregated zone near the boundaries of small grains and a fine distribution of particles, which is typical of a completely divorced eutectic. It was shown that the segregated zone is coherent with the primary -Mg grain core even if the increased aluminium content produces a deformation of the hexagonal crystal lattice, which was estimated through diffraction patterns (SADP). The variation of the alloying elements content through the grain boundaries was shown by means of EPMA line scanning. The particle composition was quantitatively investigated and the results show that, in comparison with the equilibrium phase diagram, the non-equilibrium phase boundary of the Mg17Al12 region is moved some percent towards the lower aluminium content, at the high cooling rate that occurs in high pressure die castings. The cubic structure of the phase was revealed by diffraction pattern. The presence of small Al–Mn particles both inside the grain and in the boundary region was also put in evidence by TEM

Effect of solution heat treatments on the microstructure and mechanical properties of a die-cast AlSi7MgMn alloy

The influence of solution heat treatment time and temperature on the microstructure and mechanical properties and the mode of fracture of a high-pressure die-cast AlSi7MgMn alloy is reported. Metallographic and image analysis techniques have been used to quantitatively examine the microstructural changes occurring during solution heat treatment. A solution heat treatment of 15 minutes at 475 degrees C, or even more at 525 degrees C, is sufficient to spheroidize the eutectic Si, as well as coarsen and increase the interparticle distance of the eutectic Si. Increasing the solutionizing temperature from 475 degrees C to 525 degrees C improves the mechanical properties

Formation of the surface layer in hypoeutectic Al-alloy high-pressure die castings

A layer of distinctive microstructure known as the surface layer or the skin is often observed near the casting surface of high-pressure die cast (HPDC) parts. With its different microstructure, the surface layer could influence mechanical performance, corrosion properties and also pressure tightness of the whole cast component. This research aims to develop a better understanding of surface layer formation, which is essential to control the microstructure and therefore properties of HPDC components. In this study, microstructural characterization has been performed on HPDC specimens cast from an HPDC-specific Al alloy for structural applications, AlMg5Si2Mn. Most regions in the samples investigated contain ∼10-30 μm globular-rosette primary α-Al grains and [Al + MgSi] eutectic, while very dendritic primary α-Al grains are also present in the surface layer. The surface layer was observed in the region where the alloy did not directly impinge on the die surface during die filling (the so-called indirect impingement zone). In the region between the surface layer and inner regions (the so-called surface-layer edge), the only primary grain population is the very dendritic grains also observed in the surface layer. The surface layer formation is related to shearing at the interface between two parts of material containing different solid fractions (f) which can occur either during the die-filling or during the intensification stage

An overview of defect bands in high pressure die castings

Bands of positive macrosegregation, porosity and/or cracks commonly follow the outer contour of high pressure die cast (HPDC) components. This paper overviews defect bands in hot- and cold-chamber HPDC components manufactured using Mg, Al and Zn-based alloys. A hot chamber HPDC AM50 magnesium steering wheel is used as a case study, and defect band formation is discussed based on recent research on strain localization in partially solid alloys

Agglomeration and bending of equiaxed crystals during solidification of hypoeutectic Al and Mg alloys

Agglomeration and bending of equiaxed crystals were studied by microstructural characterization of specimens produced by near-static cooling, high-pressure die casting and Thixomolding®, where the solidifying crystals experience different levels of mechanical stresses. EBSD was used to acquire statistical grain misorientation data which is linked to crystal agglomeration and bending behavior during solidification. An aluminum alloy and two magnesium alloys were used to compare grain misorientations for different crystal structures. The length fraction of low-energy grain boundaries in HPDC and Thixomolded samples was substantially higher than in “statically cooled” samples. This is attributed to the high shear stresses and pressure applied on the solidifying alloy, which promote crystal collisions and agglomeration. In-grain misorientations were found to be significant only in branched dendritic crystals which were subjected to significant shear stresses. This is related to the increased bending moment acting on long, protruding dendrite arms compared to more compact crystal morphologies