J. A. Taylor

Strength-Ductility Behaviour of Al-Si-Cu-Mg Casting Alloys in T6 Temper

A comparative study of the mechanical properties of 20 experimental alloys has been carried out. The effect of different contents of Si, Cu, Mg, Fe and Mn, as well as solidification rate, has been assessed using a strength-ductility chart and a quality index-strength chart developed for the alloys. The charts show that the strength generally increases and the ductility decreases with an increasing content of Cu and Mg. Increased Fe (at Fe/Mn ratio 0.5) dramatically lowers the ductility and strength of low Si alloys. Increased Si content generally increases the strength and the ductility. The increase in ductility with increased Si is particularly significant when the Fe content is high. The charts are used to show that the cracking of second phase particles imposes a limit to the maximum achievable strength by limiting the ductility of strong alloys. The (Cu + Mg) content (at.%), which determines the precipitation strengthening and the volume fraction of Cu-rich and Mg-rich intermetallics, can be used to select the alloys for given strength and ductility, provided the Fe content stays below the Si-dependent critical level for the formation of pre-eutectic α-phase particles or β-phase plates.

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 on Mg 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.

Interactions between iron, manganese, and the Al-Si eutectic in hypoeutectic Al-Si alloys

Sand-cast plates were used to determine the effect of iron and manganese concentrations on porosity levels in Al-9 pct Si-0.5 pct Mg alloys. Iron increased porosity levels. Manganese additions increased porosity levels in alloys with 0.1 pct Fe, but reduced porosity in alloys with 0.6 and 1 pct Fe. Thermal analysis and quenching were undertaken to determine the effect of iron and managanese on the solidification of the Al-Si eutectic. At high iron levels, the presence of large β-Al5FeSi was found to reduce the number of eutectic nucleation events and increase the eutectic grain size. The preferential formation of α-Al15Mn3Si2 upon addition of manganese reversed these effects. It is proposed that this interaction is due to β-Al5FeSi and the Al-Si eutectic having common nuclei. Porosity levels are proposed to be controlled by the eutectic grain size and the size of the iron-bearing intermetallic particles rather than the specific intermetallic phase that forms.

Crystallography of recently developed grain refiners for Mg–Al alloys

The grain refinement potency of some recently reported grain refiners for Mg–Al based casting alloys, including TiC, TiN, SiC and TiB2, was investigated from a crystallographic viewpoint using the edge-to-edge matching model. The results show that TiB2 is more likely to form an energetically favourable orientation relationship with the Mg matrix, and it is predicted that TiB2 should have the best grain refinement efficiency. The other three compounds may also serve as heterogeneous nucleation sites, but are not likely to be as effective as TiB2. The present analysis provides a better rationale of the superior grain refinement performance of TiB2 compared with SiC.

The Surface Tension of Pure Aluminum and Aluminum Alloys

The surface tension of high purity and commercial purity aluminum in vacuo was determined using the sessile drop method and the results were found to compare favorably with published data. The effects of holding atmosphere, substrate, and “surface fracture” of the sessile drop on the measured surface tension values were investigated together with the effects of different solute elements commonly present in commercial aluminum alloys. The results obtained suggest that the nature of the surface oxide film formed on the droplets (affected by alloy composition and atmosphere) and the rupture of this film are the dominant factors influencing the surface tension values obtained. Changes in surface tension values of up to 60 pct were observed. The possible effect of this variable surface tension on practical casting processes, such as direct chill casting, is suggested.

Alloy Composition and Dendrite Arm Spacing in Al-Si-Cu-Mg-Fe Alloys

Six Al-Si-Cu-Mg-(Fe/Mn) alloys with two levels of each of Cu, Si, and Fe/Mn were cast in the form of quasi-directionally solidified plates. The secondary dendrite arm spacing (SDAS) was measured as a function of the distance from the chill end for each composition and related to the local cooling rate as determined by thermocouples embedded in one of the cast plates. For a given cooling rate, Si has a strong, consistently refining effect on the SDAS per unit of solute content. Cu showed its strongest refining effect at low-Si and high-Fe contents. It is argued that the scale of the SDAS is determined by a combination of five main factors: constitutional undercooling; the fraction of Al-Si eutectic; and the amount, morphology, and distribution of the various intermetallic phases. The first two factors affect the early stages of the dendrite structure and SDAS formation, whereas the ones involving intermetallics affect the dendrite-coarsening mechanisms in the post-eutectic stage. The latter ones are more sensitive to cooling rate than the ones involving solute in solution. The scales of both, SDAS and intermetallics, can be predetermined to a measurable extent through the solute content to best suit particular casting conditions.

Theoretical and Practical Considerations of Grain Refinement of Mg-Al Alloys

The fundamentals of grain refinement are reviewed with particular focus on magnesium alloys. This is followed by considerations of the theoretical and practical aspects of grain refinement of Mg-Al alloys by carbon-based grain refiners. Finally, experimental results using Al4C3 as a potential grain refiner are presented and discussed.

An empirical analysis of trends in mechanical properties of T6 heat treated Al-Si-Mg casting alloys

The Al-Si-Mg family of alloys is typically used in the T6 heat treated condition for high integrity casting applications. There are several variables that can influence the tensile properties of these alloys besides the actual ageing treatment employed. These are alloy composition, particularly Mg and Fe content, and secondary dendrite arm spacing or SDAS. Tensile data from samples covering a wide range of Mg content (0.3–0.7%), Fe content (0.05–0.20%), SDAS (20–60 μn) and two ageing treatments (under- and peak-aged), have been collected and analysed empirically. Concise relationships have been determined for yield strength, ultimate tensile strength and elongation to fracture in terms of one or more of the experimental variables. The observed trends are explained in terms of basic metallurgical principles and the predictive use of the empirically-determined equations is qualified. A comparison of experimentally determined properties and those calculated using the derived relationships highlights some important and clear trends.

The Effect of Solute Elements on the Grain Refinement of Cast Zn

The effect of both peritectic-forming elements (Cu and Ag) and eutectic-forming elements (Mg and Al) on the grain refinement of cast pure Zn was investigated. It is found that these four alloying elements lead to effective grain refinement of cast pure Zn, although they have different values of growth restriction factor (Q). Mg and Al seem to have better grain refining efficiency for cast pure Zn than Cu and Ag. These results raise questions regarding the mechanisms of grain refinement in Zn-based alloys, and therefore further studies are required.

Mass balance characterisation of Al-7Si-Mg alloy microstructures as a function of solution treatment time

Mass balance calculations were performed to model the effect of solution treatment time on A356 and A357 alloy microstructures. Image analysis and electron probe microanalysis were used to characterise microstructures and confirm model predictions. In as-cast microstructures, up to 8 times more Mg is tied up in the π-phase than in Mg2Si. The dissolution of π is accompanied by a corresponding increase in the amount of β-phase. This causes the rate of π dissolution to be limited by the rate of β formation. It is predicted that solution treatments of the order of tens of minutes at 540 °C produce near-maximum T6 yield strengths, and that Mg contents in excess of 0.52 wt% have no advantage.