Investigation of mechanical properties of advanced Al–Zn–Mg–Cu alloy

Abstract

The mechanical properties of the rapidly solidified 7000 series powder alloy CW 67 were investigated for various extrusion and heat treatment conditions. The principal aim of the work was to ascertain the optimum processing route for peak aged (T6) material. The highest proof stress in the T6 condition was found to be 572 MN m^?2 for material extruded at 325°C and aged for 13·5 h at 120°C after solutionising. The ductility of this material was found to be 13·5%. The fracture toughness was measured in two orientations and found to be approximately 21 MN m^?3/2 in the short transverse direction and 44 MN m^?3/2 in the longitudinal direction. Degassing and hot compaction was found to improve the fracture toughness of the material substantially.

MST/1504     

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.     

Microstructure and mechanical properties of Al–Si–Mg–Cu–Ti alloy with trace amounts of scandium

The effect of Sc on the microstructure and mechanical properties of Al–Si–Mg–Cu–Ti alloy was investigated. Results obtained in this research indicate that, with increasing Sc content, the microstructure of the investigated alloys exhibits finer equiaxed dendrites with rounded edges and the morphology of the eutectic Si shows a complete transition from a coarse needle-like structure to a fine fibrous structure upon modification of eutectic Si. Subsequent T6 heat treatment had further induced the precipitation of nano-scaled secondary Al₃(Sc, Ti) phase, as well as spheroidisation of eutectic Si. Combined with T6 heat treatment, the ultimate tensile strength, yield strength, percentage elongation and hardness were achieved in 0.20?wt-% Sc-modified alloy.     

Mechanism of grain refinement of an Al–Zn–Mg–Cu alloy prepared by low-frequency electromagnetic casting

Low-frequency electromagnetic casting (LFEC) process had been developed and is being used for the past several years with the application of an induction coil placed outside the conventional direct chill (DC) casting mould. It has been demonstrated that the LFEC process has a significant grain refining effect on aluminium alloys. In the present study, temperature measurement and direct quenching from liquid and/or semi-solid were carried out to study the temperature field during casting process and to understand the mechanism of the grain-refining effect of the LFEC process. The experimental results showed that in contrast to the conventional DC casting process, the liquid melt from the launder, during the LFEC process, is cooled with very high cooling rate directly to 3–6 °C below the liquidus, and the temperature field of the entire melt in the mould, and the hot top is quite uniform, which results in the enhanced heterogeneous nucleation and improved survival rate of the nuclei. This is believed to be the main reason why the LFEC process can significantly refine the grain size of aluminium alloys.     

Effect of Zn addition on two-step aging behaviour of Al–Mg–Si–Cu alloy

This study investigates the effect of Zn addition two-step behaviour in an Al–Mg–Si–Cu alloy. During pre-aging at 100°C for 3?h, the Zn can partition into clusters because of the strong Zn–Mg interaction, prompting the formation of clusters. During subsequent artificial aging at 180°C for up to 240?min (peak hardness condition), the Zn does not significantly partition into clusters or precipitates, and the majority of Zn remains in the Al matrix. However, the presence of Zn in the matrix stimulates the transformation from clusters to GP zones to β′′ phases. The enhanced formation of GP zones and β′′ phases correlates well with the remarkable age-hardening response.     

High-pressure homogenization treatment of Al–Zn–Mg–Cu aluminum alloy

Absract The microstructures and aging hardening response of Al–12Zn–3.5Mg–3.0Cu–0.14Zr aluminum alloy after a high-pressure homogenization treatment at 750 °C for 45 min under 5 GPa were investigated. The results showed that the constituent phases dissolved completely and formed α-Al single-phase solid solution comparable to that formed after ambient-pressure homogenization at 450 °C/96 h + 460 °C/128 h. The complete dissolution of the constituent phases increased the solubility of the alloying elements, as well with the over-burning temperature and aging hardness.     

Microstructural evolution of Al–Zn–Mg–Cu alloy during homogenization

In this study, the microstructural evolution of an as-cast Al–Zn–Mg–Cu alloy (AA7085) during various homogenization schemes is investigated. It is found that in a single-stage homogenization scheme, some of the primary eutectic gets transformed into the Al₂CuMg phase at 400 °C, and the primary eutectic and Al₂Cu phase gradually dissolve into the alloy matrix at 450 °C. The Al₃Zr particles are mainly precipitated at the center of the grain because Zr is peritectic. However, the homogeneous distribution of the Al₃Zr particles improves and the fraction of Al₃Zr particles increases in two-stage homogenization scheme. At the first low-temperature (e.g., 400 °C) stage, the Al₃Zr particles are homogeneously precipitated at the center of the grain by homogeneous nucleation and may be heterogeneously nucleated on the residual second-phase particles at the grain boundary regions. At the second elevated-temperature (e.g., 470 °C) stage, the Al₃Zr nuclei become larger. A suitable two-stage homogenization scheme for the present 7085-type Al alloy is 400 °C/12 h + 470 °C/12 h.     

Precipitation in Cu–Ni–Si–Zn alloy for lead frame

The aging of Cu–Ni–Si–Zn alloy for lead frame is investigated. The results showed that the peak of hardening effect occurs after aging for about 1 h and the electrical conductivity increases continuously with aging times. The hardness of the alloy reached a peak at 430–460 °C for 2 h and electrical conductivity reached a peak at 500–550 °C and continuously decreased afterwards. The cold rolling prior to the aging treatment was used to increase the precipitation rate. The precipitates responsible for the age-hardening effect are disc-shaped δ-Ni₂Si, which has an orthorhombic structure.     

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.     

Non-isothermal aging of a high-Zn-containing Al–Zn–Mg–Cu alloy: microstructure and mechanical properties

The non-isothermal aging behaviour of a newly developed Al–Zn–Mg–Cu alloy containing 17?wt-% Zn was investigated. Hardness and shear punch tests demonstrated that during non-isothermal aging, the mechanical properties of the alloy first increased and then decreased. The best properties were obtained in a sample which was non-isothermally aged upto 250°C with heating rate of 20°C?min^?1, due to the presence of η′/η (MgZn₂) phases. This was confirmed by differential scanning calorimetery. After homogenisation, residual eutectic phases remained at triple junctions or in a spherical form. During aging, these phases transformed into rodlike S (Al₂CuMg)-phase at 400°C, with sizes ranging from 50 to 250?nm. The precipitation sequence in this high-Zn alloy was similar to that for conventional Al–Zn–Mg–Cu alloys.     

Effects of Cu content on microstructure and mechanical properties of Al–14.5Si–0.5Mg alloy

This study elucidates how Cu content affects the microstructure and mechanical properties of Al–14.5Si–0.5Mg alloy, by adding 4.65 wt.% and 0.52 wt.% Cu. Different Fe-bearing phases were found in the two alloys. The acicular β-Al₅FeSi was found only in the high-Cu alloy. In the low-Cu alloy, Al₈Mg₃FeSi₆ was the Fe-bearing phase. Tensile testing indicated that the low-Cu alloy containing Al₈Mg₃FeSi₆ had higher UTS and elongation than the high-Cu alloy containing the acicular β-Al₅FeSi. It is believed that the presence of the acicular β-Al₅FeSi in the high-Cu alloy increased the number of crack initiators and brittleness of the alloy. Increasing Cu content in the Al–14.5Si–0.5Mg alloy also promoted solution hardening and precipitation hardening under as-quenched and aging conditions, respectively. The hardness of the high-Cu alloy therefore exceeded that of low-Cu alloy.     

Microstructure and mechanical properties of a new Al–Zn–Mg–Cu alloy joints welded by laser beam

A new type of Al–Zn–Mg–Cu alloy sheets with T6 temper were welded by laser beam welding (LBW). Microstructure characteristics and mechanical properties of the joints were evaluated. Results show that grains in the heat affected zone (HAZ) exhibit an elongated shape which is almost same as the base metal (BM). A non-dendritic equiaxed grain zone (EQZ) appears along the fusion line in the fusion zone (FZ), and grains here do not appear to nucleate epitaxially from the HAZ substrate. The FZ is mainly made up of dendritic equiaxed grains whose boundaries are decorated with continuous particles, identified as the T (AlZnMgCu) phase. Obvious softening occurs in FZ and HAZ, which mainly due to the changes of nanometric precipitates. The precipitates in BM are mainly η′, while plenty of GPI zones exist in FZ and HAZ adjacent to FZ, in the HAZ farther away from FZ, η phase appears. The minimum microhardness of the joint is always obtained in FZ at different times after welding. The ultimate tensile strength of the joint is 471.1 MPa which is 69.7% of that of the BM. Samples of the tensile tests always fracture at the FZ.     

Compressive and impression creep behavior of a cast Mg–Al–Zn–Si alloy

Creep behavior of an Mg–6Al–1Zn–0.7Si cast alloy was investigated by compression and impression creep test methods in order to evaluate the correspondence of impression creep results and creep mechanisms with conventional compression test. All creep tests were carried out in the temperature range 423–523 K and under normal stresses in the range 50–300 MPa for the compression creep and 150–650 MPa for impression creep tests. The microstructure of the AZ61–0.7Si alloy consists of β-Mg₁₇Al₁₂ and Mg₂Si intermetallic phases in the α-Mg matrix. The softening of the former at high temperatures is compensated by the strengthening effect of the latter, which acts as a barrier opposing recovery processes. The impression results were in good agreement with those of the conventional compressive creep tests. The creep behavior can be divided into two stress regimes, with a change from the low-stress regime to the high-stress regime occurring, depending on the test temperature, around 0.009 < (σ/G) < 0.015 and 0.021 < (σ_imp/G) < 0.033 for the compressive and impression creep tests, respectively. Based on the steady-state power-law creep relationship, the stress exponents of about 4–5 and 10–12 were obtained at low and high stresses, respectively. The low-stress regime activation energies of about 90 kJ mol⁻¹, which are close to that for dislocation pipe diffusion in the Mg, and stress exponents in the range of 4–5 suggest that the operative creep mechanism is pipe-diffusion-controlled dislocation viscous glide. This behavior is in contrast to the high-stress regime, in which the stress exponents of 10–12 and activation energies of about 141 kJ mol⁻¹ are indicative of a dislocation climb mechanism similar to those noted in dispersion strengthening mechanisms.     

Characterisation of interfaces in Al–Zn–Mg alloy reinforced with Sicabo fibres

Abstract

The microstructure of a metal matrix composite consisting of an Al–Zn–Mg alloy reinforced with SiC coated boron fibres has been examined by electron microscopy, electron probe microanalysis, and by optical microscopy. Considerable amounts of Mg₂Si phase were found to be segregated at the fibre/matrix interface. This intermetallic was not formed by a reaction between the fibre and matrix during the fabrication process, a liquid infiltration technique, but as a result of silicon impurities present as contaminants in the melt. It was concluded that the interface phase was precipitated from the metal matrix in the later stages of solidification without any nucleation role being played by the fibre. The Mg₂Si phase appears to be brittle and was present in amounts likely to have a deleterious effect on the strength of the composite.

MST/871     

Synthesis and properties analysis of char-reinforced Al–13.5Si–2.5Mg alloy composites

The re-evaluation of previous and existing methods in materials processing is becoming ever more critical because of processing and starting materials cost factors. A study on the synthesis and properties investigation of hypereutectic Al–13.5Si–2.5Mg alloy reinforced with carbon chars using coconut shell as the organic precursor has been carried out. The low-cost, double compaction solid-state technique was used. Reinforcing the hypereutectic alloy with coconut shell char particles (size:<140 m) at 2 vol % and consolidating by reaction sintering at 600 °C in vacuum for 15 min, followed by near net-shape compaction at 250 MPa, increased the hardness of the alloy 6% while reducing its strength (UTS) by only 3%. The use of palm kernel shell char as the dispersed phase was found to yield identical results. At 2 vol % char, the mechanical properties, sintered density and dimensional changes were optimally found to be suitable for lightweight anti-friction electromechanical applications. Attempts to reinforce the alloy with 2 vol % coconut shell chars activated in CO2 reduced its strength in the range of 19 to 26% at different burn-off percentages. This is attributed to the higher amount of oxide products formed during the activation process. At 600 °C, formation of the brittle Al4C3 phase in the different sintered composites containing activated and unactivated chars was identified by X-ray studies. © 1998 Chapman & Hall     

High-temperature deformation behavior of Cu–6.0Ni–1.0Si–0.5Al–0.15?Mg–0.1Cr alloy

The hot compression deformation behavior of Cu–6.0Ni–1.0Si–0.5Al–0.15?Mg–0.1Cr alloy with high strength, high stress relaxation resistance and good electrical conductivity was investigated using a Gleeble1500 thermal–mechanical simulator at temperatures ranging from 700 to 900?°C and strain rates ranging from 0.001?to 1?s^?1. Working hardening, dynamic recovery and dynamic recrystallization play important roles to affect the plastic deformation behavior of the alloy. According to the stress–strain data, constitutive equation has been carried out and the hot compression deformation activation energy is 854.73?kJ/mol. Hot processing map was established on the basis of dynamic material model theories, and Prasad instability criterion indicates that the appropriate hot processing temperature range and strain rate range for hot deformation were 850~875?°C and 0.001~0.01?s^?1, which agreed well with the hot rolling experimentation results.     

Effect of precipitates on properties of cold-rolled Al–Mg–Si–Sc–Zr alloy with higher temperature aging

The aging hardening behaviours of the cold-rolled Al–Mg–Si–Sc–Zr alloy were investigated. The microstructure, hardness and electrical conductivity (EC) of the Al–Mg–Si–Sc–Zr alloy were measured. The relationship between the microstructure and the properties of the Al–Mg–Si–Sc–Zr alloy with cold-rolling and aging processes was studied. The result shows that the addition of Sc and Zr elements significantly refines the grains of the Al–Mg–Si alloy during casting. The cold rolling promotes the Al–Sc(Zr) precipitation. The precipitate strengthening increases with increasing roll reduction. The EC of the cold rolling?+?aging Al–Mg–Si–Sc–Zr alloy increases with increasing rolling reductions. The combination effects of the precipitation hardening and DRX softening during the aging process lead to the similar peak hardness values of around 70?HV of the rolled Al–Mg–Si–Sc–Zr alloy with the different reductions.     

Investigation on the mechanical properties and frictional performance of Ni–Cu–Si alloy

The microstructure evolution, mechanical properties and dry sliding behaviour of Ni–30Cu–xSi alloy have been investigated systematically. As the volume fraction of microscale second-phase particles and nanoscale precipitates increases, the hardness, yield strength and ultimate tensile strength of alloy are improved significantly but elongation is reduced. Through confocal laser scanning microscope and atomic force microscope, it is suggested that the wear mode changes from the mixture of abrasive and adhesive wear to single abrasive wear. Owing to the existence of netlike microscale second-phase particles which are more likely to split the matrix, the Ni–30Cu–5.5Si alloy exhibits an abnormal higher wear rate even with the highest hardness. The netlike structure which deteriorates the friction performance should be avoided in wear-resistant materials.     

Damping properties in Mg–Zn–Y alloy with dispersion of quasicrystal phase particle

The effect of the interface structure between the matrix and the particle on the damping capacity was investigated using Mg–Zn and Mg–Zn–Y alloys in this study. The damping capacity was not affected by the interface structure at room temperature. However, the onset of temperature, which was higher in the Mg–Zn–Y alloy than in the Mg–Zn alloy despite their similar grain sizes, increased the damping capacity through grain boundary relaxation by grain boundary sliding. Compared to the Mg–Zn alloy, the existence of the quasicrystal phase particles, which had the coherent interface with low interface energy, was likely to have suppressed and delayed the grain boundary sliding in the Mg–Zn alloy.