Hot workability and processing maps of aluminium based discontinuously reinforced composites

Abstract

Hot deformation behaviour under tensile conditions of Al6061/20%Al₂O₃ , Al2618/20%Al₂O₃, and Al2618/20%SiC particle reinforced composites and of an unreinforced 6061 aluminium alloy was investigated in the temperature range 350–500°C and in the strain rate range 10^–3-10^–1s^–1. Processing maps were generated from the obtained data and the domains of optimal hot workability were identified for the different materials. Microstructural analyses on deformed specimens allowed it to be stated that dynamic recovery was the main restoration mechanism for the composites investigated. The concurrent evaluation of ductility and flow stress data also suggested that differences in reinforcement properties can play a significant role on property definition.     

The workability evaluation of wrought AZ80 magnesium alloy in hot compression

Workability, an important parameter in metal forming process, can be evaluated by means of processing maps on the basis of dynamic materials model (DMM), constructed from experimentally generated flow stress variation with respect to strain, strain rate and temperature. To obtain the processing maps of wrought AZ80 magnesium alloy, hot compression tests were performed over a range of temperatures 523–673 K and strain rates 0.01–10 s⁻¹. As the true strain is 0.25, 0.45, 0.65, 0.85 respectively, the response of strain-rate sensitivity (m-value), power dissipation efficiency (η-value) and instability parameter (ξ-value) to temperature and strain rate were evaluated. By the superimposition of the power dissipation and the instability maps, the stable, metastable and unstable regions were clarified clearly. In further, in the stable area the regions having the highest efficiency of power dissipation were identified and recommended. The optimal working parameters identified by the processing maps contribute to designing the hot forming process of AZ80 magnesium alloy without any defect.     

A study on the hot workability of wrought NiTi shape memory alloy

The hot workability of a wrought 49.8 Ni-50.2 Ti (at pct) alloy was assessed using the hot compression tests in temperature range of 700-1000 °C, strain rate of 0.001-1 s⁻¹, and the total strain of 0.7. The constitutive equations of Arrhenius-type hyperbolic-sine function was used to describe the flow stress as a function of strain rate and temperature. The preferable regions for hot workability of the alloy were achieved at Z (Zener-Holloman parameter) values of about 10⁹-10¹³ corresponding to the peak efficiency of 20-30% in the processing map. However, a narrow area in the processing map including the deformation temperature of 1000 °C and strain rate of 1 s⁻¹ is inconsistent with the related Z values. A flow instability region was observed at high Z values. Further instability regions were found at low temperature of 700 °C and low strain rates of 0.01-0.001 s⁻¹ as well as at high temperature of 1000 °C and high strain rate of 1 s⁻¹. The apparent feature of flow curves, the low value of peak efficiency, the similarity between the estimated apparent activation energy of deformation and that of the self diffusion of Ti in Ni, and the stress exponent of higher than 5, suggested that dynamic recovery (DRV) is the dominant restoration phenomenon during the hot working of the alloy.     

High-resolution elemental maps for three directions of Mg2Si phase in Al-Mg-Si alloy

Elemental maps of the Mg and Si sub-lattices of the Mg₂Si phase in an Al-1.0mass% Mg₂Si alloy were produced using an energy-filtering transmission electron microscope (EFTEM). Low magnification elemental maps were obtained using both low and high energy loss edges, and the intensities of the high energy loss edges were sufficiently high to allow the Mg₂Si phase to be observed at high magnification. High-resolution core-loss images of Mg and Si-K edges were taken parallel to [001], [111] and [110] of the Mg₂Si phase. In the [110] direction, Mg and Si atoms were successfully identified as sub-lattices. The Mg atoms formed a 0.39 nm diamond network, whereas the Si atoms formed a 0.32 nm by 0.22 nm rectangular network. This result is in good agreement with the projected potential of the Mg₂Si phase in the [110] direction. This is the first report of magnesium and silicon atoms in the Mg₂Si phase being successfully identified at the atomic level by EFTEM.     

Research on the hot deformation behavior of Ti40 alloy using processing map

The deformation behavior of a Ti40 titanium alloy was investigated with compression tests at different temperatures and strain rates to evaluate the activation energy and to establish the constitutive equation, which reveals the dependence of the flow stress on strain, strain rate and deformation temperature. The tests were carried out in the temperature range between 900 and 1100 °C and at strain rates between 0.01 and 10 s⁻¹. Hot deformation activation energy of the Ti40 alloy was calculated to be about 372.96 kJ/mol. In order to demonstrate the workability of Ti40 alloy further, the processing maps at strain of 0.5 and 0.6 were generated respectively based on the dynamic materials model. It is found that the dynamic recrystallization of Ti40 alloy occurs at the temperatures of 1050-1100 °C and strain rates of 0.01-0.1 s⁻¹, with peak efficiency of power dissipation of 64% occurring at about 1050 °C and 0.01 s⁻¹, indicating that this domain is optimum processing window for hot working. Flow instability domains were noticed at higher stain rate (≥1 s⁻¹) and stain (≥0.6), which located at the upper part of the processing maps. The evidence of deformation in these domains has been identified by the microstructure observations of Ti40 titanium alloy.     

Compressive deformation behavior of Al 2024 alloy using 2D and 4D processing maps

The hot deformation behavior of Al 2024 was studied by isothermal hot compression tests in the temperature range of 250–500 °C and strain rate range of 10⁻³ to 10² s⁻¹ in a computer-controlled 50 kN servo-hydraulic universal testing machine (UTM). The results show that the flow stress of Al 2024 alloy increases with strain rate and decreases after a peak value, indicating dynamic recovery and recrystallization. The processing map exhibits two domains of optimum efficiency for hot deformation at different strains, including the low strain rate domain at 500 °C and between 10⁻² and 10⁻¹ s⁻¹ and the high strain rate domain in 250 and 300 °C in the strain rate range of 10¹ to 10² s⁻¹. An attempt has been made in this article to generate a new hybrid 4D process map which illustrates contours of power dissipation and instability in the 3D space of strain rate, temperature, and strain.     

The effect of shock-loading on the aging behavior of an Al-Mg-Si alloy

The impact of pre-shock loading on the precipitation reactions in an Al-Mg-Si alloy (AA6022) was studied by means of differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and hardness measurements. The samples were solutionized and quenched in water prior to subsequent shock-loading and aging treatment. The TEM and DSC results show that, while shock-loading prior to aging facilitated the precipitation of Q ′ and β, no significant effect on β′′ precipitates was observed. The hardness studies indicate that pre-shock loading strengthens the material by forming a high concentration of microstructural defects, however the resultant mechanical properties of the shocked sample are comparable to those without shock processing at the peak of aging. It was found that the rate of overaging is higher in shocked samples, which is in agreement with the DSC and TEM results.     

Hot workability analysis with processing map and texture characteristics of as-cast TX32 magnesium alloy

Hot deformation behavior of as-cast TX32 (Mg–3Sn–2Ca) alloy has been studied in uniaxial compression in the temperature and strain rate ranges of 300–500 °C and 0.0003–10 s^?1 with a view to characterize the evolution of microstructure and texture. On the basis of the temperature and strain rate dependence of flow stress, a processing map has been developed and the crystallographic orientation information on the deformed specimens has been obtained from electron back scatter diffraction micro-texture analysis. The processing map revealed two domains of dynamic recrystallization in the temperature and strain rate ranges of (1) 300–350 °C and 0.0003–0.001 s^?1 and (2) 390–500 °C and 0.005–0.6 s^?1. Specimens deformed at peak in Domain 1 exhibited maximum intensity of basal poles located at about 35–45° to the compression axis while those deformed at peak in Domain 2 showed near-random texture. Schmid factor analysis of different slip systems operating in the two domains suggests that basal + prismatic slip causes the basal texture in Domain 1 while second-order pyramidal slip randomizes the texture in Domain 2.     

Hot workability of an Al-Mg alloy AA5182 with 1 wt% Cu

A comparative study of the hot workability of two aluminium alloys, alloy AA5182 used for automotive applications and a variant modified with 1 wt% copper, has been carried out. Hot torsion tests were performed on both alloys subjected to two different heat treatments: a low temperature preheat to 450 °C and a high temperature preheat at 540 °C. The results from the torsion experiments are interpreted in terms of microstructural features. Both treatments produce the same strength, but the high temperature preheat leads to better ductility. This improvement is related to the homogenization of solute elements in the matrix; and, concerning AA5182 + Cu, also to the dissolution of a non-equilibrium Al-Mg-Cu ternary eutectic present in the as-cast microstructure. The precipitation of (Fe, Mn)Al₆ precipitates in the matrix of both alloys is induced by the high temperature heat treatment. Comparison of the results obtained by hot torsion shows that at low deformation rates AA5182 + Cu has better ductility than the classical alloy, but its ductility is lower at strain rates above 0.6–0.8 s^–1. The null ductility transition temperature is lower compared with that in the classical alloy, restricting the range of hot working temperatures. Inside this range the strength of both alloys is approximately the same, although the strain rate sensitivity coefficient is increased by copper additions. The experimental strength values follow the classical sinus-hyperbolic constitutive equation for hot working.     

Evaluation of hot workability of particle reinforced aluminum matrix composites by using deformation efficiency

The high temperature deformation behavior of Al 6061 composites reinforced with SiC and Al₂O₃ particles has been studied in the temperature range of 300–550°C and the strain rate range of 0.1–3.0/sec by hot torsion test. The deformation efficiency , given by (2m/m + 1), where m is the strain rate sensitivity, is calculated as a function of temperature and strain rate to obtain iso-efficiency contour map. The composite reinforced with SiC particle exhibited a domain of dynamic recrystallization (DRX) with a peak efficiency of 40% at the temperature range of 450–500°C and strain rate range of 0.2–0.5/sec. On the other hand, the composite reinforced with Al₂O₃ particle showed the DRX domain at the temperature range of 450–480°C and strain rate range of 0.1–0.2/sec. The characteristics of these domain have been investigated with the help of microstructural observation and hot ductility measurements.     

Hot working of Ti-17 titanium alloy with lamellar starting structure using 3-D processing maps

Isothermal compression of Ti-17 titanium alloy with lamellar starting structure at the deformation temperatures ranging from 780 °C to 860 °C, the strain rates ranging from 0.001 to 10 s⁻¹, and the height reductions ranging from 15% to 75% with an interval 15% were carried out. Based on experimental results, 3-D processing maps including strain were developed and used to identify various microstructural mechanisms and distinguish the safe and unsafe domains. The processing maps exhibit two maximum power dissipation efficiency domains and dynamic globularization takes place in this two domains. The first domain occurs at 800–860 °C and at strain rates lower than 0.01 s⁻¹, and the second occurs at 780–800 °C and at strain rates lower than 0.01 s⁻¹. With the increasing of the strains, the values of maximum power dissipation efficiency in this two domains increase. One flow instability domain due to adiabatic shear bands and lamellar kinking occurs at strain rates higher than 0.487 s⁻¹, lower temperature, and higher strain above 0.2. The instability deformation region increases with increasing strain, strain rate, and decreasing temperature.     

Optimising sulfuric acid hard coat anodising for an Al-Mg-Si wrought aluminium alloy

This research evaluates the effects of sulfuric acid hard coat anodising parameters, such as acid concentration, electrolyte temperature, current density and time, on the hardness and thickness of the resultant anodised layers. A small scale anodising facility was designed and set up to enable experimental investigation of the anodising parameters. An experimental design using the Taguchi method to optimise the parameters within an established operating window was performed. Qualitative and quantitative methods of characterisation of the resultant anodised layers were carried out. The anodised layer’s thickness, and morphology were determined using a light optical microscope (LOM) and field emission gun scanning electron microscope (FEG-SEM). Hardness measurements were carried out using a nano hardness tester. Correlations between the various anodising parameters and their effect on the hardness and thickness of the anodised layers were established. Careful evaluation of these effects enabled optimum parameters to be determined using the Taguchi method, which were verified experimentally. Anodised layers having hardness varying between 2.4–5.2 GPa and a thickness of between 20–80 μm were produced. The Taguchi method was shown to be applicable to anodising. This finding could facilitate on-going and future research and development of anodising, which is attracting remarkable academic and industrial interest.     

TEM study of microstructure development during low-cycle fatigue of an overaged Al-Mg-Si alloy

The evolution of dislocation structure during room-temperature, uniaxial, low-cycle fatigue of an overaged Al-Mg-Si alloy is studied. Ageing at 450°C produces a fine dispersion of Mg₂Si precipitate particles. During fully reversed strain-controlled cyclic tests, these fine particles restrict deformation to local regions and a stable dislocation substructure is developed early in fatigue life. Substructural observations of hardening and saturation by transmission electron microscopy reveal extensive dislocation band formation on Mg₂Si precipitate rods. Various microstructural features such as configuration of tangled dislocations, dislocation cells, precipitate morphologies, sizes, precipitate-free zones, etc., have been examined during cyclic hardening and saturation. The results have been analysed in terms of kinematic and isotropictype microstructural mechanisms.     

Hot deformation and processing maps of extruded ZE41A magnesium alloy

The hot deformation behavior and microstructure evolution of extruded ZE41A magnesium alloy has been studied using the processing map. The compression tests were conducted in the temperature range of 250–450 °C and the strain rate range of 0.001–1.0 s⁻¹ to establish the processing map. The dynamic recrystallization (DRX) and instability zones were identified and validated through micrographs. The observations were performed in order to describe the behavior of the material under hot forming operation in terms of material damage and micro-structural modification.     

Optimization of hot extrusion process for AZ61 magnesium alloy carriers

This study applies fuzzy-base Taguchi method to investigate the optimal process parameters of the multiple performance characteristics index (MPCI) for the hot extrusion process of AZ61 magnesium alloy products. Flattening strength, fracture strength and extrusion load are taken as the input data of MPCI. Since the optimal combination of process parameters for Taguchi method varies with individual quality characteristic, the optimal combination of parameters for different quality characteristics may be contradictory to each other. In order to consider the larger-the-better quality characteristic of flattening strength and T-slot fracture strength values as well as the smaller-the-better quality characteristic of extrusion load, the fuzzy-base Taguchi method is used to analyze and to obtain the optimal combination of process parameters for the MPCI. First of all, orthogonal array is applied to arrange the experimental combination of extrusion process. The signal-to-noise (S/N) ratios of the three quality characteristics of flattening strength, T-slot fracture strength and extrusion load for the products acquired from experiments are calculated. The S/N ratio serves as the input variable of fuzzy control unit, whereas MPCI serves as a single output variable. The acquired MPCI is employed to analyze the optimal process parameters. In this study, an optimal combination of process parameters of AZ61 carrier for MPCI is obtained, and the verification experiments are conducted to prove the accuracy. Moreover, mechanical properties of AZ31 and AZ61 magnesium alloy carriers are tested for further comparison.     

Failure analysis of a hot extrusion die used for Al-Li alloy processing

Al-Li alloys being developed as lighter, substitutes for conventional high strength Al alloys are to be processed by routine methods. During extrusion of a 8090 Al-Li alloy, the extrusion die container failed causing some alarm. This failed die container was analysed to examine if the failure was caused by interaction of Li diffusing out of Al-Li alloy with the carbides of die steel. The evidence, although not conclusive, is sufficient to exercise caution during such processing.     

Simultaneously enhancing the hot workability and room-temperature strength of Ti-6Al-4V alloy via adding Mo and Fe

Reducing the hot working temperature and high-temperature deformation resistance of titanium alloy to improve hot rolling and hot extrusion workability of products with thin walls and complex section shapes has always been an important topic in the field of titanium alloy processing.This paper proposed a strategy of adding Mo and Fe elements to simultaneously reduce the hot working temperature and high-temperature deformation resistance of Ti-6Al-4V alloy.The effects of Mo and Fe contents on the mi-crostructure,β transus temperature(Tp),and high-temperature flow stress(HFS)of Ti-6Al-4V-xMo-xFe(x=0-5)alloys were investigated.The results showed that adding Mo and Fe can substantially reduce the Tp and HFS of the alloy,and greatly improve its room-temperature strength.Compared with com-mercial Ti-6Al-4V samples,the Tβ of Ti-6Al-4V-2Mo-2Fe and Ti-6Al-4V-3Mo-3Fe samples was decreased by 68-98 ℃,and the HFS at 800-900 ℃ was decreased by 37.8％-46.0％.Compared with hot-rolled Ti-6Al-4V samples,the room-temperature tensile strength of hot-rolled Ti-6Al-4V-2Mo-2Fe samples was increased by about 30％,while the elongation hardly decreased.The increased strength was mainly at-tributed to fine grain strengthening and solid solution strengthening.The hot workability and room-temperature strength of Ti-6Al-4V alloy can be significantly improved by adding 2-3 wt.％Mo and Fe simultaneously.