X-Ray Videomicroscopy Studies of Eutectic Al-Si Solidification in Al-Si-Cu

Al-Si eutectic growth has been studied in-situ for the first time using X-ray video microscopy during directional solidification (DS) in unmodified and Sr-modified Al-Si-Cu alloys. In the unmodified alloys, Si is found to grow predominantly with needle-like tip morphologies, leading a highly irregular progressing eutectic interface with subsequent nucleation and growth of Al from the Si surfaces. In the Sr-modified alloys, the eutectic reaction is strongly suppressed, occurring with low nucleation frequency at undercoolings in the range 10 K to 18 K. In order to transport Cu rejected at the eutectic front back into the melt, the modified eutectic colonies attain meso-scale interface perturbations that eventually evolve into equiaxed composite-structure cells. The eutectic front also attains short-range microscale interface perturbations consistent with the characteristics of a fibrous Si growth. Evidence was found in support of Si nucleation occurring on potent particles suspended in the melt. Yet, both with Sr-modified and unmodified alloys, Si precipitation alone was not sufficient to facilitate the eutectic reaction, which apparently required additional undercooling for Al to form at the Si-particle interfaces.     

Microstructural effects on the tensile and fracture behavior of aluminum casting alloys A356/357

The tensile properties and fracture behavior of cast aluminum alloys A356 and A357 strongly depend on secondary dendrite arm spacing (SDAS), Mg content, and, in particular, the size and shape of eutectic silicon particles and Fe-rich intermetallics. In the unmodified alloys, increasing the cooling rate during solidification refines both the dendrites and eutectic particles and increases ductility. Strontium modification reduces the size and aspect ratio of the eutectic silicon particles, leading to a fairly constant particle size and aspect ratio over the range of SDAS studied. In comparison with the unmodified alloys, the Sr-modified alloys show higher ductility, particularly the A356 alloy, but slightly lower yield strength. In the microstructures with large SDAS (>50 μm), the ductility of the Sr-modified alloys does not continuously decrease with SDAS as it does in the unmodified alloy. Increasing Mg content increases both the matrix strength and eutectic particle size. This decreases ductility in both the Sr-modified and unmodified alloys. The A356/357 alloys with large and elongated particles show higher strain hardening and, thus, have a higher damage accumulation rate by particle cracking. Compared to A356, the increased volume fraction and size of the Fe-rich intermetallics (π phase) in the A357 alloy are responsible for the lower ductility, especially in the Sr-modified alloy. In alloys with large SDAS (>50 μm), final fracture occurs along the cell boundaries, and the fracture mode is transgranular. In the small SDAS (<30 μm) alloys, final fracture tends to concentrate along grain boundaries. The transition from transgranular to intergranular fracture mode is accompanied by an increase in the ductility of the alloys.     

Fluidized bed heat treatment of cast Al-Si-Cu-Mg alloys

The effects of fluidized bed heat treatment on the microstructural and mechanical properties of Al-Si-Cu-Mg cast alloys, namely, 354 and 319, were studied. The heating rate in fluidized beds (FBs) is greatervis-à-vis conventional electrical resistance furnaces (CFs). The high heating rate in FBs increases the kinetics of metallurgical phenomena such as Si fragmentation and spherodization during solution heat treatment, as well as the precipitation rate of phases such as Al₅Cu₂Mg₈Si₆ and Al₂Cu during aging. It is observed that the dissolution rate of phases such as Mg₂Si and Al₅Cu₂Mg₈Si₆ takes place very rapidly. The solution heat treatment of 319 alloy using FB results in complete dissolution of Mg₂Si and Al₅Cu₂Mg₈Si₆ particles within 45 minutes. However, for phases such as Al₂Cu and Ferich intermetallics, the dissolution rate is relatively slow. Even on prolonged solution heat treatment for 6 hours, these phases do not dissolve completely. It is observed that incomplete dissolution of the Al₂Cu phase does not significantly affect tensile properties of T4-treated alloys. The optimum solution heat-treatment time in FB for both 354 and 319 alloys is 45 minutes at 527 °C and 493 °C, respectively. Thermal analysis shows an exothermic peak owing to recrystallization and coarsening of eutectic grains during solution heat treatment. The high heating rate in FB causes this transformation to take place at a lower temperature than in CF. It is observed that the nucleation rate of Al₅Cu₂Mg₈Si₆ during aging in FB is greater than using CF. Thermal analysis of samples during the ramp-up stage while aging using FB did not show any phase transformation, while those using CF show two endothermic transformations, which are most likely due to the dissolution of GP zones or the co-cluster of solutes. Aging at 200 °C results in a greater number density of precipitates than those at 240 °C. The tensile strength of samples aged at 200 °C is greater than those aged at 240 °C, because the amount of precipitates formed at 200 °C is greater than that at 240 °C. The total heat-treatment time for T6 temper is less than 2 hours in FBs, which is a significant reduction in heat-treatment time, as well as energy consumption.     

The effect of Mg on the microstructure and mechanical behavior of Al-Si-Mg casting alloys

The microstructure and tensile behavior of two Al-7 pct Si-Mg casting alloys, with magnesium contents of 0.4 and 0.7 pct, have been studied. Different microstructures were produced by varying the solidification rate and by modification with strontium. An extraction technique was used to determine the maximum size of the eutectic silicon flakes and particles. The eutectic Si particles in the unmodified alloys and, to a lesser extent, in the Sr-modified alloys are larger in the alloys with higher Mg content. Large Fe-rich π-phase (Al₉FeMg₃Si₅) particles are formed in the 0.7 pct Mg alloys together with some smaller β-phase (Al₅FeSi) plates; in contrast, only β-phase plates are observed in the 0.4 pct Mg alloys. The yield stress increases with the Mg content, although, at 0.7 pct Mg, it is less than expected, possibly because some of the Mg is lost to π-phase intermetallics. The tensile ductility is less in the higher Mg alloys, especially in the Sr-modified alloys, compared with the lower Mg alloys. The loss of ductility of the unmodified alloy seems to be caused by the larger Si particles, while the presence of large π-phase intermetallic particles accounts for the loss in ductility of the Sr-modified alloy.     

Microstructure Evolution during Semi-solid Isothermal Holding of Liquidus Cast Strontium-Modified A356 Alloy

An attempt has been made to understand the microstructural evolution during semi-solid isothermal holding of liquidus cast Sr-modified A356 alloy. It was observed that the liquidus casting is effective in producing non-dendritic microstructure with average sphericity of 0.8 and 0.82 for unmodified and modified alloys, respectively. Isothermal holding of the samples at a temperature corresponding to solid fraction of 0.3 for 15 min enhanced the sphericity of unmodified and modified alloys to 0.84 and 0.85, respectively. Further increase in holding resulted grain coarsening and reduction in sphericity. The grain coarsening rate (K) of samples was analysed with Lifshitz–Slyozov–Wanger equation, and the values of K were found to be 58.3 and 98.3 µm³/s for unmodified and modified alloys, respectively. The grain coarsening in unmodified alloy was dominated by Ostwald ripening, whereas in case of modified alloy, grain coarsening was governed by grain coalescence.     

Localized Recrystallization in Cast Al-Si-Mg Alloy during Solution Heat Treatment: Dilatometric and Calorimetric Studies

During heat treatment, the work piece experiences a range of heating rates depending upon the sizes and types of furnace. When the Al-Si-Mg cast alloy is heated to the solutionizing temperature, recrystallization takes place during the ramp-up stage. The effect of heating rate on recrystallization in the A356 (Al-Si-Mg) alloy was studied using dilatometric and calorimetric methods. Recrystallization in as-cast Al-Si alloys is a localized event and is confined to the elasto-plastic zone surrounding the eutectic Si phase; there is no evidence of recrystallization in the center of the primary Al dendritic region. The size of the elasto-plastic zone is of the same order of magnitude as the Si particles, and recrystallized grains are observed in the elasto-plastic region near the Si particles. The coefficient of thermal expansion of Al is an order of magnitude greater than Si, and thermal stresses are generated due to the thermal mismatch between the Al phase and Si particles providing the driving force for recrystallization. In contrast, recrystallization in Al wrought alloy (7075) occurs uniformly throughout the matrix, stored energy due to cold work being the driving force for recrystallization in wrought alloys. The activation energy for recrystallization in as-cast A356 alloy is 127 KJ/mole. At a slow heating rate of 4.3 K/min, creep occurs during the heating stage of solution heat treatment. However, creep does not occur in samples heated at higher heating rates, namely, 520, 130, and 17.3 K/min.     

Solution Treatment Effects on Microstructure and Mechanical Properties of Al-(1 to 13 pct)Si-Mg Cast Alloys

The effects of solution treatment time and Si content and morphology on microstructures and mechanical properties of heat-treated Al-Si-Mg cast alloys were investigated systematically. Five alloys, with Si levels ranging from 1 to 13 pct, were tested in as-cast, T4, and T61 conditions. The eutectic Si was both unmodified and Sr-modified. Results show that the microstructures are affected significantly by alloy composition, eutectic Si morphology, and solution treatment time. Si content has significant effects on ultimate tensile strength (UTS), yield strength (YS), and elongation as well as a strong influence on solution treatment response. In T61 treatment with different solutionizing times, UTS and YS reach their maximum values in ~1 hour of solutionizing followed by a decrease, then a slight increase, and finally, a plateau close to the maximum level. Elongation of alloys with a high Si content, 7 pct and 13 pct, increases rapidly at solutionizing times of 1 to 2 hours then varies in a wide range, showing improvements in the 4 to 10 hours range. The data indicate that a solution treatment time of ~1 hour is sufficient to achieve maximum strength. The changes in mechanical properties were correlated to changes in microstructure evolution—Mg-Si precipitation, Si particle fragmentation, and microstructure homogenization. Empirical models uniquely relating Si content to UTS and YS are given for T61 heat-treated alloys.     

Aging Effects on Heat Treatment Response and Mechanical Properties of Al-(1 to 13 pct)Si-Mg Cast Alloys

The effects of solution treatment time and Si content and morphology on microstructures and mechanical properties of heat-treated Al-Si-Mg cast alloys were investigated systematically. Five alloys, with Si levels ranging from 1 to 13 pct, were tested in as-cast, T4, and T61 conditions. The eutectic Si was both unmodified and Sr-modified. Results show that the microstructures are affected significantly by alloy composition, eutectic Si morphology, and solution treatment time. Si content has significant effects on ultimate tensile strength (UTS), yield strength (YS), and elongation as well as a strong influence on solution treatment response. In T61 treatment with different solutionizing times, UTS and YS reach their maximum values in ~1 hour of solutionizing followed by a decrease, then a slight increase, and finally, a plateau close to the maximum level. Elongation of alloys with a high Si content, 7 pct and 13 pct, increases rapidly at solutionizing times of 1 to 2 hours then varies in a wide range, showing improvements in the 4 to 10 hours range. The data indicate that a solution treatment time of ~1 hour is sufficient to achieve maximum strength. The changes in mechanical properties were correlated to changes in microstructure evolution—Mg-Si precipitation, Si particle fragmentation, and microstructure homogenization. Empirical models uniquely relating Si content to UTS and YS are given for T61 heat-treated alloys.     

The effect of heat treatment on the fatigue and corrosion fatigue behavior of a CuNiCr alloy

Fatigue experiments were conducted on a CuNiCr alloy (IN838) in air and in 0.5 N NaCl solutions under conditions of free corrosion and of applied anodic currents. The alloy was heat treated to produce a solutionized structure and also to produce a precipitation hardened structure. The fatigue behavior of the solutionized alloy was unaffected by free corrosion although increased corrosion rates resulted in a decrease in fatigue resistance for small applied anodic currents. The age hardened alloy showed a decrease in fatigue resistance under free corrosion conditions and a further decrease in resistance with small applied anodic currents. In both heat treatments fatigue in air resulted in mixed transgranular-intergranular crack initiation and propagation while corrosion increased the relative amount of intergranular cracking. These results can be explained by a consideration of previously developed fatigue and corrosion fatigue models of pure copper and copper aluminum alloys. H. N. Hahn, formerly with Rensselaer Polytechnic Institute     

Investigation of the microstructure of the Al–Si eutectic in binary aluminium–7 wt% silicon alloys by electron backscatter diffraction (EBSD)

Binary aluminium–7 wt% silicon alloys with and without strontium modification have been cast with a cooling rate of 0.2–0.6°C/s. The level of impurities has been kept at a minimum. The crystallographic orientation of the dendritic and eutectic aluminium as well as the eutectic silicon has been studied using electron backscatter diffraction (EBSD). The crystallographic orientation of the aluminium within the eutectic is found to be strongly influenced by the orientation of neighbouring dendrites in unmodified and Sr-modified alloys. The crystallographic orientation of the silicon phase in the eutectic shows that silicon flakes/fibers within one eutectic colony can often be related to each other by the misorientation characteristic of twinning. Within one Sr-modified eutectic colony, silicon fibers are often found to have a common 〈1 1 0〉 direction. Aluminium has been found to have a 〈1 0 0〉 or a 〈1 1 0〉 direction parallel to the 〈1 1 0〉 direction of the Si fibers.     

The influence of strontium on porosity formation in Al-Si alloys

Strontium modification is known to alter the amount, characteristics, and distribution of porosity in Al-Si castings. Although many theories have been proposed to account for these effects, most can be considered inadequate because of their failure to resolve contradictions and discrepancies in the literature. In an attempt to critically appraise some of these theories, the amount, distribution, and morphology of porosity were examined in sand-cast plates of Sr-free and Sr-containing pure Al, Al-1 wt pct Si, and Al-9 wt pct Si alloys. Statistical significance testing was used to verify apparent trends in the porosity data. No apparent differences in the amount, distribution, and morphology of porosity were observed between Sr-free and Sr-containing alloys with no or very small eutectic volume fractions. However, Sr modification significantly changed the amount, distribution, and morphology of porosity in alloys with a significant volume fraction of eutectic. The addition of Sr reduced porosity in the hot spot region of the casting, and the pores became well dispersed and rounded. This result can be explained by considering the combined effect of the casting design and the differences in the pattern of eutectic solidification between unmodified and Sr-modified alloys.     

Feeding and Distribution of Porosity in Cast Al-Si Alloys as Function of Alloy Composition and Modification

Unmodified, Na-modified, and Sr-modified castings of Al-7?pct Si and Al-12.5?pct Si alloys were cast in molds in which it was possible to create different cooling conditions. It is shown how solidification influences the distribution of porosity at the surface and the center of the castings as a function of modification and Si content in sand- and chill-cast samples. Eutectic modification, Si content, and cooling conditions have a great impact on the distribution of porosity. Unmodified and Na-modified castings are more easily fed with porosity tending to congregate near the centerline of the casting, while Sr-modified castings solidify in a mushy manner that creates a more homogeneous distribution of porosity in the casting. The amount of porosity was highest in the Sr-modified alloys, lower in the Na-modified alloys, and lowest in the unmodified alloys. The size of the porosity-free layer and the effectiveness of the feeders were greater in the castings made with the steel chills due to the increased thermal gradients and consequent increase in the directionality of solidification.     

A1-Si-Cu合金低温力学性能的研究

主要以Al-Si-Cu合金为研究对象,研究了低温变化对锶变质后的Al-Si-Cu合金组织及性能的影响。变质处理后的Al-si-Cu合金的抗拉强度和屈服强度随温度的降低均升高,同时伸长率和断面收缩率随温度的降低也略有上升。通过对Al-si-Cu合金金相观察,加入Sr元素后,合金的组织得到显著细化。变质后的Al-si-Cu合金在低温下具有更好的力学性能。     

枝晶凝固过程中的微观偏析半解析数学模型

基于典型微观凝固单元体内的溶质质量守恒,结合前人的研究工作,建立了一个适合于枝晶凝固方式的二元合金微观偏析半解析模型.本模型同时考虑了反向扩散和粗化对微观偏析的影响,并对枝晶臂间距的粗化直接进行计算,因此更为精确.若只考虑反向扩散的影响,本模型可以简化为BF模型形式;而如果只考虑粗化的影响,本模型可简化为Mortensen模型.本模型完整地统一了以BF模型为代表的反向扩散类模型和以Mortensen模型为代表的粗化模型.利用本模型同样可以对多元合金的微观偏析进行很好预测.以Fe-C-X（Si,Mn,P,S）合金体系为例对本模型的求解过程进行了详细的阐述.本模型可以很好地预测Al-4.9%Cu二元合金的共晶分数以及Fe-C-X（Si,Mn,P,S）多元合金体系的零强度温度和零塑性温度,并与实测值吻合良好.      

Simultaneous Primary Si Refinement and Eutectic Modification in Hypereutectic Al–Si Alloys

A solid?Cliquid duplex casting process was applied to achieve simultaneous refinement and modification of silicon phases in hypereutectic Al?CSi alloys. In this process P-treated Al-24Si (wt%) solid alloy is mixed with Sr-treated eutectic Al-12.6Si molten alloy to provide an Al-18Si alloy in the Liquid?+?Primary Si phase field and then cast. By using this process the mean particle size of primary silicon was reduced from 50???m to less than 15???m (refinement) and the eutectic structure changed from a plate-like to a fibrous structure (modification) when compared with conventional casting process. The results of mechanical testing showed that the tensile strength increased by about 14?% from 153.5 to 175.1?MPa and the elongation more than doubled from 1.64 to 3.76?%. The improvement of mechanical properties is attributed to the combination of refined primary silicon and the fibrous structure of the modified eutectic Al?CSi matrix.     

Solution Treatment Enhances Both Static and Damping Properties of Al–Si–Mg alloys

The static properties of Al–Si–Mg alloys have been widely studied, but their damping behavior is generally neglected. Although there is an increasing concern regarding the mitigation of vibration, the correlation between the damping properties and the load bearing capacity of an alloy is frequently neglected. This study explores this relationship using A356 alloy as an example. It is concluded that, owing to eutectic Si coarsening/spheroidization, Mg₂Si/π-phase dissolution, and α-Al solution strengthening, the solution treatment can enhance both static (yield strength) and dynamic (damping ratio) mechanical properties.     

The effect of solidification microstructure on the corrosion behavior of a columnar aluminum-copper alloy

The morphological and kinetic nature of corrosion of directionally solidified aluminum-4.5 wt pct copper alloy in the as-cast, solutionized and solutionized-and-aged conditions in air-saturated aqueous 3.5 wt pct NaCl solution were evaluated. In the solutionized and solutionized-and-aged conditions the intergranular attack and pitting are similar to those occurring in solutionized wrought alloys; the extent of attack at long times increases with increasing severity of solidification rate. The as-cast alloy exhibits a cored dendritic structure with significant formation of interdendritic nonequilibrium eutectic. Extensive inter dendritic corrosion of the α-phase containing more than 3.2 wt pct copper is seen; α containing less than 3.2 wt pct copper and the θ-Al₂Cu phase are cathodic. Corrosion of the as-cast alloy is parabolic with time and increases with increasing severity of solidification rate in proportion to the amount of nonequilibrium second phase.     

Phenomenological observations on mechanical and corrosion properties of thixoformed 357 alloys: A comparison with permanent mold cast 357 alloys

In the present study, the mechanical and corrosion properties of thixoformed 357 alloys were examined with different reheating temperatures, and the results were compared with those of permanent mold cast (PMC) 357 alloys. It was found that the thixoforming process significantly improved the mechanical properties (i.e., tensile elongation, impact energy, and resistance to fatigue crack propagation) and the corrosion resistance of 357 alloys. A 380 pct increase in tensile elongation and a 120 pct increase in impact energy were, for example, observed with the thixoforming process of 357 alloy in the T1-tempered condition, as compared to the PMC counterparts. The impact energy was extremely sensitive to reheating temperature due to the coarsening of eutectic Si particles. The resistance to fatigue crack propagation was also much higher for the thixoforming process than the PMC process in the T1-tempered condition. The resistance to both general corrosion and stress corrosion cracking was also greatly improved with thixoforming process. The present observations strongly suggest that the enhancement with thixoforming 357 alloy is largely associated with the size and shape of eutectic Si particles.     

Grain oriented silicon- iron with a unique inhibition system for texture development

The new high induction grain oriented silicon-iron alloys are characterized, as compared to the conventional alloy, by a heavier final cold reduction and increased restraint to normal grain growth. It is shown that the conventional alloy possesses sufficient restraint to normal grain growth to undergo secondary recrystallization when it contains manganese sulfide but not when it contains sulfur only as solute. However, the presence of boron, nitrogen and uncombined sulfur leads to strong grain growth inhibition and, when the strip is heavily cold rolled, to a high induction. A deficiency in any one of the three elements results in grain coarsening rather than a nearly constant grain size over a broad temperature range prior to secondary recrystallization.     

Effects of Pb and Cu additives on microstructure and wear corrosion resistance behaviour of PM Al–Si alloys

Abstract

The wear and wear corrosion resistance of Al–20Si–XPb–YCu (X=0–10 wt-%, Y=0–3 wt-%) alloys fabricated using powder metallurgy technique and subsequent heat treatments were evaluated using a block on ring tribotest. The microstructures of all aluminium alloys were observed using an optical microscope, a scanning electron microscope and an X-ray energy dispersive spectroscope. The evaluation studied the effects of applied potential and environments of dry air and 3·5 wt-%NaCl aqueous solution. The microstructural analysis showed that Pb was bimodally distributed in Pb containing alloys, and Cu particles formed the intermetallic phase CuAl₂. Additionally, the hardness of both Pb and Cu containing alloys increased significantly. The wear and corrosion results showed that the addition of both lead (Pb) and copper (Cu) increased the wear resistance and the corrosion rate, while heat treatments reduced the corrosion rate of most alloys except the Al–Si alloy. Furthermore, comparison of all alloys following heat treatment shows that the wear corrosion resistance of Al–Si alloy is inferior to that of the other alloys. Therefore, addition of Pb and Cu further improved the wear corrosion resistance. Additionally, at anodic potential, the wear corrosion rate and current density of both Al–Si and Al–Si–Cu alloys containing particle Pb were significantly lower than those of alloys containing no Pb, because the layer produced by corrosion comprised Al, O and Pb elements.