Cast A356 Research Articles

Effects of eutectic silicon particles on tensile properties and fracture toughness of A356 aluminum alloys fabricated by low-pressure-casting, casting-forging, and squeeze-casting processes

Effects of eutectic silicon particles on tensile properties and fracture toughness of three A356 aluminum alloys were investigated in this study. These A356 alloys were fabricated by casting processes such as low-pressure-casting, casting-forging, and squeeze-casting, and their tensile properties and fracture toughness were analyzed in relation with microfracture mechanism study. All the cast A356 alloys contained eutectic Si particles segregated along solidification cells, and the distribution of Si particles was modified by squeeze-casting and casting-forging processes. Microfracture observation results showed that eutectic Si particles segregated along solidification cells were cracked first, but that aluminum matrix played a role in blocking crack propagation. The cast-forged alloy had the best hardness, strength, ductility, and fracture toughness because of the matrix strengthening and homogeneous distribution of eutectic Si particles due to forging process. The squeeze-cast alloy was considerably refined and densified, but this refined microstructure resulted in the reduced spacing between eutectic Si particles which worked as fracture initiation sites, thereby leading to the lower fracture toughness than the PC alloy.

Effect of microstructural variables on tensile behaviour of A356 cast aluminium alloy

The effects of microstructural variables, including secondary dendrite arm spacing (SDAS), the size of primary α phase, the aspect ratio of eutectic Si particle and the thickness of eutectic wall structure, on tensile behaviour of A356 cast aluminium alloy, were quantitatively identified using linear regression analysis method. For systematic microstructural control of A356 specimen, directional solidification method was used with different solidification rates of 5, 25, 50 and 100 μm s−1 respectively. The linear regression analysis suggests that each microstructural variable affects tensile strength and tensile elongation of A356 cast aluminium alloy in a similar fashion. The change in tensile behaviour with varying microstructural variables in A356 cast aluminium alloy is discussed based on fractographic and micrographic observations.

Comparison of Age-Hardening Characteristic in Thixo-Cast and Rheo-Cast T5-A356 Alloys by Nanoindenter and AFM

The nano/microstructure and mechanical/tribological properties of the eutectic regions in thixo/rheo-cast A356 alloy parts were investigated using nano/micro-indentation and mechanical scratching, combined with optical microscopy and atomic force microscopy (AFM). Most eutectic silicon crystals in the A356 alloy showed a modified morphology as fine-fibers. The aging responses of the eutectic regions in both the thixo/rheo cast A356 alloys aged at 150 0C for different times (0, 2, 4, 8, 10, 16, 24, 36, and 72h) were investigated. Both Vickers hardness (HV) and indentation hardness (HIT) test results showed almost the same trend of aging curves, the peak was obtained at the same aging time of 10 h, and in tribological properties of thixo/rheo-cast, the friction coefficient of thixo-cast was investigated lower than that of rheo-cast.

Effect of equal channel angular pressing on the microstructure of a semisolid aluminum alloy

The microstructure evolution during semisolid equal channel angular pressing and semisolid isothermal heating of a cast A356 aluminum alloy is reported. It is demonstrated that partially melted dendritic cast structure undergoes substantial plastic deformation during semisolid equal channel angular pressing. Coarsening of the fragmented dendritic microstructure during subsequent semisolid isothermal heating gives rise to spheroidal solid phase surrounded by liquid metal.

Substructures and Fatigue Crack Growth in HIPing and Semi-Liquid Die Casting A356 Alloys

Permanent mold and semi-liquid die A356 cast alloys were used to examine the roles of microstructures and aging conditions on fatigue crack growth. HIP treatment to the A356 alloy generates substructure like dendritic arm boundaries as well as reduction of pores, which improves fracture elongation and fatigue fracture toughness. The similar substructure occur at primary α-Al and inter Si particles of semi-liquid die cast, too. Fracture elongation of HIPed permanent mold cast is comparable to that semi-liquid die cast, the fatigue crack growth is faster than in semi-liquid die cast. Plastic hardening occurs around fatigue crack flank, which decreases fatigue crack growth rate, and such effect appears highly in under aged alloy.

A357 thixoforming feedstock produced by cooling slope casting

Cooling slope (CS) casting process was employed in the present work to produce A357 thixoforming feedstock. The dendritic primary phase in the conventionally cast A357 alloy has readily transformed into a non-dendritic one in ingots cast over a cooling plate from pouring temperatures between 620 and 640 °C and with cooling lengths between 200 and 400 mm. Dissipation of the melt superheat required longer cooling lengths for higher pouring temperatures. Primary phase in the ingots thus produced has attained a globular morphology in only as much as 5 min during isothermal holding at 580 °C. The minor effect the pouring temperature and the cooling length had on the CS-cast morphologies was not evident after reheating, implying that a wide range of CS casting parameters are capable of providing the required microstructural features for thixoforming.

Effect of pre/post T6 heat treatment on the mechanical properties of laser welded SSM cast A356 aluminium alloy

Aluminium alloy A356 was treated by the Rheo semi solid metal (SSM) process, developed recently by CSIR-Pretoria, and cast in plates using a high pressure die casting (HPDC) machine. Plates in as cast condition (F) and heat treated T6 condition (pre HT) were butt welded, using an Nd:YAG laser. In another experiment, as cast welded samples were heat treated to T6 condition (post HT). The base metal and weld microstructures were presented. The effect of heat treatments on microstructure and mechanical properties of the welds was investigated. It was found that the fine dendrite structure of the weld metal contributed to the mechanical properties of the joint. The mechanical properties of the post HT samples were found to be higher than the pre HT and as cast materials.

Mechanical properties of cast A356 alloy, solidified at cooling rates enhanced by phase transition of a cooling medium

Phase transition cooling (PTC), using the absorbed latent heat during the melting of phase transition cooling medium to cool and solidify alloys in the process of casting, is a new fast cooling technology. Specimens of A356 casting aluminum alloy were prepared by this method. For comparison, specimens made by water-cooling copper mould (WCCM) were prepared too. The mechanical properties of A356 alloy made by PTC and WCCM were measured by microhardness and tensile strength testing methods. Microstructures of A356 alloy were investigated by scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and X-rays diffraction (XRD). The results show that both microhardness and ultimate tensile strength (UTS) of A356 alloy made by PTC method are much higher than those made by WCCM method. This can be attributed to the refined microstructure and the extended solubility of silicon in α-Al. The solubility of silicon in α-Al was 2.7 at.% in specimens solidified in phase transition cooling medium and 2.4 at.% in specimens solidified in water-cooling copper mould.

Semisolid Roll Casting of Aluminum Alloy Strip and Its Properties

A356 was cast into the strip by the semisolid strip casting using an unequal diameter twin roll caster. This process was one of the rheocasting. The semisolid slurry was made by a cooling slope. Solid fraction was about 5%. The casting speed was 20 m/min, and the thickness was 4.4 mm. The as-cast strip could be cold rolled down to 1 mm. The result of tension test of 1mm-thickness-T4 heat-treated strip was as below. Tensile stress was 223 MPa, 0.2 proof stress was 123 MPa and elongation was 21.8%. Deep drawing could be operated to 1 mm-thickness T4 sheet, and L.D.R was attained to 2.0. In some condition, 180 degrees bending could be operated without crack at outer surface. These results mean that semisolid roll cast A356 strip may be able to use for press forming.

Laser Welding of SSM Cast A356 Aluminium Alloy Processed with CSIR-Rheo Technology

Samples of aluminium alloy A356 were manufactured by Semi Solid Metals HPDC technology, developed recently in CSIR-Pretoria. They were butt welded in as cast conditions using an Nd:YAG laser. The base metal and weld microstructure were presented. The effect of different heat treatments on microstructure and mechanical properties of the welds were investigated. It was found that the fine dendrite structure of the weld metal contributed for equalizing the mechanical properties of the joint.

Study on Microstructures and Mechanical Properties of Rheo-Die Casting Semi-Solid A356 Aluminum Alloy

In this paper, with a newly self-developed rotating barrel rheomoulding machine(RBRM), microstructures and mechanical properties of rheo-die casting A356 alloy were studied. In order to clearly show the characteristic of rheo-die casting, liquid die casting and semi-solid casting were done too. The experimental results showed that microstructures of rheo-die casting were composed of solid grains, which were finer and rounder, and had fewer pores. In the three technologies, integrated mechanical properties of semi-solid rheo-die casting were the best.

Mechanical and wear properties of rheocast and conventional gravity die cast A356 alloy

A356 alloy produced by means of conventional gravity die casting and rheocasting has been investigated and their microstructure, mechanical and tribological properties were compared. The microstructure of conventional cast sample is fully dendritic in contrast to spheroidal morphology in rheocast sample. The mechanical properties of the rheocast samples are considerably higher than the conventional cast samples. The volumetric wear loss and coefficient of friction in rheocast samples are always less than those in conventional cast samples at all loads. The wear occurs mainly by ploughing mechanism.

Effect of friction stir processing on the microstructure of cast A356 aluminum

Friction stir processing (FSP) was applied to cast A356 Al to modify the as-cast microstructure. FSP homogenizes and refines the cast microstructure, completely eliminates porosity, and creates a microstructure with fine Si particles (0.25–0.42 μm) distributed in a fine grain aluminum matrix (3–4 μm). Further, FSP parameters significantly influence microstructural development in the processed zone of cast A356 Al. Generally, higher tool rotation rate creates a more homogeneous microstructure. At lower tool rotation rates, a macroscopically visible banded structure characterized by a low density of coarse particles, was detected in the nugget zone. However, FSP parameters did not significantly influence the Al matrix grain size. The varied distribution pattern, size, and volume fraction of Si particles at different locations within the FSP zone indicates inhomogeneous material flow. A post-FSP T6 heat treatment did not alter the Si particle distribution, but did significantly coarsen the Si particles.

A Validated Through Process Model to Predict the Fatigue Life of a Cast A356 Automotive Wheel

Models describing the critical steps in the manufacturing process of aluminum alloy wheels were integrated with a model of in-service loading to predict component fatigue performance. A multiscale solidification model was coupled with models of the subsequent heat treatment and machining to predict the residual stress distribution in a finished wheel. A second multiscale model was incorporated into a service model to relate the pore size and local stress state to fatigue performance. The predictions resulting from this through process modeling approach have been validated by comparisons with a series of measurements: 1) thermocouple measurements during casting; 2) characterization of pore size and distribution using X-ray microtomography and optical metallography; 3) residual strain measurements on finished wheels; 4) in-service strain measurements during a rotating bend test; and 5) in-service fatigue performance during rotating bend tests.

Effect of multiple-pass friction stir processing on microstructure and tensile properties of a cast aluminum–silicon alloy

Five-pass friction stir processing (FSP), with 50% overlap, was conducted on cast A356. Overlapping FSP did not exert a significant effect on the size and distribution of the Si particles. In the as-FSP condition, the strength and ductility of the transitional zones between two FSP passes were slightly lower than those of the nugget zones. Further, in the multiple-pass material the strength of the previously processed zones was lower than that of the subsequent processed zones due to overaging from the FSP thermal cycles. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The influence of Mn and Cr on the tensile properties of A356–0.20Fe alloy

In the present work, Mn and Cr were added to a commercial A356 (Al–7.0Si–0.38Mg–0.12Ti–0.02Sr in wt.%)–0.20Fe cast alloy to control the crystallization of β-Al5FeSi. The tensile properties are compared in terms of the way the microstructures changed before and after the addition of 0.20Mn and 0.13Mn–0.13Cr. The addition of 0.20Mn causes the acicular β-Al5FeSi to be replaced by granular or skeletal α-Al(Mn,Fe)Si in solidification, and the additions of 0.13Mn–0.13Cr acts as an effective replacing element of β-Al5FeSi. Moreover, the addition of 0.20Mn functionally improves the strength, and the addition of 0.13Mn–0.13Cr improves the strain as well as the strength compared to a commercial A356 cast alloy.

Effect of Porosity on Ductility Variation in Investment Cast 17-4PH

The stainless steel alloy 17-4PH contains a martensitic microstructure and second phase delta ({delta}) ferrite. Strengthening of 17-4PH is attributed to Cu-rich precipitates produced during age hardening treatments at 900-1150 F (H900-H1150). For wrought 17-4PH, the effects of heat treatment and microstructure on mechanical properties are well-documented [for example, Ref. 1]. Fewer studies are available on cast 17-4PH, although it has been a popular casting alloy for high strength applications where moderate corrosion resistance is needed. Microstructural features and defects particular to castings may have adverse effects on properties, especially when the alloy is heat treated to high strength. The objective of this work was to outline the effects of microstructural features specific to castings, such as shrinkage/solidification porosity, on the mechanical behavior of investment cast 17-4PH. Besides heat treatment effects, the results of metallography and SEM studies showed that the largest effect on mechanical properties is from shrinkage/solidification porosity. Figure 1a shows stress-strain curves obtained from samples machined from castings in the H925 condition. The strength levels were fairly similar but the ductility varied significantly. Figure 1b shows an example of porosity on a fracture surface from a room-temperature, quasi-static tensile test. The rounded features represent the surfaces ofmore » dendrites which did not fuse or only partially fused together during solidification. Some evidence of local areas of fracture is found on some dendrite surfaces. The shrinkage pores are due to inadequate backfilling of liquid metal and simultaneous solidification shrinkage during casting. A summary of percent elongation results is displayed in Figure 2a. It was found that higher amounts of porosity generally result in lower ductility. Note that the porosity content was measured on the fracture surfaces. The results are qualitatively similar to those found by Gokhale et al. and Surappa et al. in cast A356 Al and by Gokhale et al. for a cast Mg alloys. The quantitative fractography and metallography work by Gokhale et al. illustrated the strong preference for fracture in regions of porosity in cast material. That is, the fracture process is not correlated to the average microstructure in the material but is related to the extremes in microstructure (local regions of high void content). In the present study, image analysis on random cross-sections of several heats indicated an overall porosity content of 0.03%. In contrast, the area % porosity was as high as 16% when measured on fracture surfaces of tensile specimens using stereology techniques. The results confirm that the fracture properties of cast 17-4PH cannot be predicted based on the overall 'average' porosity content in the castings.« less

An investigation on semi-solid Al–7Si–0.3Mg alloy produced by mechanical stirring

In this research an aluminum alloy Al–7Si–0.3Mg (A356.1) was cast by a semi-solid casting technique. The parameters like stirring time, temperature and heat treatment conditions on mechanical properties were investigated and obtained optimum conditions. The cylindrical specimens with a height of 40 mm and a diameter of 160 mm were cast in a permanent mould. Mechanical properties plus fracture surfaces of specimens were identified as brittle or ductile fracture. Results showed that the optimum temperature and time for semi-solid cast A356.1 aluminum alloy, with 700-rpm shear rate were 590 °C and 20 min, respectively. The most appropriate heat treatment cycle, to achieve the best tensile properties was annealing at 520 °C for 6 h, followed by quenching in 40 °C water and then aging at 160 °C for 6 h. The most important feature of this study was the conversion of fracture mechanism mode from brittle to ductile in result of changing the morphology of globules and the matrix.

Effects of friction stir processing on mechanical properties of the cast aluminum alloys A319 and A356

Abstract Surfaces of A319 and A356 castings were treated by friction stir processing to reduce porosity and to create more uniform distributions of second-phase particles. Dendritic microstructures were eliminated in stir zones. The ultimate tensile strengths, ductilities, and fatigue lives of both alloys were increased by the friction stir processing.

Effects of grain refiner additions on the grain structures in HPDC A356 castings

In the cold chamber high pressure die casting process (HPDC) solidification begins when the metal is poured into the shot sleeve and impinges on the relatively cold shot sleeve wall and plunger. Therefore, a mixture of liquid and externally solidified crystals (ESCs) is injected into the die cavity. The mechanisms that control the formation of ESCs are not fully understood. In the work presented here, the microstructures of thin walled A356 aluminium alloy die castings have been investigated. The castings were produced by varying the melt superheat and constitutional conditions. It was found that the area fraction of ESCs (fsESC) increases when decreasing the melt superheat; a low superheat generates coarser, more globular ESCs, whilst a larger superheat results in branched, dendritic crystals; additions of Ti in solution increase the fsESC and additions of Al–5Ti–1B grain refiner increase the number of globular, coarse ESCs and generate a finer grain size in the casting. The results are discussed with special emphasis on the shot sleeve solidification conditions and the mechanisms that control the formation of the ESCs.