The effects of mischmetal, cooling rate and heat treatment on the hardness of A319.1, A356.2 and A413.1 Al–Si casting alloys

The present study investigated the effect of mischmetal as a modifier, as well as the effects of cooling rate and heat treatment on the hardness of non-modified and Sr-modified A319.1, A356.2 and A413.1 Al–Si casting alloys. The main aim of the study was to determine the effect of mischmetal in terms of mischmetal-containing intermetallic phases, as well as the effects of the chemical composition of the alloys, cooling rate and heat treatment on the corresponding hardness values obtained for the alloys in question. Two cooling rates were employed to provide estimated hardness levels of 85 and 110–115 BHN, levels conforming to levels most commonly observed in commercial applications of these alloys.

The hardness measurements revealed that the hardness values of the as-cast alloys were higher at high cooling rates than at low cooling rates. Non-modified alloys (i.e. those with no Sr addition) displayed slightly higher hardness levels compared to the Sr-modified alloys. Also, the hardness decreased with the addition of mischmetal at both cooling rates.

Two peak hardness values were observed at 200 °C/5 h and 240 °C/5 h at high cooling rates in the non-modified A319.1 alloy after aging at different temperatures between 155 °C/5 h and 240 °C/5 h, while the Sr-modified alloy showed only one peak at 200 °C/5 h. Two maximum hardness values were observed at 155 °C/5 h and 180 °C/5 h in both non-modified and Sr-modified alloys at low cooling rates. The alloys containing 0 and 2 wt% mischmetal additions exhibited the highest hardness values at both cooling rates; the hardness decreased with further mischmetal additions.

Peak hardness was observed at 180 °C/5 h in the non-modified and Sr-modified A356.2 alloys under both cooling rate conditions after aging at different temperatures between 155 °C/5 h and 240 °C/5 h. The alloys free of mischmetal exhibited relatively higher levels of hardness than those containing mischmetal. The hardness decreased with increasing mischmetal addition. At the high cooling rates, the non-modified alloys displayed higher hardness values than the Sr-modified alloys, while an opposite trend was observed at the low cooling rate.

The decrease in the hardness values may be attributed to the interaction of the mischmetal with the alloying elements Cu and Mg to form the various intermetallic phases observed. In tying up these elements, the volume fraction of the precipitation-hardening phases formed in the A319.1 and A356.2 alloys (i.e. the Al₂Cu and Mg₂Si phases) is significantly reduced, thereby decreasing the hardness. The addition of mischmetal was also reported to change the precipitation sequence of the Mg₂Si phase in the A356.2 alloy. In the case of the A413.1 alloy, the low content of alloying elements resulted in a weak response of the alloy to the age-hardening process at all aging temperature/time conditions (155 °C/5 h–240 °C/5 h), and at both cooling rates. Thus, no peak hardness was observable in these alloys.     

In-situ Observation of Tensile Fracture in A357 Casting Alloys

The mechanism of damage evolution and fracture in A357 casting alloys was investigated by in-situ scanning electron microscopy （SEM） tensile testing. Different microstructures of A357 casting alloys were produced by eutectic Si modification and T6 heat treatment. It is shown that microcracks in these alloys are predominantly formed in eutectic Si particles. Large and elongated eutectic Si particles in unmodified alloy show the greater tendency to cracking, whereas cracking of small and round eutectic Si particles in Sr modified and T6 heat treated alloys is relatively lag. The crack mainly propagates along the broken eutectic Si particles in unmodified and Sr modified alloys or along the deepened shear bands in T6 heat treated alloy with accumulating the applied strain. The results were discussed in terms of Weibull statistics and the fracture models were established.     

A study on the mechanical properties of cryorolled Al–Mg–Si alloy

The mechanical properties of a precipitation hardenable Al–Mg–Si alloy subjected to cryorolling and ageing treatments are reported in this present work. The severe strain induced during cryorolling of Al–Mg–Si alloys in the solid solutionised state produces ultrafine microstructures with improved mechanical properties such as strength and hardness. The improved strength and hardness of cryorolled alloys are due to the grain size effect and higher dislocation density. The ageing treatment of cryorolled Al–Mg–Si alloys has improved its strength and ductility significantly due to the precipitation hardening and grain coarsening mechanisms, respectively. The reduction in dimple size of cryorolled Al–Mg–Si alloy upon failure confirms the grain refinement and strain hardening mechanism operating in the severely deformed samples.     

Microstructure and mechanical property developments in Al–12Si gravity die castings after Ti and/or Sr additions

This study evaluates the influence that modification and refinement have on the mechanical properties of the eutectic Al–Si alloy and on the quantitative and qualitative correlations with the microstructure. A general improvement in the mechanical properties of the alloys was observed after the additions. The best mechanical values were obtained when both Ti and Sr were present, in particular for the experimental composition with the higher Ti/Sr ratio. The results also indicate the development of a ductility trough for the alloy with lower Ti/Sr ratio, indicating a poisoning effect of Ti over Sr.     

Wear behaviour of an Al–12% Si alloy reinforced with a low volume fraction of SiC particles

Aluminium–silicon alloys reinforced with low volume fractions of SiC particles were prepared by the compocasting process. The wear behaviour of the unreinforced Al–12Si alloy and metal-matrix composites (MMCs) was investigated by using a block-on-ring test at room temperature under dry conditions. The results showed that the addition of a low volume fraction of SiC particles (2–8 vol%) is a very effective way of increasing the wear resistance of the matrix alloy. Metallographic examinations revealed that the wear zone of the Al–12Si alloy consists of both hardened and deformation layers. The depth of the hardened layer depended on the applied load and was in the vicinity of 10–50 μm. The formation of the hardened layer was related to the alignment and redistribution of fragmented eutectic phase to the surface region during sliding wear. Furthermore, the delamination of debris from the hardened layer was responsible for a higher wear loss observed in the Al–12Si alloy. The thickness of the hardened layer formed on the MMC specimens was reduced considerably by the incorporation of fragmented SiC particles. This layer exhibited higher hardness and wear resistance than that developed in the unreinforced alloy.     

Effect of Si content on microstructure and mechanical properties of Al–Si–Mg alloys

Microstructure and mechanical properties of as-cast and as-extruded Al–Si–Mg alloys with different Si content are investigated by tensile test, microstructure observation. High density of Si particles in the Al alloys can induce dynamic recrystallization during hot extrusion and it becomes more matured with an increase in the density of Si particles. The tensile strength of as-cast and as-extruded alloys can be improved with the increase of Si content and hot extrusion make the elongation of alloys increase dramatically. Considerable grain refining effect caused by recrystallization occurred during hot extrusion of S2 (equivalently commercial A356 alloy) and S3 (near eutectic alloy) alloys plays an important role in the improvement of elongation. A good combination of strength and elongation for the as-extruded S3 alloy indicates that near eutectic Al–Si alloys can be hot-extruded to produce aluminum profiles with high performance.     

Effect of modification and ceramic particles on solidification behavior of aluminum-matrix composites

Thermal analysis is used to establish the relationship between solidification history and the microstructure of SiC particulate reinforced Al-Si alloy-matrix composites. The results show that cooling curves are influenced by the presence of SiC particles and by strontium modification. The eutectic growth temperature of SiC_P/359 composites modified with Sr lies in the range of 840 to 843 K, i.e., about 5 to 7 K higher than that of Sr-modified unreinforced 359. For the same composite, the eutectic undercooling is higher with Sr modification than without. The eutectic solidification time of the composites is shorter than that of the unreinforced base alloy because of the presence of the ceramic particles. Strontium modification has the tendency to extend the eutectic solidification time. Microstructure analysis reveals that Sr modification has a refining effect on eutectic silicon for the composites, and SiC particles in the composite melt serve as the substrates for eutectic Si phase nucleation.     

合金元素对 Nb2Mo2Si合金相平衡及Nb2 Nb 5Si 3共晶组织形态的影响

采用电弧熔炼法制备了 Nb220Si210Mo、Nb220Si210Mo23M (M = Cr , Al , Ti) (原子分数) 四种 Nb2Mo2Si基超高温合金。利用 SEM、EDS、XRD等实验技术对铸造合金的相组成与组织形态进行了观察和分析。Nb220Si210Mo 合金由铌固溶体 (Nb SS) 与βNb 5Si 3化合物两相构成 , 其铸造组织包含大量片层状共晶 (Nb SS 2βNb 5Si 3) 组织。少量合金元素 Cr (3 at %) 能够改变 Nb220Si210Mo 合金的相平衡关系 , Nb220Si210Mo23Cr 的铸造组织中不仅存在 Nb SS和βNb 5Si 3 , 而且还出现少量 Cr 2Nb相 ; 而添加合金元素 Al、Ti (3 at %) 并不改变 Nb220Si210Mo 合金的相平衡关系。添加 Cr 使 Nb SS 2 βNb 5Si 3共晶组织失去了平直片层特征 ; Al 有利于共晶组织中片层状共晶形成 ; 添加 Ti使共晶组织呈现羽毛状特征。合金化使 Nb与βNb 5Si 3的晶格常数发生变化 : Nb的晶格常数均变小; Nb220Si210Mo23Cr合金中βNb 5Si 3的 c/ a值减小 , 其它 3种合金中βNb 5Si 3的 c/ a值增大。     

The Role of Bismuth as Trace Element on the Solidification Path and Microstructure of Na-Modified AlSi7Mg Alloys

The effects of Bi content as a trace element on the microstructure and the solidification path of A356.2 alloy have been investigated. The alloys containing different Bi levels (0, 20, and 200 ppm) have been modified by sodium. The experimental alloys have been thermally analyzed by using the two-thermocouple method. Metallographic and image analysis techniques have been used to quantitatively examine the microstructural changes occurring at different Bi and Na concentrations. The results indicate how the presence of Bi as a trace element affects the eutectic structure. Upon increasing the Bi level, the nucleation and growth temperatures of eutectic Si raise, and the eutectic Si particles appear coarser in Na-modified alloys. The EBSD analyses show that the crystallographic orientation between eutectic Al and surrounding primary Al dendrites becomes identical in Na-modified Bi-containing alloy. Furthermore, an irregular Bi + (Mg,Na)₃Bi₂ eutectic is formed prior to the precipitation of the eutectic Si, thus reducing the efficiency of Na addition to fully modify the eutectic Si.     

Effect of grain refining and Sr-modification interactions on the impact toughness of Al–Si–Mg cast alloys

The present work investigates the effects of various types of grain refiners on the impact properties of Sr-modified A356.2 alloys in both the as-cast and heated-treated conditions. The results showed that the addition of Ti and B greatly improves the alloy toughness, but only when the alloy was in a fully modified state; moreover, the right type of master alloy and addition levels must be used. The highest values of the total absorbed energy recorded for T6-tempered alloys were obtained using Al–5%Ti–1%B and Al–10%Ti master alloys in addition to 0.04%Ti. A significant deterioration in the impact properties is observed due to the Sr–B interaction (in some cases). The improvements in toughness may be attributed to the change in Si particle morphology as well as to the dissolution and fragmentation of a number of the intermetallics formed during the T6 temper.     

Microstructural,mechanical and tribological properties of Al–7Si–(0–5)Zn alloys

A series of Al–7Si–(0–5)Zn alloys were produced by permanent mould casting and their microstructure, mechanical and tribological properties were investigated in as-cast state. The microstructure of Al–7Si alloy consisted of α-Al dendrites surrounded by eutectic Al–Si mixture and a small amount of primary silicon particles. Addition of zinc into Al–7Si alloy resulted in the formation of α-solid solution and an increase in size and volume fraction of primary silicon particles. Moreover, these particles gathered inside interdendritic regions of the ternary Al–7Si–Zn alloys. The density, strength and hardness of Al–7Si–Zn alloys increased continuously with increasing zinc content, but their elongation to fracture and impact energy showed a reverse trend. It was also observed that zinc had no significant effect on the friction coefficient of the alloys, but their wear volume decreased with increasing zinc content up to 4%, above which the trend reversed. The wear surfaces of the alloys were characterized mainly by smearing layer with some degree of oxidation. In addition, delamination and fine scratches were observed on the worn surface. It was concluded that the addition of zinc up to 4% improves both mechanical and wear behaviour of Al–7Si alloy.     

Microstructure Control and Performance Evolution of Hypereutectic Al–20Mg2Si Alloy by Novel Al–Ca–Sb Masteralloy

Herein, the microstructure control and performance evolution of hypereutectic Al–20Mg₂Si alloy with the addition of novel Al–3.3Ca–10Sb master alloy are investigated. It is found that AlCa₁₁Sb₉ and CaSb₂ compounds are successfully synthesized through in situ melt reaction of masteralloy. With 0.45 wt% Al–3.3Ca–10Sb master alloy addition, primary Mg₂Si particles in hypereutectic Al–20Mg₂Si alloy are significantly refined from more than 150 μm to 10.7 μm, which are accompanied with the 3D morphologies changing from dendrites to octahedrons. After heat treatment, Brinell hardnesses of Al–20Mg₂Si alloys are remarkably improved to 112 HB. Furthermore, it is also found that the cooling rate of Al–3.3Ca–10Sb master alloys has certain influence on the refinement effect of Al–Mg₂Si alloys. The excellent complex modification of this master alloy on Al–20Mg₂Si alloy can be attributed to the existence of CaSb₂ particles as the heterogeneous nucleation sites of Mg₂Si particles and the inhibiting growth effect of residual Ca atoms adsorbed on the surface of Mg₂Si phase.     

Refinement of Mg2Si reinforcement in a commercial Al–20%Mg2Si in-situ composite with bismuth,antimony and strontium

Refinement by addition elements of Al–Mg₂Si alloys is known to result in a change of primary Mg₂Si morphology. In this paper, the effects of Bi, Sb and Sr on the characteristic parameters of Al–20%Mg₂Si in-situ composite have been investigated by computer aided cooling curve thermal analysis and microstructural inspection. Size, density and aspect ratio measurements showed that additions of 0.4 wt.% Bi, 0.8 wt.% Sb and 0.01 wt.% Sr refined the Mg₂Si reinforcement. Exceeding these concentrations, however, resulted in coarsening of Mg₂Si particles with no change in the morphology. The results also showed that addition elements caused a decrease in the nucleation and growth temperatures of Mg₂Si particles. The refining effect of Bi, Sb and Sr is likely to be related to the effect of oxide bifilms suspended in the composite melt as favored nucleation substrates for Mg₂Si particles.     

Impact toughness in Al–12Si and Al–12Si–3Cu cast alloys—Part 1: Effect of process variables and microstructure

The charpy impact energy of Al–12Si and Al–12Si–3Cu cast alloys was measured in terms of the total absorbed energy. The standard charpy specimens 10×10×55 mm with a 2 mm V-notch were prepared from the castings. Effect of process variables and microstructural changes on the impact toughness of Al–12Si and Al–12Si–3Cu cast alloys was investigated. The results indicate that combined grain refined and modified Al–12Si–3Cu cast alloys have microstructures consisting of uniformly distributed α-Al dendrites, eutectic Al–Si and fine CuAl₂ particles in the interdendritic region. These alloys exhibited better impact toughness in the cast condition compared with the same alloy subjected to only grain refinement or modification.     

A new modification effect of eutectic Si in selective laser melted AlSi10Mg

A distinct modification of eutectic Si was observed in the selective laser melted AlSi10Mg. The eutectic Si particles were modified substantially and demonstrated a nanoscale size, a fibrous morphology with obvious ‘necking effect’. The modification mechanism and the influence of heat treatment on eutectic Si growth were explored. It was suspected that Si experienced the necking effect under a tensile environment due to the large temperature gradient and α-Al erosion during the SLM process. Upon heat treatment, the Si particles were spheroidised and distributed homogeneously. Under the synergistic action of Si crystal fragmentation and the continuous precipitation of saturated Si from the Al matrix, the coarsened and spheroidised eutectic Si were distributed homogeneously in the Al matrix.     

Recent developments in hypoeutectic Al-Si A-S4G alloy

The effect of adding Sr in the form of the Al-5 Sr master alloy to commercial A-S4G and high purity Al-4Si alloys on the modification process has been investigated. The volume fraction of the eutectic matrix decreased by modification due to the movement of the eutectic point to the higher Si content side. The tensile properties, especially elongation, have been increased by modification. The elongation of the A-S4G alloy is increased from a value of 0.9 to 15% by modification. Additionally, the elongation of the modified high purity alloy reached a value of 34%. The fracture path of the modified alloys circumvents the -phase while it is not yet known if it propagates intergranularly or transgranularly through the eutectic matrix. The fracture surface revealed dimple and smooth ripple patterns reflecting the high ductility of the modified alloys.     

Combined grain refining and modification of conventional and rheo-cast A356 Al–Si alloy

This paper describes a comprehensive study on the combined addition of Ti–B grain refiner and Sr modifier elements to A356 Al–Si alloy. Using different qualitative and quantitative techniques in conventional and semi-solid metal castings, it is shown that, while the refiner and modifier elements affect respectively the nucleation and eutectic reactions, the combined addition not only replicates both individual element effects but also gives the added bonus of better globularity in the semi-solid metal process. A new innovative concept is introduced for fluidity measurement by using the magnitude of remaining liquid in the form of drainage, which is increased by combined treatment.     

The influence of modified intermetallics and Si particles on fatigue crack paths in a cast A356 Al alloy

Mechanical fatigue tests were conducted on uniaxial specimens machined from a cast A356-T6 aluminium alloy plate at total strain amplitudes ranging from 0.1 to 0.8% ( R = − 1). The cast alloy contains strontium-modified silicon particles (vol. fract. ~6%) within an Al–Si eutectic, dispersed α intermetallic particles, Al₁₅ (Fe,Mn)₃ Si₂ (vol. fract. ~1%), and an extremely low overall volume fraction of porosity (0.01%). During the initial stages of the fatigue process, we observed that a small semicircular fatigue crack propagated almost exclusively through the Al–1% Si dendrite cells. The small crack avoided the modified silicon particles in the Al–Si eutectic and only propagated along the α intermetallics if they were directly in line with the crack plane. These growth characteristics were observed up to a maximum stress intensity factor of ~ K trmax = 7.0 MPa m^1/2 (maximum plastic zone size of 96 μm). When the fatigue crack propagated with a maximum crack tip driving force above 7.0 MPa m^1/2 the larger fatigue crack tip process zone fractured an increased number of silicon particles and α intermetallics ahead of the crack tip, and the crack subsequently propagated preferentially through the damaged regions. As the crack tip driving force further increased, the area fraction of damaged α intermetallics and silicon particles on the fatigue fracture surfaces also increased. The final stage of failure (fast fracture) was observed to occur almost exclusively through the Al–Si eutectic regions and the α intermetallics.     

Structural and mechanical properties of Al–Si alloys obtained by fast cooling of a levitated melt

Data on the structure and mechanical properties of cast Al–Si alloys in a wide compositional range from hypo-to high hyper-eutectic composition are scares. These properties depend on many factors during solidification of the alloys. In the present work, samples were obtained by rapid cooling of levitated melts of various compositions from 11.5 to 35 wt.% Si. The measurements revealed linear concentration dependences of density and Young's modulus. The average temperature coefficient of Young's modulus in the range from room temperature to 500 °C and the yield point for bending both had maxima at about 20 wt.% Si. The hysteresis of the temperature dependence of Young's modulus had a minimum at about 20 wt.% Si as well. Changing Young's modulus temperature coefficient and Young's modulus hysteresis as a function of the Si content are connected with the creation of the Guinier–Preston zones. Values of the yield point are explained by the plasticity of components of the eutectic structure, primary crystals and grain boundaries. The extrema of the concentration dependences of the mechanical properties occurred for the fine-grained structure arisen from coupled eutectic-like growth. Solidification at other conditions led to formation of primary crystals of solid solution or primary Si crystals.     

Impact toughness and fractography of Al–Si–Cu–Mg base alloys

Critical automotive applications using heat-treatable alloys are designed for high impact toughness which can be improved using a specified heat treatment. The alloy toughness and fracture behavior are influenced by the alloy composition and the solidification conditions applied. The mechanical properties of alloys containing Cu and Mg can also be enhanced through heat treatment. The present study was undertaken to investigate the effects of Mg content, aging and cooling rate on the impact toughness and fractography of both non-modified and Sr-modified Al–Si–Cu–Mg base alloys. Castings were prepared from both experimental and industrial 319 alloy melts containing 0–0.6wt% Mg. Test bars were cast in two different cooling rate molds, a star-like permanent mold and an L-shaped permanent mold, with dendrite arm spacing (DAS) values of 24 and 50 μm, respectively. Test bars were aged at 180 °C and 220 °C for 2–48 h. Charpy Impact test was used to provide the impact energy. It was observed that high cooling rates improve the impact toughness whereas the presence of Cu significantly lowers the impact properties which are determined mainly by the Al₂Cu phase and not by the eutectic Si particles. The addition of Mg and Sr were also seen to decrease the impact toughness. The crack initiation energy in these alloys is greater than the crack propagation energy, reflecting the high ductility of Al–Si–Cu–Mg base alloys.