Grain refinement of magnesium alloys: a review of recent research, theoretical developments and their application

This paper builds on the ‘‘Grain Refinement of Mg Alloys’’ published in 2005 and reviews the grain refinement research onMg alloys that has been undertaken since then with an emphasis on the theoretical and analytical methods that have been developed. Consideration of recent research results and current theoretical knowledge has highlighted two important factors that affect an alloy’s as-cast grain size. The first factor applies to commercial Mg-Al alloys where it is concluded that impurity and minor elements such as Fe and Mn have a substantially negative impact on grain size because, in combination with Al, intermetallic phases can be formed that tend to poison the more potent native or deliberately added nucleant particles present in the melt. This factor appears to explain the contradictory experimental outcomes reported in the literature and suggests that the search for a more potent and reliable grain refining technology may need to take a different approach. The second factor applies to all alloys and is related to the role of constitutional supercooling which, on the one hand, promotes grain nucleation and, on the other hand, forms a nucleation-free zone preventing further nucleation within this zone, consequently limiting the grain refinement achievable, particularly in low solute-containing alloys. Strategies to reduce the negative impact of these two factors are discussed. Further, the Interdependence model has been shown to apply to a broad range of casting methods from slow cooling gravity die casting to fast cooling high pressure die casting and dynamic methods such as ultrasonic treatment

The skin effect in a high pressure die cast Mg-9%Al alloy

Cross sectional microhardness maps of cast-to-shape tensile specimens of rectangular and circular cross-sections have been obtained for a Mg-9 mass% Al alloy. The hardness is generally higher near the surface and at the corners of the cross-section in comparison with that at the centre. The lower hardness values at the casting core are accounted for by the coarser solidification microstructure and the concentration of porosity. The cross sectional mapping shows that the harder surface layer is generally uneven, questioning the concept of a well defined, uniform and continuous skin. Physical reasons for these features of the casting's skin are discussed in terms of the grain microstructure. © (2010) Trans Tech Publications

INTERMETALLICS DISTRIBUTION IN TWO AND THREE DIMENSIONS IN HIGH PRESSURE DIE CAST AZ91 ALLOY

Scanning Electron Microscope (SEM) and DUalBeam FIB (Focussed Ion Beam) were used to characterize the intermetallic microstructure in 2D and 3D, respectively. The intermetallic appears as a closely interconnected network with very fine scale near the surface, while it takes a more open structure in the core regions of the casting. The possible effects of this difference in the scale of the intermetallic structure on the mechanical behaviour of the material are discussed

Growth restriction factor effects near the surface of high pressure die cast Mg-Al alloys

The grain microstructure at the corner regions of the cross-section of thigh pressure die cast Mg-Al alloys has been compared to those of the core and flat surfaces of the castings, for Al contents between ~ 5 and 12 mass%, using the electron back scattered diffraction technique. The average grain size at the corner is somewhat smaller than that of the surface for the higher concentrations of Al, and it appears to be controlled by the amount of solute in the same way as quiescent castings

An experimental verification of the finite element modelling of equal channel angular pressing

Strain hardening of pure copper and friction between the sample and die channels is considered for finite element modelling. To validate the FEM results, the FEM calculated effective strain variations were compared with the hardness measurements. Simulated load-stroke curve and peak load calculations were also compared with the experimentally recorded load-stroke curve and peak load. Different stages of the load-stroke curve of the ECAP process was explained in detail. In over all, good conformity is observed between the FEM calculations and experimental results

Calculated phase diagrams and the corrosion of die-cast Mg-Al alloys

The corrosion of commercial die-cast Mg-Al alloys was elucidated by a study, of the corrosion in 3% NaCl, of(i) high-pressure die-cast (HPDC) model Mg-Al alloys, (ii) low-purity Mg, (iii) high-purity (HP) Mg and (iv) HP Mg heat treated at 550 degrees C. HPDC is the most important route for the production of Mg components. The corrosion of the model alloys was dominated by the Fe impurity element. The present research identified the appearance of the Fe-rich particles in the microstructure. In high magnification (similar to 1000x to 5000x) secondary electron images, they appear as small white features, typically less than 1 pm in diameter. In order to understand the impurity tolerance limits, (i) the appropriate corrosion literature was summarised and reviewed and (ii) Mg phase diagrams were calculated using the Pandat software package. Calculated phase diagrams can explain (i) the tolerance levels for Fe and Cu and (ii) the production of high-purity castings by means of control of melt conditions; this has high significance for the production of quality castings from recycled Mg. A full analysis requires that the Mg database be extended to include Ni, Co and some RE. The Fe tolerance limit is similar to 5-10 ppm for cast HP Mg heat treated at 550 degrees C. Analysis of the Mg corrosion literature indicates that several studies have been dominated by the Fe impurity content and have not dealt with the stated aims: it means that the full chemical composition should be reported in all studies of the corrosion of Mg alloys. (C) 2008 Elsevier Ltd. All rights reserved