Effect of ultrasonic treatment on the Fe-intermetallic phases in ADC12 die cast alloy

The ultrasonic treatment temperatures were varied from about 100 °C above the liquidus temperature down to the Al–Si eutectic temperature, for different treatment times (0–15 s). The results showed that the ultrasonic melt treatment was very effective to convert the long plate-like Fe-intermetallic phases (up to 200 μm length) to a highly compacted fine polyhedral/globular form (<15 μm size). The critical ultrasonic treatment temperature to affect the morphology of Fe intermetallics was found to be in the range of 596–582 °C. The eutectic Si was mostly not affected by ultrasonic treatments carried out in this study (in the temperature range of 670–581 °C and for up to 10 s). It was also observed that the nucleation undercooling, which is a measure of nucleation efficiency, at the start of solidification was lowered from ～2.9 to ～0.4 °C by ultrasonic treatment. The variation of horn temperature within 20 °C above pouring temperature to 10 °C below it had no noticeable effect. The ultrasonically treated samples showed better tensile properties than the untreated samples, due to the change in morphology of the Fe-intermetallic particles.     

Muon kinetics in heat treated Al (–Mg)(–Si) alloys

Al–Mg–Si alloys are heat-treatable and rely on precipitation hardening for their mechanical strength. We have employed the technique of muon spin relaxation to further our understanding of the complex precipitation sequence in this system. The muon trapping kinetics in a material reveals a presence of atom-sized defects, such as solute atoms (Mg and Si) and vacancies. By comparing the muon kinetics in pure Al, Al–Mg, Al–Si and Al–Mg–Si when held at different temperatures, we establish an interpretation of muon trapping peaks based on different types of defects. Al–Mg–Si samples have a unique muon trapping peak at temperatures around 200 K. This peak is highest for samples that have been annealed at 70–150 °C, which have microstructures dominated by a high density of clusters/Guinier–Preston zones. The muon trapping is explained by the presence in vacancies inside these structures. The vacancies disappear from the material when the clusters transform into more developed precipitates during aging.     

High Temperature Deformation Behavior of Al–Zn–Mg-Based New Alloy Using a Dynamic Material Model

High temperature compression tests for newly developed Al–Zn–Mg alloy were carried out to investigate its hot deformation behavior and obtain deformation processing maps. In the compression tests, cylindrical specimens were deformed at high temperatures (300–500 °C) and strain rates of 0.001–1/s. Using the true stress–true strain curves obtained from the compression tests, processing maps were constructed by evaluating the power dissipation efficiency map and flow instability map. The processing map can be divided into three areas according to the microstructures of the deformed specimens: instability area with flow localization, instability area with mixed grains, and stable area with homogeneous grains resulting from continuous dynamic recrystallization (CDRX). The results suggest that the optimal processing conditions for the Al–Zn–Mg alloy are 450 °C and a strain rate of 0.001/s, having a stable area with homogeneous grains resulting from CDRX.     

Effects of Heat Treatment on the Microstructures and High Temperature Mechanical Properties of Hypereutectic Al–14Si–Cu–Mg Alloy Manufactured by Liquid Phase Sintering Process

Hypereutectic Al–Si alloy is an aluminum alloy containing at least 12.6 wt.% Si. It is necessary to evenly control the primary Si particle size and distribution in hypereutectic Al–Si alloy. In order to achieve this, there have been attempts to manufacture hypereutectic Al–Si alloy through a liquid phase sintering. This study investigated the microstructures and high temperature mechanical properties of hypereutectic Al–14Si–Cu–Mg alloy manufactured by liquid phase sintering process and changes in them after T6 heat treatment. Microstructural observation identified large amounts of small primary Si particles evenly distributed in the matrix, and small amounts of various precipitation phases were found in grain interiors and grain boundaries. After T6 heat treatment, the primary Si particle size and shape did not change significantly, but the size and distribution of CuAl₂ (θ) and AlCuMgSi (Q) changed. Hardness tests measured 97.36 HV after sintering and 142.5 HV after heat treatment. Compression tests were performed from room temperature to 300 °C. The results represented that yield strength was greater after heat treatment (RT?~?300 °C: 351?~?93 MPa) than after sintering (RT?~?300 °C: 210?~?89 MPa). Fracture surface analysis identified cracks developing mostly along the interface between the primary Si particles and the matrix with some differences among temperature conditions. In addition, brittle fracture mode was found after T6 heat treatment.     

Effect of post-heat treatment on the fatigue behaviour of a friction stir spot-welded Al–Mg–Si alloy

In this paper, the effect of post heat treatment on fatigue behaviour of friction stir spot welded Al–Mg–Si aluminium alloy was investigated. The microstructure of the weld zone was classified into two regions: stir zone (SZ) and mixed zone (MZ), where fine equiaxed grains due to dynamic recrystallization were observed. Two kinds of post heat treatment, namely aging and T6 treatment, were applied to the as-welded joints. The grains in the SZ and MZ were extremely enlarged only by T6 treatment, but some fine grains still remained near the boundary of MZ. Fatigue tests were conducted using lap-shear specimens at a stress ratio R = 0.1. Post heat treatments exhibited little influence on fatigue strength, but fatigue fracture morphology was dependent on both load level and post heat treatment. At high applied loads, fatigue fracture took place through the MZ in the as-welded and aged joints, while along the boundary of MZ in the T6 treated joint. At low applied loads, the fatigue crack initiated at the edge of the nugget and then propagated through the upper sheet in the as-welded joint, but the lower sheet in the aged and T6 treated joints. The dependence of fracture morphology on post heat treatment was attributed to the change of microstructures and hardness distribution around the nugget by post heat treatment.     

Processing,microstructure and property aspects of a spraycast Al–Mg–Li–Zr alloy

This paper describes a microstructural and property investigation of an Al–5.31Mg–1.15Li–0.28Zr alloy produced by spraycasting and downstream processing. Following a dispersoid precipitation treatment of 4 h at 400 °C, samples were hot compressed at strain rates of 2, 1, 0.2 and 0.1 × 10⁻² s⁻¹ at temperatures between 250 and 475 °C. Electron backscattered diffraction showed a strong dependence of recrystallised grain size on deformation temperature. At 250 °C and faster strain rates at 325 °C, a network of fine recrystallised necklace grains formed by progressive lattice rotation. At 325 °C at slow strain rates and at 400 °C and higher, dynamic recrystallisation occurred by discontinuous nucleation and growth at regions of microscopic strain localisation such as grain boundaries and triple points. The microstructures from small-scale hot compression experiments were compared with larger forgings under similar conditions and microstructural evolution was broadly similar. Mechanical properties of larger-scale forgings exceeded the targets for mechanically alloyed Al–Mg–Li alloy AA5091.     

Effects of homogenisation conditions on semisolid microstructures of Al–Mg–Si–Mn alloys produced by D-SSF process

Abstract

The effects of different heating rates to a homogenisation temperature on the semisolid microstructure of Al–Mg–Si–Mn alloys are investigated. It is found that the size, morphology and distribution of the α-Al₁₂Mn₃Si₂ intermetallic compound (Mn containing dispersoid) depend on the heating rate in the homogenisation process. Fine spherical and homogeneously distributed Mn containing dispersoid particles are found in the slow heated samples (0˙7°C min^?1), while inhomogeneously distributed coarser particles with a rod-like shape are found in the rapid heated samples (110°C min^?1). The homogenised sample is deformed by 60% cold rolling. It is found that the recrystallised and semisolid grain sizes of the rapid heated sample are smaller than those of the slow heated sample in all conditions. Compared with the M4 alloy (0˙4 mass-%Mn), the M7 alloy (0˙72 mass-%Mn) has much finer semisolid grain size and smaller values of the shape factor close to 1. The Mn containing dispersoid greatly affects the semisolid grain size of the alloys. The results in this work show that the rapid heating in the homogenisation process is useful to produce high quality semisolid products of the Al–Mg–Si–Mn alloys.     

The microstructures of polycrystalline Al–0.1Mg after hot plane strain compression

The microstructures of Al–0.1Mg polycrystals deformed in plane strain compression at temperatures of 20–400 °C have been investigated by electron backscatter diffraction. At all temperatures, the microstructures are characterized by two types of banded substructure, primary bands of aligned low angle boundaries whose alignment to the rolling plane is a function of strain and temperature, and secondary bands which develop into grain-scale shear bands with increasing strain. Several aspects of the deformation microstructures, including the orientation dependence of subgrain parameters and the inhomogeneity of the microstructure, differ from those of single crystals of similar orientation, and this it attributed to local grain interactions.     

Effect of alloying elements on heat treatment response of aluminium high pressure die castings

Abstract

High pressure die cast (HPDC) aluminium components that respond to age hardening cannot normally be solution treated at high temperatures because the presence of internal porosity and entrapped gases leads to the formation of surface blisters. Parts may also become dimensionally unstable due to swelling. These factors that prevent heat treatment present significant limitations to the utilisation of HPDC components. Now it has been found that blistering and dimensional change can be avoided by using much shorter solution treatment times and lower temperatures. Experiments with alloys 360 (Al–9·5Si–0·5Mg) and 380 (Al–8·5Si–3·5Cu) have shown that strong responses to age hardening are still possible following these modified solution treatments. In the current paper, the role of critical alloying elements is considered in both current specification Al–Si–Cu–(X) alloys, and also in newly developed alloy compositions. It is shown that 0·2% proof strengths over 400 MPa may be readily achieved by heat treating conventionally produced die castings.     

Morphology and formation of Fe–Al intermetallic layers on iron hot-dipped in Al–Mg–Si alloy melt

We have examined the morphology and the growth of Fe–Al intermetallic layers of η-Fe₂Al₅ and θ-FeAl₃ phases formed on pure Fe sheets dipped in an Al-8.2Mg-4.8Si (wt.%) alloy melt and pure Al melt at 750 °C. The η phase layer grows one order of magnitude slower in the Al–Mg–Si alloy melt than in the pure Al melt. The change in thickness of Fe sheets with dipping time is less pronounced in the Al–Mg–Si alloy melt than in a pure Al melt. Microstructure observations suggest that the retarded interfacial reaction between solid Fe and liquid Al–Mg–Si alloy is associated with a continuous θ phase layer formed in the Al–Mg–Si alloy melt, which acts as the diffusion barrier.     

Microstructure characterization of Al-cladded Al–Zn–Mg–Cu sheet in different hot deformation conditions

Al-cladded Al–Zn–Mg–Cu sheets were compressed up to 70% reduction on a Gleeble–3500 thermo-mechanical simulator with temperatures ranging from 380 to 450 °C at strain rates between 0.1 and 30 s^?1. The microstructures of the Al cladding and the Al–Zn–Mg–Cu matrix were characterized by electron back-scattered diffraction (EBSD) and X-ray diffraction (XRD). The microstructure is closely related to the level of recovery and recrystallization, which can be influenced by deformation temperature, deformation pass and deformation rate. The level of recovery and recrystallization are different in the Al cladding and the Al–Zn–Mg–Cu matrix. Higher deformation temperature results in higher degree of recrystallization and coarser grain size. Static recrystallization and recovery can happen during the interval of deformation passes. Higher strain rate leads to finer sub-grains at strain rate below 10 s^?1; however, dynamic recovery and recrystallization are limited at strain rate of 30 s^?1 due to shorter duration at elevated temperatures.     

Modelling yield strength of heat treated Al–Si–Mg casting alloys

Abstract

A model for the yield strength of artificially aged Al–Si–Mg casting alloys has been developed. The model includes Mg concentrations between 0·2 and 0·6 wt-% and aging temperatures between 150 and 210°C. Spherical precipitates with the composition Mg₅Si₆, which grow by diffusion of Mg from the surrounding α-Al matrix, are assumed in the model. Nucleation is assumed to be instantaneous and growth of the precipitates is modelled using Fick’s second law and mass balance. When supersaturation is lost the continued precipitate growth is modelled using the Lifshitz–Slyozov–Wagner coarsening law. An average precipitate radius is calculated and a precipitate size distribution is introduced by using a relation between the average radius and its standard deviation. The strength contribution from precipitates is calculated using coherency strengthening and Orowan strengthening. The agreement between the model and experimental data is generally good; however, modelling the underaged condition needs further refinement.     

Reduction of intermetallic compounds in ultrasonic-assisted semi-solid brazing of Al/Mg alloys

ABSTRACT

An ultrasonic-assisted semi-solid brazing method was used to control Mg/Al intermetallic compounds (IMCs). Zn–20.95Al, which has a large semi-solid temperature range of 60°C, was chosen as filler metal. Oxide films on the Al/Mg surfaces were removed despite the semi-solid state of the filler metal at 430°C. Al–Mg–Zn IMCs were observed inside the joint at 667?W and 5?s. The IMCs was reduced as ultrasonication time was prolonged. At high power of 1000?W, short ultrasonication time was needed to guarantee that no IMCs remained inside the joint. Otherwise, new IMCs formed because of serious cavitation erosion on substrates. Ultrasonication time of 20?s was unable to completely eliminate IMCs and solid phases at 423°C.     

Peak temperatures during friction stir welding of Ti–6Al–4V

Abstract

Experimental measurements were made to determine the peak temperatures during friction stir welding of Ti–6Al–4V alloy as a function of the processing conditions such as tool rotation speed and feedrate. It was found that the spindle speed has a dominant effect on peak temperatures, while feedrate controls exposure time. Low spindle speed conditions lead to peak temperatures near, or below, the beta transus temperature of the material, 1000°C (1800°F), while high spindle speed welds result in peak temperatures above 1200°C (2100°F). Weld microstructures were also evaluated as a function of the weld parameters. Higher spindle speeds and lower federate lead to increased grain size.     

Prediction of porosity characteristics of aluminium castings based on X-ray CT measurements

Porosity is a main factor limiting the fatigue performance of aluminium castings. Using micro X-ray computed tomography, size and morphology characteristics of porosity distributions are analysed for material from a cast Al–8Si–3Cu–(Sr) crankcase as well as from cast Al–8Si–3Cu–(Sr), Al–7Si–0·5Cu–Mg–(Sr) and Al–7Si–0·5Cu–Mg–(Na) cylinder heads. Correlations are developed between the porosity volume percentage and mean and maximum pore sizes. Two characteristic size measures of the porosity distribution are identified: the volume weighted spherical mean diameter and the volume weighted mean envelope diameter. Both correlate linearly with the corresponding diameters of the largest pore. The pore morphology is described by a volume weighted mean sphericity. This mean sphericity and the local amount of porosity are used to predict the mean and maximum pore sizes of the porosity distributions. These correlations will find applications in integrated computational materials engineering.     

Thermal stability evaluation of nanostructured Al6061 alloy produced by cryorolling

Grain growth of nanostructured Al6061 produced by cryorolling and aging process was investigated during isothermal heat treatment in 100–500 °C temperature range. Transmission electron microscopy (TEM) observations demonstrate that after cryorolling and aging at 130 °C for 30 h, the microstructure contains 61 nm grains with dispersed 50–150 nm precipitates and 0.248% lattice strain. In addition, an increase in tensile strength up to 362 MPa because of formation of fine strengthening precipitation and nano-sized grains was observed. Thermal stability investigation within 100–500 °C temperature range showed release of lattice strain, dissolution of precipitates and grain growth. According to the X-ray diffraction (XRD) analysis, Mg₂Si precipitates disappeared after annealing at temperatures higher than 300 °C. According to the results, due to the limited grain growth up to 200 °C, there would be little decrease in mechanical properties, but within 300–500 °C range, the grain growth, dissolution of strengthening precipitates and decrease in mechanical properties are remarkable. The activation energies for grain growth were calculated to be 203.3 kJ/mol for annealing at 100–200 °C and 166.34 kJ/mol for annealing at 300–500 °C. The effect of precipitation dissolution on Al lattice parameter, displacement of Al6061 (111) XRD peak and Portevin–LeChatelier (PLC) effect on stress–strain curves is also discussed.     

Effect of Heat Treatment on the Microstructure and Wear Properties of Al–Zn–Mg–Cu/In-Situ Al–9Si–SiCp/Pure Al Composite by Powder Metallurgy

This study examined the effects of heat treatment on the microstructure and wear properties of Al–Zn–Mg–Cu/in-situ Al–9Si–SiCp/pure Al composites. Pure Al powder was used to increase densification but it resulted in heterogeneous precipitation as well as differences in hardness among the grains. Heat treatment was conducted to solve this problem. The heat treatment process consisted of three stages: solution treatment, quenching, and aging treatment. After the solution treatment, the main dissolved phases were η′(Mg₄Zn₇), η(MgZn₂), and Al₂Cu phase. An aging treatment was conducted over the temperature range, 100–240 °C, for various times. The GP zone and η′(Mg₄Zn₇) phase precipitated at a low aging temperature of 100–160 °C, whereas the η(MgZn₂) phase precipitated at a high aging temperature of 200–240 °C. The hardness of the sample aged at 100–160 °C was higher than that aged at 200–240 °C. The wear test was conducted under various linear speeds with a load of 100 N. The aged composite showed a lower wear rate than that of the as-sintered composite under all conditions. As the linear speed was increased to 1.0 m/s, the predominant wear behavior changed from abrasive to adhesive wear in all composites.     

Improving the properties of cold-rolled Al–6%Ni sheets by alloying and heat treatment

The mechanical properties of a cold-rolled binary eutectic Al–Ni alloy can be considerably improved by additional alloying with Zr and heat treatment of a billet. Two-stage annealing of billets results in the favourable morphology changes in the eutectics structure and in the substantial hardening with participation of Al₃Zr dispersoids. The cold-rolled material starts to soften only at temperatures as high as 350 °C. The main reason of softening at higher temperatures is the beginning of recrystallization with the formation of subgrains and grains of submicron size.     

Effects of ultrasonic treatment on the formation of iron-containing intermetallic compounds in Al-12%Si-2%Fe alloys

In general, the iron impurity is detrimental to the mechanical properties of Al–Si alloys. The α-phase and β-phase are the most important and common iron-containing intermetallic compounds (IMCs) in Al–Si alloys. During conventional casting, the acicular β-phase is stable, and considered to be harmful. In this paper, the Al-12%Si-2%Fe alloy was treated by power ultrasound and solidified under different cooling conditions. The effects of ultrasonic treatment (UST) and cooling rate on morphology and composition of IMCs were investigated. The results showed that UST can change the morphology and composition of iron-containing IMCs and promote the formation of metastable α-phase. When the ultrasound was applied at 720 °C, the amount of starlike α-phase increases and the acicular β-phase decreases with increasing applied time of UST. In addition, the polygonal α-phase is formed and substitutes for the β-phase when quenching after UST for 60 s and 120 s, suggesting that the formation of β-phase can be suppressed under this condition. For the case of UST at 610 °C which the β-phase has been nucleated, the β-phase transforms from an acicular shape to the rod-like morphology, indicating that the cavitation-induced fracture of β-phase.     

Microstructures and mechanical properties of Fe–Al–Ta alloys with strengthening Laves phase

The addition of Ta to Fe–Al alloys results in the formation of a stable Ta(Fe,Al)2 Laves phase with hexagonal C14 structure in the Fe–Al phase at temperatures of 800, 1000 and 1150 °C. It was found that the solubility of Ta in Fe–Al is generally low and the solubility of Ta varies with Al content. Respective isothermal sections of the Fe–Al–Ta system have been established. Particular attention has been given to precipitation in the Fe₃Al phase with a small addition of Ta. At intermediate temperatures, 600–750 °C, an additional Heusler-type phase with L2₁-structure precipitates, which transforms at longer times and high temperatures to the stable C14 Laves phase. The yield stress in compression and the creep behaviour of the Fe–Al–Ta alloys with various microstructures were studied. Due to the presence of the L2₁-Heusler phase, the yield stress and the creep resistance at temperatures below 700 °C was increased considerably.