Melt Thermal Treatment Research Articles

Impact of Melt Thermal Treatment and Artificial Aging on Microstructure and Mechanical Properties of A356 Alloy

Impact of Melt Thermal Treatment and Artificial Aging on Microstructure and Mechanical Properties of A356 Alloy

Comparative Study of Microstructure and Properties of Hypoeutectic Al-Si Alloys Being Cast With and Without Melt Thermal Treatment

This study investigated the microstructure and mechanical properties of several hypoeutectic Al-xSi (x = 0, 1, 3, 5, 7, 9) alloys prepared with and without melt thermal treatment. The overall study showed a trend of effective utilization of grain refiner until the transition point (3 wt.% Si) and the slope of graph between grain size vs. wt.% of Si remain similar for the samples prepared with or without chemical refinement. This also indicates that the role of Si in decreasing grain size is more dominating than the poisoning of Ti above transition point. Melt thermal-treated alloys exhibited less significant changes in grain size, especially below the transition point, but significant changes in their mechanical properties. The melt thermal treatment also causes partial refinement of eutectic Si particles by which the coarse silicon particles are broken up into smaller pieces and this has neither been observed in unrefined alloys nor refined alloys.

Effects of melt thermal treatment on cast Al-Si alloys: A review

The shape and size of primary phases (α-Al and primary Si in hypoeutectic and hypereutectic alloys, respectively), and that of eutectic Si phase significantly affect the mechanical properties of cast Al-Si alloys. The Al-Si alloys prepared by the conventional casting method have large-sized primary phases and needle-shaped eutectic Si phases along with their uneven distribution in the microstructure. This leads to poor mechanical properties in those alloys. This paper presents a review on a modified casting process known as melt thermal treatment, which has the potential to positively change the shape and size of primary and eutectic phases in Al-Si alloy without any foreign additives. Various researches that have been carried out to study the effect of melt thermal treatment on microstructure and different properties of Al-Si alloys are discussed here.

Correlation of composition, cooling rate and superheating temperature with solidification behaviors and microstructures of Al–Bi–Sn ribbons

In this paper, Al85Bi6Sn9, Al75Bi9Sn16, Al60Bi14.4Sn25.6 and Al45Bi19.8Sn35.2 immiscible alloys ribbons were prepared using a single-roller melt-spinning technique. The influences of the composition, the rotating speed of copper roller and the melt superheating temperature were investigated in terms of the thickness, width, phase constitution and microstructures of Al–Bi–Sn ribbons. The results showed that the (Al), (Bi) and (Sn) solid solution phases existed in all the investigated Al–Bi–Sn ribbons. When the rotating speed was larger than 2000 revolutions per minute (rpm), the amorphous phase also was formed. The thickness of ribbons increased and the width decreased with the increase in the aluminum content or with the descent in the melt superheating temperature. The grain sizes of Sn–Bi-rich phase and Al-rich phase decreased gradually with the increases in the rotating speed, the aluminum content or the distance to the free surface. Although there was no obvious macrosegregation under the rapid solidification, the microsegregation still occurred under the rapid solidification. In addition, the grain size of Al75Bi9Sn16 ribbons had a minimum value after the melt superheating treatment at 910 °C. The impact of melt thermal treatment on the microstructure was analyzed from the views of thermodynamics and kinetics. The changes in the grain size and morphology were attributable to the differences in migration rate, solidification rate and the time of liquid-liquid phase separation (L-LPS). The results are conducive for understanding the effects of composition and melt treatment on the microstructures of immiscible alloys.

Melt holding time as an important factor on the formation of quasicrystal phase in Mg67Zn30Gd3 alloy

In the present work, the content of icosahedral quasicrystal phase (I-phase) and melt holding time shows a mono peak curve: a small amount of I-phase and lots of Mg3(Gd,Zn) phase are presented for t < 21 min, however a great deal of I-phase is observed for t > 21 min, and the volumetric fraction of I-phase has the maximum of 49.50% for t = 41 min. This strategy of formation of quasicrystal phase makes us realize that melt thermal treatment could significantly affect the phase types in MgZnGd alloy.

New Method of Eutectic Silicon Modification in Cast Al–Si Alloys

The aim of the present work was to investigate various means of obtaining a fine eutectic silicon structure in A356.2 alloy and therefore improve the alloy mechanical properties. The effects of solidification rate, Sr modification and melt thermal treatment (MTT) on the silicon particle characteristics of A356.2 (Al–7 %Si–0.4 %Mg) alloy were studied. The particle characteristics measured were the average particle area, average particle length, average particle roundness and average particle aspect ratio, using image analysis and optical microscopy. The results showed that Sr modification-, superheat- and Sr modification-MTT (SrMTT)-processed castings provide fine eutectic Si particles, the SrMTT process giving the best modification results. Both size and morphology of the eutectic silicon particles are affected by the modification process used. The Sr-grain-refined, Sr-melt superheat and SrMTT castings show well-modified fibrous Si particles, whereas the MTT casting exhibits Si particles that, although refined to a certain extent, still retain their acicular morphology. Solidification rate affects the eutectic Si particle size in that a higher solidification rate produces finer Si particles. However, within the range of solidification rates provided by the end-chill mold used in this work, the solidification rate does not affect the morphology of the Si particles.

Role of modification and melt thermal treatment processes on the microstructure and tensile properties of Al–Si alloys

Abstract The present study was performed on an Al–7%Si–0.35%Mg alloy (A356 alloy) with the primary objective of improving the alloy performance through modification of the microstructure. Ultimate tensile strength (UTS) can be improved by the addition of strontium (Sr), superheating or Sr modified melt thermal treatment. The melt thermal treatment process alone has no apparent influence on the UTS. Both Sr-modified and Sr-modified melt thermal treatment can help to improve the percentage elongation of A356 alloy castings. A higher percentage elongation can be reached at a higher cooling rate. The effect of solution heat treatment on the tensile properties of various A356.2 alloy castings can be summed up as follows: (i) the yield strength of the A356.2 castings is significantly improved after 8 h solution heat treatment due to the precipitation of Mg2Si, (ii) the yield strength remains more or less the same with further increase in solution treatment time to 80 h, and (iii) the UTS is greatly improved within the first 8 h of solution heat treatment and continues up to 80 h, where this improvement is attributed to Mg2Si precipitation, dissolution of silicon within the Al-matrix and change in the Si particle morphology (spheroidization). The ductility of the A356.2 alloys can also be considerably enhanced with solution heat treatment (e.g. from ∼6% in the non-modified casting in the as-cast condition to ∼10% after 80 h solution treatment).

Effect of Melt Thermal Treatment on Eutectic Silicon Particles Characteristics in Cast Al-Si-Mg Alloys

A new means of obtaining fine Si particles is through the use of a melt thermal treatment (MTT), where the mixing of low and high temperature alloy melts produces a fine Si structure. Modification is achieved by nuclei resulting from the degeneration of large atom clusters and some refractory solids in the low temperature melt when it is heated by the high temperature melt. This is a relatively recent technique which demonstrates promise as an alternative to Sr-modification, as it requires no element addition, thus reducing the risk of increased porosity normally associated with the addition of strontium to the melt. The use of melt superheat is also found to produce refinement of the eutectic Si structure. In this case, the high melt temperature assists in the degeneration of atom clusters, providing more nuclei for α-Al dendrite formation, and a resulting refinement of the microstructure. KeywordsMelt Thermal Treatment; Melt Superheat; Sr Modification Eutectic Si Particle Characteristics

Combined effect of melt thermal treatment and solution heat treatment on eutectic Si particles in cast Al–Si alloys

The present results showed that in Al&hyphen ;Si alloys, Sr modified (SrM), superheat (SH) and SrM melt thermal treatment (SrMTT) processed castings provide fine eutectic Si particles, with the SrMTT process giving the best modification results. Both size and morphology of the eutectic silicon particles are affected by the modification process used. The SrM, SH and SrMTT castings show well modified fibrous Si particles, whereas the melt thermal treatment (MTT) casting exhibits Si particles that, although refined to a certain extent, still retain their acicular morphology. Cooling rate affects the eutectic Si particle size in that a higher cooling rate produces finer Si particles. However, within the range of cooling rates provided by the end chill mould used in this work, the cooling rate does not affect the morphology of the Si particles. During solution heat treatment at 540°C, the eutectic Si particles undergo fragmentation, spheroidisation and coarsening, affecting the Si particle morphology. The spheroidisation process is determined by the size and morphology of the Si particles in the as cast condition. The SrM, SH and SrMTT processed castings with their refined Si particles require much less solution treatment time for the spheroidisation process to take place than do the non-modified and MTT castings.

Influence of thermal rate treatment and low temperature pouring on microstructure and tensile properties of AlSi7Mg alloy

The combination of thermal rate treatment and low temperature pouring was proposed in the present work, and the effects of the novel melt thermal treatment on microstructure and tensile properties of AlSi7Mg alloy have been investigated. The grain size and microstructure of AlSi7Mg alloy were examined by optical microscopy (OM) and scanning electron microscopy (SEM). It was found that the grain size obviously reduced and the growth of the columnar dendrites changed into equiaxed ones by low temperature pouring, especially the novel melt thermal treatment, by which a maximum microstructure refining effect could be obtained. The refinement can be attributed to the multiplication of the nuclei in the melt. Furthermore, the morphology of eutectic silicon changed little, but the size of silicon phase was finer due to the refinement of the grain size and higher cooling rate. Because of the refinement of the grain size and eutectic silicon, the tensile properties were improved, and the ultimate tensile strength and elongation increased by 11.7% and 35.3%, respectively.

Grain refinement of Al–Si alloy (A356) by melt thermal treatment

In order to increase the casting quality and material plasticity of hypoeutectic Al–Si alloys, the effects of melt thermal treatment on the solidification structure of the A356 alloy were analyzed. Unlike the conventional melt addition treatment, no refinements are required during this novel processing. The structural characteristics and mechanical properties (plasticity and strength) were demonstrated. Results of quantitative metallorgraphic analysis showed that the primary dendrite size obviously reduces and the dendrite grains change into equiaxed ones. However, the secondary dendrite arm spacing changes little. Experimental results show that there is an optimal cooling rate range for thermal treatment, in which a maximum microstructure refining effect can be obtained. By the optimized parameters obtained with orthogonal experiments, the elongation ratio of the castings increases by 46.2% and the tensile strength by 8.6%. These results are explained by means of microheterogeneous melt theory, which is a result of the multiplication of the nuclei in the melt thermal treating procedure.

Effects of melt thermal treatment on hypoeutectic Al–Si alloys

A new refining method—melt thermal treatment without additive to melt is studied in this paper. Microstructure analysis and property evaluation of hypoeutectic Al–Si alloys treated with this method show that the solidification microstructure can be refined significantly with a considerable increase in elongation ratio and strength. Effects such as cooling rate, holding time and alloy composition on the solidification microstructure and mechanical properties have been evaluated. It is shown that the strengthening and toughening effects on the treated samples vary with alloy composition. The property increment of the alloy rich of iron is relatively more remarkable than those rich of Cu or Mg element. Specifically, the structure of the low temperature melt is identified as a primary factor, on which the solidification structure of the treated melt is dependent.

A356 alloy refined by melt thermal treatment

In order to increase the casting quality and material plasticity of hypoeutectic Al-Si alloys, the effects of melt thermal treatment on the solidification structure of the A356 alloy were analyzed. The structural characteristics and mechanical properties (plasticity and strength) were demonstrated. Results of quantitative metallurgraphic analysis showed that the first dendrite size reduces obviously and the dendrite grains change into equiaxed ones. However, the second dendrite arm spacing changes little. By the optimized parameters obtained with orthogonal experiments, the elongation ratio of the castings increases by 46% and the tensile strength by 8%. These results are explained by means of micro heterogeneous melt theory, which is a result of the multiplication of the nuclei in the melt thermal treating procedure.