Analysis of crack in aluminium alloy AA2219 weldment

Aluminium alloy AA2219 is used for fabrication of propellant tanks of launch vehicles. After welding of one of the heat treated plate (T87 temper condition) with a rolled ring (T851 temper condition), crack has been noticed near the weldment. A thorough metallographic analysis was carried out to investigate the cause for development of crack, using optical and scanning electron microscope. Morphology of cracks alongwith the attributable factors are discussed and the remedial measures are suggested.     

Fatigue behaviour of post weld heat treated electron beam welded AA2219 aluminium alloy joints

This paper reports the effect of post weld heat treatment on fatigue behaviour of electron beam welded AA2219 aluminium alloy. An attempt has been made to enhance the fatigue strength of the electron beam welded joints through post weld heat treatment methods such as solution treatment, artificial aging, solution treatment and artificial aging. Electron beam welding machine with 100 kV capacity has been used to fabricate the square butt joints. Servo hydraulic controlled fatigue testing machine with a capacity of 100 kN has been used to evaluate the fatigue life of the welded joints. Of the three post weld heat treated joints, the solution treated and aged joints are enduring higher number of cycles under the action of cyclic loads.     

Structure-property correlation on AA 2219 aluminium alloy weldments

AA 2219 aluminium alloy is the material proposed for the construction of tanks for liquid cryogenic fuels like liquid oxygen and liquid hydrogen, in Indian launch vehicle programmes. To meet the material requirement, ISRO has developed the alloy AA 2219 indigenously on an industrial scale. Process development and structure-property correlation of auto TIG weldments of AA 2219 Al alloy reported in this paper establishes that postweld reaging of AA 2219 weldments in T62 condition enhances both strength and ductility.     

Fabrication and properties of WC–10Co cemented carbide reinforced by multi-walled carbon nanotubes

High-velocity parting-off has been applied to 80 mm bars of pearlitic 100CrMn6, resulting in shear localisation and white-etching bands in a severely deformed region below the fracture surface. Electron microscopy showed that going from the bulk material towards the fracture surface the grains become elongated and refined. The region below the fracture surface can be divided into three subzones: 50–100 μm below the surface grains are elongated, cementite lamellae are distorted, break up and the lamellar spacing decreases. <50 μm below the fracture surface the microstructure becomes a mix of cementite lamellae and carbides in a ferrite matrix. Within the white-etching band the microstructure consists of equiaxed ferrite refined to a grain size of 50–150 nm. Several twinned regions caused by the deformation can be observed. Selected area electron diffraction and low angle convergent beam electron diffraction indicate nanocrystalline cementite dispersed in the ferrite matrix.     

Effect of prestrain with a path change on the strain rate sensitivity of AA5754 sheet

The effect of prestrain with a path change on the strain rate sensitivity of AA5754 sheet was investigated. Prestrain magnitudes between 0% and 12% were applied in plane strain in either the transverse or longitudinal (rolling) material direction. Samples were then loaded in uniaxial tension in the longitudinal direction at strain rates of 0.001/s and 0.1/s. Results show that when a path change is involved between prestrain and subsequent uniaxial loading, the strain rate sensitivity of the hardening rate at 0.1/s compared to 0.001/s is reduced. The rate sensitivity of the yield stress remains constant with increasing magnitudes of prestrain, while the rate sensitivity of the elongation to failure decreases with increasing prestrain. A permanent softening of the flow stress is also observed, which is greater when the path change is combined with a change in orientation.     

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.     

High strain rate superplasticity of TiNP/2014Al composite

Superplasticity of the TiN_p/2014AI composite prepared by powder metallurgy method was investigated by tensile tests conducted at different temperatures (773, 798, 818 and 838 K) with different strain rates range from 1·7×10° to 1·7×10^?3s^?1. Results show that a maximum elongation of 351% is achieved at 818 K and 3·3·10^?1s^?1. At different deformation temperatures, the curves of m value can be divided into two stages with the variation of strain rate and the critical strain rate is 10^?1 s^?1. Superplastic deformation activation energy in the TiN_p/2014AI composite is 417 kJ mol^?1, which is related to liquid phase formation at triple points of grain boundaries and interfaces between the matrix and the reinforcement. Superplastic deformation mechanism of the TiN_p/2014AI composite is grain boundary sliding accommodate mechanism when the strain rate is lower than 10^?1 s^?1, and transfers to grain boundary sliding accommodation mechanism plus liquid phase helper accommodation mechanism when the strain rate is higher than 10^?1 s^?1     

Effects of strain rates on dynamic deformation behavior of Cu-20Ag alloy

Copper alloy is widely used in high-speed railway,aerospace and other fields due to its excellent electri-cal conductivity and mechanical properties.High speed deformation and dynamic loading under impact load is a complex service condition,which widely exists in the field of national defense,military and industrial application.Therefore,the dynamic deformation behavior of the Cu-20Ag alloy was inves-tigated by Split Hopkinson Pressure Bar (SHPB) with the strain rates of 1000-25000 s-1,high-speed hydraulic servo material testing machine with the strain rates of 1-500 s-1.The effect of strain rate on flow stress and adiabatic shear sensitivity was analyzed.The results show that the increase of strain rate will increase the flow stress and critical strain,that is to say,the increase of strain rate will reduce the adiabatic shear sensitivity of the Cu-20Ag alloy.The Cu-Ag interface has obvious orientation relationship with (111)Cu//(111)Ag;((1)11)Cu//((1)11)Ag;((2)00) Cu//((2)00)Ag and[0(1)1]Cu//[0(1)1]Ag with the increase of strain rate.The increase of strain rate promotes the precipitation of Ag and increases the number of interfaces in the microstructure,which hinders the movement of dislocations and improves the stress and yield strength of the Cu-20Ag alloy.The concentration and distribution density of dislocations and the precipitation of Ag were the main reasons improve the flow stress and yield strength of the Cu-20Ag alloy.     

Effect of strain rate and temperature on the deformation behavior in a Ti-23.1Nb-2.0Zr-1.0O titanium alloy

The compression behavior of a Ti-23.1 Nb-2.0Zr-1.0O (at.％) alloy was investigated at strain rates from 0.1 s-1 to 1000 s-1 and temperatures from 100 ℃ to 200 ℃ on a Gleeble 3800 system and Split Hopkinson Pressure Bar (SHPB) compressive tester.Optical microscopy,electron backscatter diffraction (EBSD),X-ray diffraction (XRD) and transmission electron microscopy (TEM) were employed to characterize the microstructure evolution during the deformation.Numerous deformation phenomena,including dislocation slip,twinning of both {332}<113> and {112}<111> modes,stress-induced α" martensite (SIMα") and stress-induced ω (SIω) transformations,were observed.The preferred activation of twinning and SIω transformations was observed in the sample compressed at lower temperatures and/or higher strain rates.The underlying mechanism is that twinning and stress induced phase transformations are attribute to higher stress concentrations at β grain boundaries and additional energy supplied by a higher strain rate,as well as high stacking fault energy because of higher temperature.     

Effect of texture on high temperature deformation behavior at high strain rates in a Mg–3Al–1Zn alloy

A microstructure optimization design method of the forging process is proposed. The optimization goal is a small grain size and a homogeneous grain distribution of the forgings. The optimization object is the preforming die shape. The microstructure optimization code is developed using the micro-genetic algorithm and the finite element method. The two forming steps including the preforming process and the final forging process of H-shape forgings are analyzed using the self-developed code. The optimization results show that small grain size and homogeneous grain distribution can be achieved by controlling the shape of the preforming die. Samples of the same size as in the optimization are preformed and then forged to the desired H-shape forgings under the same deformation conditions as in the optimization. Micrographs in the symmetry section of samples show that the grain sizes of the forgings almost coincide with the optimization results.     

Microstructure evolution of ZK40 magnesium alloy during high strain rate compression deformation at elevated temperatures

Abstract

Microstructure evolution of the homogenised ZK40 magnesium alloy was investigated during compression in the temperature range of 250–400°C and at the strain rate range of 0·01–50 s^?1. At a higher strain rate (?10 s^?1), dynamic recrystallisation developed extensively at grain boundaries and twins, resulting in a more homogeneous microstructure than the other conditions. The hot deformation characteristics of ZK40 exhibited an abnormal relationship with the strain rate, i.e., the hot workability increased with increasing the strain rate. However, the dynamic recrystallisation grain size was almost the same with increasing the temperature at the strain rate of 10 s^?1, while it increased obviously at the strain rates of 20 and 50 s^?1. Therefore, hot deformation at the strain rate of 10 s^?1 and temperature range of 250–400°C was desirable and feasible for the ZK40 alloy.     

变形温度与应变速率耦合作用对TWIP效应Ti-15Mo合金力学性能的影响EI北大核心CSCD

变形温度和应变速率均影响β型钛合金的力学性能,且其影响均关联塑性变形过程中变形方式的变化。利用TEM,EBSD,SEM,XRD,OM和拉伸试验机研究变形温度和应变速率耦合作用对{332}〈113〉孪生诱发塑性效应Ti-15Mo合金力学性能的影响。结果表明:在298 K和573 K下,屈服强度均随应变速率的增加逐渐升高,即依赖于位错热激活过程,且573 K下显著的位错热激活作用使得屈服强度表现出更大的应变速率依赖性。不同于298 K下,Ti-15Mo合金在573 K下通过{332}〈113〉孪生和位错滑移耦合变形;构建的流变应力模型表明位错强化成为其主要强化方式。高应变速率下,塑性变形早期形成的更多孪晶虽然会抑制孪生的进一步产生降低加工硬化率,但同时有效降低位错不均匀分布引起的局部应力集中延缓颈缩的发生;两个方面的共同作用使得Ti-15Mo合金在变形温度和应变速率耦合作用下呈现出更小的应变速率依赖性。     

Effects of Al addition on high strain rate deformation of fully austenitic high Mn steels

The effects of Al addition on dynamic flow response of the fully austenitic high Mn steel were investigated by conducting high strain rate compression tests on Fe-22Mn-xAl-0.6C steels (x = 0, 3, 6 in wt.%). While dynamic yield strength of the 0 Al steel and the 3 Al steel were comparable, the 6 Al steel exhibited the highest one. Meanwhile, strain hardenability of the 0 Al steel was the highest and that of other two steels was nearly same. Under the present dynamic loading, no obvious dynamic recrystallization by adiabatic heating was observed in all steels. Fully compressed microstructures revealed (a) ?-martensite and mechanical twin bands for the 0 Al steel, (b) multi-layer deformation bands and mechanical twin bands for the 3 Al steel, and (c) a variety of dislocation configurations such as the directional slip traces, tangled dislocations, and incomplete dislocation cells for the 6 Al steel. These findings inform that dynamic flow of the 0 Al steel was associated with both TRIP and TWIP, and that of other two steels was dominated by dislocation gliding - mainly, planar glide for the 3 Al steel and the combination of both planar glide and wavy glide for the 6 Al steel. The dynamic flow response of the present steels was discussed in terms of the stacking fault energy affected by the Al content and adiabatic heating during dynamic loading and of the strain rate effect on the critical stress for mechanical twinning.     

Effect of aging on high strain rate and high temperature properties of 7075 aluminium alloy

Abstract

The high strain rate and high temperature properties of as cast and aged 7075 aluminium alloy were examined by metallographic observation and by means of a split Hopkinson bar test at temperatures between 25 and 300°C and at strain rates of 1·3 × 10³ and 3·1 × 10³ s^-1. The effect of aging, as well as strain rate and temperature, on the dynamic mechanical response, microstructure evolution, and fracture characteristics are presented. The compressive stress–strain response of as cast and aged 7075 alloy is found to depend strongly on both the applied strain rate and the test temperature. However, the aged material is generally found to be stronger than the as cast material. The work hardening rate is seen to decrease with increasing strain, strain rate, and temperature, and its value is higher in the aged material than in the as cast material. Microscopic observation shows that aging, strain rate, and temperature have a significant influence on the microstructural evolution and the changes in grain morphologies. The average grain size can be expressed by a Hall–Petch type relationship after impact deformation. Fracture surface examination revealed that a high strain rate favours the formation of deformed shear bands that are precursors to crack formation and fracture. The aged material has a better ductility owing to the higher percentage of transgranular fracture and an increased density of microdimples.     

Microstructure and property of nitrogen-alloyed high manganese austenitic steel under high strain rate tension

The microstructures and mechanical properties of 32Mn–7Cr–1Mo–0.3N steel under high strain rate tension companied with different deformation temperature are investigated by using of the split-Hopkinson pressure bar (SHPB). The results show that with increasing the strain rate and decreasing the deformation temperature the strength increase, but the elongation and the area reduction do not obviously decrease. The fracture surfaces of the tensile specimens all exhibit ductile characters with many dimples. The X-ray diffraction analysis (XRD) results show no ′-martensite in all specimens. The transmission electron microscope (TEM) observations further confirm that the deformation microstructures are mainly composed of deformation twins and slipping bands or stacking faults.     

Mesoscopic grain boundary sliding as the rate controlling process for high strain rate superplastic deformation

Important features observed during high strain rate superplastic deformation are enumerated. Starting from the premise that the phenomenon of structural superplasticity in different classes of materials results when grain boundary sliding that develops to a mesoscopic scale (defined to be of the order of a grain diameter or more) controls the rate of flow, the particular case of high strain rate superplasticity is explained. The rate equation developed is validated using experimental results concerning 5 alloy systems in which an ultra-fine grain size is developed by thermomechanical processing and retained in a similar condition during superplastic deformation by fine, grain boundary pinning particles and 3 alloy composites in which the volume fraction of the reinforcing constituent is significant (15–25%). It is demonstrated that the analysis results in estimates for the externally measured strain rates that are within a factor of two, in addition to providing a physically meaningful free energy of activation for the rate controlling process. This approach explains superplastic flow in different classes of materials in terms of a single rate controlling mechanism of deformation, viz., mesoscopic grain boundary sliding, with the help of a few constants that have the same values for all systems. The system-dependent variables of threshold stress needed for the onset of mesoscopic boundary sliding and free energy of activation are obtained directly from superplasticity stress–strain rate data, without external inputs.     

Texture evolution and dynamic mechanical behavior of cast AZ magnesium alloys under high strain rate compressive loading

In this study, texture and compressive mechanical behavior of three cast magnesium alloys, including AZ31, AZ61 and AZ91, were examined over a range of strain rates between 1000 and 1400 s⁻¹ using Split Hopkinson Pressure Bar. Texture measurements showed that after shock loading, initial weak texture of the cast samples transformed to a relatively strong (00.2) basal texture that can be ascribed to deformation by twinning. Furthermore, increasing the aluminum content in the alloys resulted in increase in the volume fraction of β-Mg₁₇Al₁₂ and Al₄Mn phases, strength and strain hardening but ductility decreased at all strain rates. Besides, it was found for each alloy that the tensile strength and total ductility increased with strain rate. By increasing the strain rate, the maximum value of strain hardening rate occurred at higher strains. Also, it is suggested that a combination of twinning and second phase formation would affect the hardening behavior of the cast AZ magnesium alloys studied in this research.     

Evaluation of low strain rate constitutive equation of 7075 aluminium alloy at high temperature

Abstract

The 7075 aluminium alloy is one of the most important engineering alloys utilised extensively in aircraft and transportation industries due to its high specific strength. In the present research, the flow behaviour of this alloy has been investigated using hot compression test at strain rates of 0·001, 0·01, 0·1 and 1 s^?1 and temperatures of 350, 400 and 450°C. The results reveal that dynamic softening occurred in these temperatures and strain rates. The activation energy, strain rate sensitivity and two constitutive equations (hyperbolic sine law and the power law) are derived from the results. It is shown that the hyperbolic sine law has a better agreement with the experimental results.     

应变速率对(Zr72Cu16.5Ni11.5)90Al10大块非晶合金超塑性变形行为的影响

通过合理配置成分及优化工艺参数,用铜模吸铸法成功制备出具有室温超塑性变形能力的(Zr72Cu16.5Ni11.5)(90)Al(10)大块非晶合金,并对其塑变效应进行研究,结果表明,在4.2×10-4-6.0×10-5s(-1)应变速率范围内合金试样均呈现出室温超塑性,即经历85.5%工程应变或193.1%真应变仍无脆...