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.     

Effect of processing variables on structure and tensile properties of investment cast Al–Si–Mg casting alloy

Abstract

A research programme was conducted to study the effects of grain refinement, eutectic silicon modification, filtering, pouring and shell preheat temperatures, and heat treatment on the structure and tensile properties of an investment cast Al–Si–Mg alloy, LM25 (BS 1490 : 1988). The principal findings of the research were that: an increase in shell preheat temperature adversely affects the structure and, hence, the tensile properties; grain refinement was enhanced as the titanium content was increased to about 0·28% but the tensile properties were not affected; a modified eutectic silicon structure was achieved with strontium additions in the range 0·01–0·02%, with the optimum addition, based on tensile properties, being 0·01%; and, as would be expected, heat treatment improved the tensile properties. On the basis of the interrelationships between process variables, structural changes, and tensile properties observed, an optimum processing route was identified. The optimum tensile properties were obtained in fully heat treated specimens that had been both grain refined and modified and produced in moulds poured at ambient temperature.     

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 local volume fraction of microporosity on tensile properties in Al–Si–Mg cast alloy

Abstract

Porosity in Al–Si–Mg cast alloys utilised in automotive parts affects directly products quality, i.e. mechanical properties. In this study, the effect of micropores on mechanical properties has been investigated by X-ray tomography from the viewpoint of clustering micropores. The local volume fraction (LVF) of porosity was introduced to analyse the effect of clustering micropores. The statistical Weibull method was also used in order to explain strength of the alloy tested. The fracture strain decreased drastically from 17 to 3% on an inverse parabolic relationship with increasing porosity. In the case of the specimens that contain the largest pore higher than 100 m m, the ultimate tensile strength decreases monotonically. It is found that the fracture surface passes through high LVF regions. The fracture strain obviously depends on the ratio of LVF higher than 10%. It is confirmed that the LVF, which represents unevenly distribution of micropores cluster, is one of important dominant factor for managing the mechanical properties in the Al–Si–Mg cast alloy.     

Effect of aging and dispersoid content on tensile properties of Al–0·6Mg–1 Si alloys

Abstract

Slip distribution was varied in a series of Al–Mg–Si alloys by changing the amount of manganese-bearing dispersoid present and by under- and overaging, which also altered the grain–boundary precipitate-free zones and hence the grain–boundary strength. Dispersoids are shown to increase ductility by slip homogenization. Slip is more heterogeneous in underaged alloys, but these show greater ductility than overaged alloys because the grain boundaries are stronger. Work-hardening rates increase with dispersoid content, although for a given dispersoid content, the underaged alloys have higher work-hardening rates. This effect is interpreted in terms of the effect of aging upon the properties of the grain-boundary regions.

MST/340     

Effect of thermal treatments on microstructure and impact toughness of die cast Mg–Al–Mn alloys

Abstract

A study of the effects of heat treatment on an Mg–Al–Mn alloy was carried out. Die cast AM60B alloy (Mg–6.0Al–>0.25Mn–<0.010Cu–<0.002Ni–<0.005Fe (wt-%)) specimens for microstructural investigation, tensile testing, and impact toughness testing, were produced using a multispecimen die in a high pressure, cold chamber apparatus. As cast specimens were studied either in their original condition or after they had been subjected to a direct aging treatment at 175°C. Solution treatment was also carried out, producing a T4 temper and a T6 temper by subsequent additional aging, and the resulting specimens examined. The investigation allowed evaluation of modifications to microstructural and mechanical properties produced by thermal treatments. In particular, the analysis of structure and solidification defect evolution showed that, despite an increase in void volume fraction and size induced by thermal treatments, a significant improvement of toughness during the crack growth process could be achieved with the appropriate tempers. Marked modifications to high strain rate loading conditions were detected, with improvements of total absorbed impact energy of up to 40% with respect to the as cast condition. This was associated with changes in fracture mechanisms, promoting a transition to a completely ductile mode in solution treated specimens.     

Effect of cooling rate and Mg content on the Al–Si eutectic for Al–Si–Cu–Mg alloys

This study investigates and clarifies the qualitative and quantitative effects of Mg content and cooling rate (ranging from 0.5 to 4 °C/s), on the modification of the silicon eutectic structure and on the undercooling of the silicon eutectic growth temperature (ΔT_Si-eut) in the series of Al–Si–Cu–Mg alloys. The critical Mg content to produce a notable improvement in the silicon eutectic by 1.5 modification levels (regardless of the cooling rate) is 0.6 wt.% Mg. A similar increase in the modification level was also observed when the cooling rate was increased to a maximum of 4 °C/s, regardless of the Mg content. Measurements of the area and roundness of the silicon particles showed a good correlation with the modification level. The undercooling (ΔT_Si-eut) increased by up to ~ 23 °C at a relatively high Mg content and cooling rate and up to ~ 14 °C when the Mg content was increased from 0.4 to 0.6 wt.%.     

The effects of Mg amount on the microstructure and mechanical properties of Al–Si–Mg alloys

In this study, the effects of magnesium (Mg) addition to A356 aluminum alloy at different amounts on the microstructure and mechanical properties of this alloy were examined. For the experimental studies, three different alloys (0.43, 0.67 and 0.86 wt%) having various amounts of Mg were prepared through casting process in the form of plates. The plates were homogenized and cooled in the furnace. All the samples were treated with aging process (T6) and then tensile samples were prepared from the homogenized samples. The samples treated with T6 process were characterized by optical microscopy, laser confocal microscopy, Scanning Electron Microscope (SEM), Energy Dispersive Spectrometer (EDS) and X-Ray Diffraction (XRD) examinations as well as hardness measurements and tensile tests. The phases which were formed in the microstructures for different amounts of Mg were examined. It was observed that iron-rich intermetallic compounds were also formed in addition to the phases resulting from the aging process. Fe-rich intermetallic compounds, observed from the fracture surfaces, were found to reduce the tensile strength the alloy. The results also indicate that the tensile strength and hardness of the alloy increase with increasing Mg amount.     

Effect of Al addition on the grain refinement and mechanical properties of as-cast Mg–5Y–4Sm alloys

Journal of Materials Science - In recent years, the utilization of Al as a grain refiner in Mg–RE alloys has gained widespread attention because of its advantages such as low cost. In this...     

Effect of Fe content on the fracture behaviour of Al–Si–Cu cast alloys

Castings were prepared from 319.2 alloy melts, containing Fe levels of 0.2–1.0 wt%. Sr-modified (∼200 ppm) melts were also prepared for each alloy Fe level. The end-chilled refractory mold used provided directional solidification and a range of cooling rates (or dendrite arm spacings, DAS) within the same casting. Impact test samples were machined from specimen blanks sectioned from the castings at various heights above the chill end provided DASs of 23–85μm. All samples were T6-heat-treated before testing keeping with Aluminum Association recommendations. The results show that at low Fe levels and high cooling rates (0.4% Fe, 23 μm DAS), crack initiation and propagation in unmodified 319 alloys occurs through the cleavage of β-Al₅FeSi platelets (rather than by their decohesion from the matrix). The morphology and the size of the platelets (individual or branched) are important in determining the direction of crack propagation. Increasing the DAS to 83μm leads to cleavage fracture. In this case, the fracture path follows a transgranular plane that is usually a well-defined crystallographic plane as judged by the relatively large smooth surfaces of the β-Al₅FeSi phase platelets. Cracks also propagate through the fracture of undissolved CuAl₂ or other Cu-intermetallics, as well as through fragmented Si particles. In Sr-modified 319 alloys, cracks are mostly initiated by the fragmentation or cleavage of perforated β-phase platelets, in addition to that of coarse Si particles and undissolved Cu-intermetallics.The amount of undissolved Cu- intermetallics is directly related to the applied cooling rate. Slow cooling rate (DAS ≈83µm) results in the precipitation of Cu- containing phases on the β-platelets, amplifying the likehood for crack propagation through these loacations.     

Effect of casting techniques on tensile properties of cast aluminium alloy (Al–Si–Mg) and TiB2 containing metal matrix composite

Abstract

An analysis of the statistical distribution of the tensile strength of a TiB₂ containing aluminium matrix composite and its matrix alloy (Al-7Si-0.35Mg) was carried out using different casting techniques. The scatter of the tensile strength data was assessed by Weibull statistics. Results for the metal matrix composite (MMC) and the matrix alloy in as cast and heat treated conditions were compared. It was found that a low turbulence casting technique resulted in less scatter of tensile values, confirming the greater reliability of the cast material. Fractographic examination of the fractured faces of lower strength specimens showed that entrained oxide films play an important role in failure of the specimens. Heat treatment causes increased scatter in strength values, reflected in the lower Weibull modulus.     

Effect of grain size and indium addition on tensile characteristics of Al–1Si alloy

Abstract

The effect of grain size and indium addition on the workhardening characteristics of Al–1Si (wt-%) alloy has been investigated at room temperature (RT). The samples were preaged at different temperatures in the range 523–623 K. The yield stress, the fracture stress, the fracture time and the linear workhardening coefficient generally decreased with increasing temperature and/or grain size, while the fracture strain and dislocation slip distance increased. The yield and fracture stresses for different grain sizes at different temperatures were found to be linearly related to grain diameters. Indium addition caused general increase for all the measured strength parameters. As concluded from transmission electron microscope (TEM) investigations, In addition to Al–Si alloy may retard the coarsening of Si particles. The energies activating the operating fracture mechanisms were found to be 79·6±0·4 and 32·4±0·4 kJ mol^?1 for alloys Al–1Si and Al–1Si–0·2In respectively. This suggests a value of 47·2 kJ mol^?1 as a binding energy between Si and In atoms in Al matrix.     

Microstructure and tensile properties of squeeze cast Al–Zn–Mg–Cu alloy

Abstract

It is well known that wrought aluminium alloys have tensile properties superior to those of the cast products. Wrought grade alloys cannot usually be produced by conventional casting processes to attain the same level of tensile properties. However, progress in casting methods in recent years has made it possible to produce wrought alloys by means of squeeze casting techniques. In the present study an Al–Zn–Mg–Cu alloy has been produced by squeeze casting. Tensile properties close to those of wrought products have been achieved by controlling the microstructure, pressure, and other processing parameters.     

Effect of microstructure on fatigue behaviour of cast Al–7Si–Mg alloy

Abstract

The fatigue behaviour of a cast Al–7Si–Mg alloy, conforming to A356, has been studied. Specimens of this material were tested in both the as cast condition and a solution treated and aged condition. It was observed that the size, number, and position of casting defects influenced the fatigue life very strongly. This marked effect nearly hides that of the heat treatment. Nevertheless, if the analysis is carried out considering only results obtained from sound specimens it is revealed that the heat treatment causes an improvement in the fatigue resistance of the alloy.     

Effects of yttrium and heat treatment on the microstructure and tensile properties of Al–7.5Si–0.5Mg alloy

The effects of yttrium (Y) additions (0, 0.1, and 0.3 wt.%) and T6 heat treatment on the microstructure and tensile properties of Al–7.5Si–0.5Mg alloy have been investigated in the present work. The microstructures and fracture surfaces of as-cast and heat treated samples were examined by scanning electron microscopy (SEM). It was found that Y modified the eutectic silicon from a coarse plate-like and acicular structure to a fine branched and some fibrous one with a better uniform distribution. In addition, T6 heat treatment played a crucial role in the fragmentation and spheroidization of eutectic silicon, especially in the well modified alloys. The tensile properties were improved by the addition of Y followed by the T6 heat treatment, and a good combination of ultimate tensile strength (353 MPa), yield strength (287 MPa) and elongation (12.1%) was obtained when the Y addition was 0.3 wt.%. Furthermore, fractographic examinations revealed that dimple-like mechanism was responsible for ductile fracture.     

Effects of Si and Cu contents on grain size of Al–Si–Cu alloys

The influence of the silicon and copper contents on the grain size of high-purity Al–Si, Al–Cu, and Al–Si–Cu alloys was investigated. In the Al–Si alloys, a poisoning effect was observed and a poor correlation between the grain size and growth restriction factor was obtained. A possible cause of the poisoning effect in these alloys is the formation of a TiSi₂ monolayer on the particles acting as nucleation sites or another poisoning mechanism not associated with TiSi₂ phase formation. In the Al–Cu alloys, a good correlation between the grain size and growth restriction factor was found, whereas in the Al–Si–Cu alloys, the correlation between these two parameters was inferior.     

Optimizing the tensile properties of Al–Si–Cu–Mg 319-type alloys: Role of solution heat treatment

The work presented in this study was carried out on Al–Si–Cu–Mg 319-type alloys to investigate the role of solution heat treatment on the dissolution of copper-containing phases (CuAl₂ and Al₅Mg₈Cu₂Si₆) in 319-type alloys containing different Mg levels, to determine the optimum solution heat treatment with respect to the occurrence of incipient melting, in relation to the alloy properties. Two series of alloys were investigated: a series of experimental Al–7 wt% Si–3.5 wt% Cu alloys containing 0, 0.3, and 0.6 wt% Mg levels. The second series was based on industrial B319 alloy. The present results show that optimum combination of Mg and Sr in this study is 0.3 wt% Mg with 150 ppm Sr, viz. for the Y4S alloy. The corresponding tensile properties in the as-cast condition are 260 MPa (YS), 326 MPa (UTS), and 1.50% (%El), compared to 145 MPa (YS), 232 MPa (UTS), and 2.4% (%El) for the base alloy with no Mg. At 520 °C solution temperature, incipient melting of Al₅Mg₈Cu₂Si₆ phase and undissolved block-like Al₂Cu takes place. At the same time, the Si particles become rounder. Therefore, the tensile properties of Mg-containing alloys are controlled by the combined effects of dissolution of Al₂Cu, incipient melting of Al₅Mg₈Cu₂Si₆ phase and Al₂Cu phase, as well as the Si particle characteristics.     

Effect of casting and homogenizing treatment conditions on the formation of Al–Fe–Si intermetallic compounds in 6063 Al–Mg–Si alloys

The effect of casting and homogenizing treatment conditions on the formation of several Al–Fe–Si intermetallic compounds in 6063 aluminum alloy was investigated using X-ray diffraction and transmission electron microscopy (TEM). The four kinds of alloys containing 0.1 to 0.5 mass% Fe were melted and then cooled at three different cooling rates ranging from 0.06 to 50 K/s, following the homogenization at 858 K for 54 ks and 2400 ks. The Al–Fe–Si compound particles were extracted from the alloy ingots using the thermal phenol method. The as-cast 0.1 mass% Fe ingot obtained at the casting cooling rate of 0.06 K/s had a largest amount of the phase among the ingots investigated. When this ingot was homogenized at 858 K for 54 ks and 2400 ks, the amount of the phase decreased, while that of the phase increased. On the other hand, the as-cast 0.5 mass% Fe ingot obtained at the casting cooling rate of 50 K/s had the largest amount of the phase among the ingots investigated. When this ingot was homogenized at 858 K for 54 ks, a large amount of the phase remained. However, the homogenization at 858 K for 2400 ks resulted in the transformation of the phase to the phase. The main phase in the as-cast 0.2 mass% Fe ingot obtained at the casting cooling rate of 5 K/s, close to the industrial cooling rates, was the phase. The phase gradually decreased, and the relative amounts of the and phases increased during homogenization at 858 K for 54 ks. Furthermore, almost all of the Al–Fe–Si intermetallic compounds were transformed into the phase in the ingots homogenized at 858 K for 2400 ks.     

Effect of extrusion parameters on properties of rapidly solidified Al–Mg powder alloys

Abstract

Three rapidly solidified Al–Mg powder alloys have been consolidated by means of cold compaction followed by hot extrusion. The extrusion conditions of temperature, reduction ratio, and ram speed were varied, and it was observed that the mechanical properties of the extrudates were strongly process related. Relationships between properties and the temperature compensated strain rates during extrusion have been established. These alloys have strength/density properties superior to the strongest conventional ingot cast alloys. Good fracture toughness has been recorded in the Al–7 Mg alloy and all three alloys possess good resistance to stress corrosion cracking.

MST/498     

Effect of heat treatment and Fe content on the microstructure and mechanical properties of die-cast Al–Si–Cu alloys

The effect of solution and ageing heat treatment on the microstructure and mechanical properties of the die-cast Al–9 wt.%Si–3.5 wt.%Cu alloys containing 0.1–1.0 wt.% Fe was investigated. The results showed that the dendritic primary α-Al phase was varied from 20 to 100 μm in size and the globular α-Al grains were smaller than 10 μm in size. The Fe-rich intermetallics exhibited coarse compact or star-like shapes with the sizes from 10 to 20 μm and the fine compact particles at an average size of 0.75 μm. The solution treatment of the alloys could be achieved in a short period of time, typically 30 min at 510 °C, which dissolved the Cu-rich intermetallics into the primary α-Al phase and spheroidised the eutectic Si phase. During the subsequent ageing treatment, numerous fine precipitates of θ′ and Q′ phases were formed to provide effective strengthening to the α-Al phase, significantly improving the mechanical properties. Therefore, Fe content in the die-cast Al–Si–Cu alloys needs to be controlled at a low level in order to obtain the improved ductility and strength under solution and aged condition.