Materials Science and Technology

Self-equilibrating residual stresses may occur in materials in the absence of external loading due to internal strain inhomogeneity. While favourable distributions of residual stress can bestow an object with the appearance of superior material properties, most welding processes leave behind residual stresses in particularly unfavourable patterns, causing a greater susceptibility to fracture based failure mechanisms and unintended deformation. Currently, heat treatment is the primary means of removing these stresses, but since the formation of residual stress is dependent upon many material and process factors, there are several other viable mechanisms (using thermal, mechanical or phase transformation effects) by which it may be modified. It is only now, using relevant advances in numerical and experimental methods, that these techniques are being fully explored. This article gives a brief introduction to weld induced residual stresses and reviews the current state of the art with regard to their reduction. Emphasis is placed on the recent development of unconventional techniques, and the mechanisms by which they act.

ZrB₂/Fe composite coating was in situ synthesised by gas tungsten arc welding cladding process on AISI 1020 steel. Zr, B₄C and Fe–B alloy powders were used as precursor powders. The phase composition and microstructure were investigated by X-ray diffraction analysis, optical microscopy, scanning electron microscopy and energy dispersive spectroscopy. Microhardness of ZrB₂/Fe composite coating at room temperature was examined. Main phases obtained from Zr and B₄C precursor are ZrB₂ and α-Fe, and those obtained from Zr and Fe–B precursor are ZrB₂ and FeB. In the upper part of these composite coatings, ZrB₂ phase mainly grows along temperature gradient direction. The middle part of these composite coatings has the highest ZrB₂ content and highest microhardness. Gradient dispersions of ZrB₂ reinforcements appeared in the composite coating from the middle to the bottom, leading to gradient dispersions of microhardness. With decreasing dilution rate, ZrB₂ content and microhardeness increase.

316LN is a type of austenitic stainless steel whose grain refinement only depends on hot deformation. The true stress–strain curves of 316LN were obtained by means of hot compression experiments conducted at a temperature range of 900–1200°C and at a strain rate range of 0·001–10 s⁻¹. The influence of deformation parameters on the microstructure of 316LN was analysed. Both the constitutive equation for 316LN and the model of grain size after dynamic recrystallisation were established, and the effect of different deformation conditions on the microstructure was analysed. The results show that the suitable working region is the one with a relatively higher deformation temperature and a lower strain rate, in which the dynamic recrystallisation is finely conducted. Moreover, the working region that should be avoided during hot deformation was indicated.

Post-heat treatment recrystallisation is a significant cause of scrap in single crystal component production. This defect could be prevented by a reduction in the mechanical strength of the shell after the metal has solidified. Addition of a zirconia layer, to produce large cracks around grain boundaries and among the large grains during phase transformations, was considered as a method of reducing sintered strength. In this study, two types of zirconia stucco, monoclinic and partially stabilised, were introduced into the shell. The effect of the incorporation of a zirconia layer on the microstructure and mechanical properties of the shell were investigated. Shell with a monoclinic zirconia additional layer showed no reduction in the green strength but a better performance in high temperature deformation tolerance, maintaining shell performance during wax removal and metal casting. The shell strength after alloy solidification is reduced, suggesting that a reduction in recrystallisation may be possible with this method.

Undercooled solidification of hypoeutectic Ni–3·3 wt-%B alloy has been studied. The microstructure evolution of Ni–3·3 wt-%B alloy indicates that a transition from regular lamellar eutectic to anomalous eutectic exists with increasing undercooling for eutectic transformation. Here, a new model was proposed to describe the cooling curves of undercooled solidification with two-step transformation. Fits of the present model to the cooling curves after different undercoolings show that the predictions of transformed fractions are compatible with the microstructure observation. Furthermore, for hypoeutectic alloy, the size of the primary phase mainly depends on the undercooling for primary solidification, while the fraction of the primary phase mainly depends on the undercooling for eutectic solidification.

The susceptibility to heat affected zone cracking of Waspaloy has been investigated in terms of its hot ductility, measured as the reduction of area (RA). Gleeble testing with on-heating as well as on-cooling test cycles was carried out to illuminate the influence of different 4 h solution heat treatments between 996 and 1080°C. A ductility maximum of between 80 and 90%RA was found at 1050–1100°C for all conditions in the on-heating tests. Although the different heat treatment conditions showed similar macrohardness, the particle size and distribution of the γ′ and M₂₃C₆ phases differed, which significantly affected the on-heating ductility in the lower temperature test region. The ductile to brittle transition was initiated at 1100°C in the on-heating testing with indications of grain boundary liquation at the higher test temperatures. Ductility recovery, as measured in the on-cooling tests from 1240°C, was very limited with <30%RA for all conditions and test temperatures except for the 1080°C/4 h treatment, which exhibited 60%RA at 980°C.

High strain rate multiple forging (HSRMF) was successfully carried out on ZK60 magnesium alloy to an accumulated strain of ∑Δϵ = 2·64 at temperature of 573 K, and the microstructure and mechanical properties of the high strain rate multiple forged samples were investigated. The results show that the initial grains were extensively refined after HSRMF due to dynamic recrystallisation (DRX). However, the DRX mechanism at initial grain boundaries was different from that at grain core. Rotation DRX and twin induced DRX were responsible for the DRX at initial grain boundaries and original grain core respectively. A novel mixed structure of honeycomb-like coarse DRX grains with average grain size of 10 μm and island-like ultrafine grains with average grain size of 1 μm formed at ∑Δϵ = 2·64. This novel mixed structure showed substantial improvements in mechanical properties. Mechanical testing gave an ultimate tensile strength of 330 MPa and an elongation of 25·7%. Therefore, HSRMF was identified as a potential technique for stronger and more ductile wrought ZK60 alloy.

This paper describes a detailed study of tube extrusion by simulation using finite element method (FEM). The finite element model used one-sixth of symmetry. The extrusion load, temperature evolution and metal flow were predicted. Innovative methods, combining both grid and surface tools, were used to define in detail the flow of material. These showed clearly the inner and outer surface formation mechanisms of the tube extrusion. The seam weld, an important quality indicator, was also evaluated by selecting an appropriate criterion.

The effects of heat treatment on the microstructure and bond strength at the interface of explosively welded titanium/304L stainless steel clad have been investigated. The microstructure of the clad interface were examined using optical and scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and X-ray diffraction (XRD) techniques. At 700°C, the formation of intermetallic phases σ and Fe₂Ti besides β-Ti were confirmed, while in samples, heat treated at 800°C and 900°C, other intermetallic phases such as λ and FeTi, NiTi, NiTi₂ phases were detected in addition to σ and Fe₂Ti phases. The shear test results show that the shear strengths of heat treated samples are overall significantly lower than that of stress relieved samples. This could be due to the formation of brittle intermetallic phases at the interface. Despite of formation of the intermetallic phase at 700°C, the shear strength of the sample is still more than the minimum standard amount of 137·9 MPa. It is also observed that the higher the heat treatment temperature, the lower the interface shear strength, which can be explained by the fact that the volume fraction of intermetallics increases with increasing temperature. The shear strength values of heat treated samples are lower than those of diffusion bonded clads. This could be attributed to the fact that the total widths of intermetallic layers achieved in heat treated samples are larger than those of for diffusion bonded ones.

An ultrafine grained (UFG) structure developed in precipitation hardenable Al alloys through cryorolling by suppression of dynamic recovery followed by low temperature aging has received great research interest because of its high strength and very good ductility. In the present work, Al 6061 alloy was solution treated and deformed by cryorolling up to an effective true strain of 2·6 and then subjected to annealing at the temperature range from 150 to 350°C to study the effect of annealing on the microstructure and mechanical properties. The evolution of microstructure and precipitates was investigated by employing X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. Vickers hardness and tensile testings were performed at room temperature to evaluate the effect of annealing on the mechanical properties. It was observed that the strength and ductility increased upon annealing at 150°C, and further annealing at high temperatures (200–350°C) results in reduction in hardness and strength but increase in ductility. A significant improvement in strength observed at low temperature annealing (150°C) is due to the precipitation of metastable phase β″. It overcompensates the reduction in hardness that occurred due to the softening caused by the recovery effect. It was found that the cryorolled Al 6061 alloy with UFG structure is thermally stable up to temperatures 250°C with slight grain coarsening. At this temperature, the TEM studies revealed that second phase Mg₂Si particles are effectively pinning the grain boundaries due to the Zener drag effect. Owing to the presence of heterogeneities in the material, a duplex structure was observed upon annealing at temperatures 150 and 200°C. Abnormal grain growth was observed after annealing at high temperatures (300°C).

Experimental and numerical studies of slurry generation using a cooling slope are presented in the paper. The slope having stainless steel body has been designed and constructed to produce semisolid A356 Al alloy slurry. The pouring temperature of molten metal, slope angle of the cooling slope and slope wall temperature were varied during the experiment. A multiphase numerical model, considering liquid metal and air, has been developed to simulate the liquid metal flow along the cooling channel using an Eulerian two-phase flow approach. Solid fraction evolution of the solidifying melt is tracked at different locations of the cooling channel following Schiel's equation. The continuity, momentum and energy equations are solved considering thin wall boundary condition approach. During solidification of the melt, based on the liquid fraction and latent heat of the alloy, temperature of the alloy is modified continuously by introducing a modified temperature recovery method. Numerical simulations has been carried out for semisolid slurry formation by varying the process parameters such as angle of the cooling slope, cooling slope wall temperature and melt superheat temperature, to understand the effect of process variables on cooling slope semisolid slurry generation process such as temperature distribution, velocity distribution and solid fraction of the solidifying melt. Experimental validation performed for some chosen cases reveals good agreement with the numerical simulations.

The influence of subrapid solidified processing parameters on the microstructure characteristic and grain refining efficiency of Al–5Ti–1B master alloy has been studied. The results show that the mean size of Al₃Ti particles in Al–5Ti–1B master alloy decreases significantly after copper mould die casting. The morphology of Al₃Ti particles changed from plate-like to blocky and petaloid. Compared with conventional Al–5Ti–1B master alloy, the phase constitution of subrapidly solidified Al–5Ti–1B master alloy does not change, and the improved master alloy shows a better grain refining efficiency. The microhardness of the pure aluminium refined by conventional and improved Al–5Tu–1B obeys the Hall–Petch relation, and the material constant k_H is in the range of 84·02–91·80 MPa μm^1/2. The mechanisms of grain refinement of aluminium by inoculation with improved Al–5Ti–1B master alloy are discussed based on the duplex nucleation theory and growth restriction effects in aluminium alloys.

The present paper investigates the grain size evolution in aluminium alloys AA 6082 and AA 7020 during hot forward extrusion process. The aim of the present work is the definition and implementation of a predictive algorithm that is able to compute the evolution of the grain shape during the process within the finite element method code Deform. Extrusion experiments were performed at two levels: at reduced scale for investigating and identifying the predictive equations and at industrial scale for validating the developed algorithm. At small scale extrusion, a complete factorial plan was performed for two alloys at three different temperatures, three extrusion ratios and two ram speeds: the discards and extrudates from the experiments were quenched immediately in order to avoid any potential recrystallisation, hence allowing measurements of transitional processing steps. At the industrial scale, instead, the 7020 alloy was extruded with two different die designs, thus producing a 20 mm diameter round bar under different extrusion ratios and strain paths. Finite element simulations were initially validated over visioplastic investigations in order to establish an accurate computation of the material flow, then experimental and numerical results were coupled, thus allowing the definition of the grain evolution model that was successfully integrated and validated on industrial scale trials.

The Mg–3·08Nd–0·27Zn–0·46Zr (wt-%) alloy was hot extruded with different extrusion ratios to refine microstructure and to optimise mechanical properties and corrosion resistance as a biodegradable magnesium alloy. The microstructure was observed by optical microscopy, the mechanical properties were tested at room temperature and the corrosion resistance was evaluated in Hanks’ solution. The results show that the microstructure becomes more and more homogeneous with increasing the extrusion ratio. The mechanical properties of the as extruded alloy are much better than those of the as cast one. Higher strength is obtained with the extrusion ratio of 18, and better elongation is obtained with the extrusion ratio of 25. Corrosion results show that corrosion resistance of the as extruded alloy immersed in Hanks’ solution is better than that of the as cast one, and higher extrusion ratio results in better corrosion resistance. Both the as cast and the as extruded alloys exhibit uniform corrosion, which was interpreted by cyclic polarisation test.

Applying fluxing method, hypercooling was achieved in Co₈₀Pd₂₀ alloys. The rapidly solidified microstructures and substructures of hypercooled Co₈₀Pd₂₀ alloys, subjected to rapid quenching and natural cooling after recalescence, have been studied. Transmission electron microscopy was used to investigate the substructures such as dislocations in the as solidified microstructures. X-ray diffraction line profile analysis was used for illuminating and determining the microstrain due to rapid solidification in the as solidified microstructures. In the undercooling range studied, the microstrain in the rapidly solidified structures increases with increasing undercooling. Quenching immediately after rapid solidification is helpful to preserving the microstrain in the microstructures.

In order to improve hydrogen storage performances of CeMg₁₂ type alloys, ball milling technology was used for preparing nanocrystalline/amorphous CeMg₁₂+100%Ni composite hydrogen storage alloys. The microstructures and morphologies of alloy samples were characterised by X-ray diffraction, scanning electron microscopy and high resolution transmission electron microscopy. The electrochemical hydrogen storage characteristics of as milled alloys were tested by an automatic galvanostatic system. The electrochemical impedance spectra were plotted by an electrochemical workstation (PARSTAT2273). The hydrogen diffusion coefficients D in the alloys were calculated by virtue of potential step method. The results show that the amount of nanocrystalline/amorphous Mg₂Ni phase and Ni phase within alloy samples increase with prolonging milling time. Prolonging of ball milled duration markedly improves the electrochemical discharge properties and cyclic stability of alloy samples. The amorphisation degree of the milling alloys increases with rising milling duration. Furthermore, the high rate dischargeability, electrochemical impedance spectra and potential step measurement all indicate that electrochemical kinetics of alloy electrodes first increases and then decreases with increasing ball milling.