The Impact of Metal Cleanliness on Mechanical Properties of Die Cast Magnesium Alloy AM50

Abstract

IntroductionAs in-house and in-cell recycling is developing (1-5) one should expect metal cleanliness issues to be of paramount importance. However, until now metal cleanliness has to some extent been a neglected aspect as low cost solutions and smart logistics often have received the main focus (6-9). The effect of inclusions on mechanical properties is pronounced and well-known in steels. Properties such as ductility, fatigue, toughness and machinablilty are affected (10-12). Also within the aluminium industry inclusions are of great concern (10). Molten magnesium alloys oxidise more readily than any other metal cast or formed into components used for construction purposes. One might expect this to trig primary producers, end-users as well as researchers interest in quantifying the impact of inclusions on important mechanical properties. However, to our knowledge only a few papers have been published addressing this topic featuring die cast magnesium alloys (13-15). The essence of these papers is that in order to detect any influence of inclusions the content must be really high (at least 0.2 %). This might have lead to the opinion that metal cleanliness represents no obstacle to go ahead with in-house/in-cell recycling. Experimental Test materialStandard die cast test bars were produced from AM50 melts deliberately contaminated with oxide inclusions. AM50 was chosen because this is a highly ductile magnesium based alloy, possibly making a reduction in tensile properties more easily visible than for instance with the less ductile AZ91. Molten metal cleanliness was adjusted in part by introducing thin-walled scrap to a reference ingot melt, and in part by turning off the gas protection for short periods allowing the melt to burn. It was not possible to predict the metal cleanliness, it could only be determined retrospectively. Molten metal cleanliness was monitored with the Hydro Magnesium Inclusion Assessment Method (HMIAM) (16,17), distinguishing between oxide particles and oxide films. Samples were withdrawn both before and after casting of the melts. The melts were stirred manually just before casting in order to interrupt the refining caused by settling. The chemical composition of the melts was controlled by spectrographic analysis of permanent mould cast specimens. Die casting of test bars was done on the 3-cavity test bar die on the 200 ton Frech machine in Hydro Magnesium's foundry laboratory. Standard die cast test bars were made from 17 Magnesium Alloys and their Applications. Edited by K. U. Kainer.

Inclusion removal and grain refinement of magnesium alloy castings

“…The stress-strain curves (Figure 2-3) for AM50 alloy containing different levels of inclusions suggest that clean melts fracture at significantly higher strain than contaminated melts [97]. An inclusion level of 500 ppm was found to be the highest acceptable inclusion content.…”

“…For castings free of defects (hot tears, porosity, folds, etc. ), the inclusion content and distribution of inclusions are assumed to be a critical factor in determining mechanical properties [97]. A significant improvement in the UTS and elongations [32,50,[53][54][55][56]75,80,81,96,99,100] and the impact strength [96,100] with the reduction of inclusion content in the melt was indeed observed by several researchers.…”

“…It is known that inclusions lead to premature failure of the material by reducing the effective cross section of metal under load and by the concentration of stresses at the inclusion. The direct impact of inclusions on the mechanical properties is, however, difficult to quantify, since casting defects such as cold shuts, hot tears and porosity will normally be present alongside inclusions and dominate over inclusions in determining the properties of a cast part [97]. This is illustrated in the work of Chen et al [98] and Yim et al [84] where Chen et al [98] studied the effect of ultrasonic treatment on the microstructural and mechanical properties of AZ91 and AM60 alloys.…”

Inclusions in magnesium and its alloys: a review

“…The direct impact of inclusions on the mechanical properties is, however, difficult to quantify, since casting defects such as cold shuts, hot tears and porosity will normally be present alongside inclusions and dominate over inclusions in determining the properties of a cast part. 28 This is illustrated in the work of Chen et al 116 and Yim et al 111 Chen et al 116 studied the effect of ultrasonic treatment on the microstructural and mechanical properties of AZ91 and AM60 alloys. It was found that the application of ultrasound to the melt led to the fragmentation of oxide films.…”

“…Less than 500 ppm 28 Addition of flux or fluxless refining (protective atmospheres, filtration, inert gas bubbling, melt sedimentation, ultrasonic purification, electromagnetic separation) promote the entrapment of oxides. Oxide defects exhibit various morphologies, from particles to films.…”