Fatigue behaviour of dissimilar Al 5052 and Mg AZ31 resistance spot welds with Sn‐coated steel interlayer

The fatigue property of dissimilar spot welds between an aluminium alloy (AA5052) and a magnesium alloy (AZ31) was studied in this research. The AA5052 and AZ31 coupons were resistance spot welded together by using an interlayer of Sn‐coated steel between the two coupons. The fatigue test results revealed that the Mg/Al joints had the same level of fatigue strength as Mg/Mg resistance spot welds. It was found that within the life range of N_f < 10⁵ cycles, Mg/Al welds degraded faster than Mg/Mg joints. This was attributed to the larger bending moment on the plane of fatigue failure in the Mg/Al welds. Three failure modes were observed under different cyclic loading regimes: Al/steel interfacial failure, Mg coupon failure and Al coupon failure. Fatigue fracture surface of Mg/Al welds consisted of two distinct regions: crack propagation region with brittle morphology and final rupture with ductile morphology.     

Improvement in the fatigue strength of chromium electroplated AISI 4340 steel by shot peening

In landing gear, an important mechanical component for high responsible applications, wear and corrosion control is currently accomplished by chrome plating or hard anodising. However, some problems are associated with these operations. Experimental results have also shown that chrome‐plated specimens have fatigue strength lower than those of uncoated parts, attributed to high residual tensile stress and microcracks density contained into the coating. Under fatigue conditions these microcracks propagate and will cross the interface coating‐substrate and penetrate base metal without impediment. Shot peening is a surface process used to improve fatigue strength of metal components due to compressive residual stresses induced in the surface layers of the material, making the nucleation and propagation of fatigue cracks difficult. This investigation is concerned with analysis of the shot peening influence on the rotating bending fatigue strength of hard chromium electroplated AISI 4340 steel. Specimens were submitted to shot peening treatment with steel and ceramic shots and, in both cases, experimental results show increase in the fatigue life of AISI 4340 steel hard chromium electroplated, up to level of base metal without chromium. Peening using ceramic shot resulted in lower scatter in rotating bending fatigue data than steel shots.     

Fatigue behaviour of dissimilar friction stir spot welds between A6061‐T6 and low carbon steel sheets welded by a scroll grooved tool without probe

A6061 and low carbon steel sheets, whose thicknesses were 2 mm, were welded by a friction stir spot welding (FSSW) technique using a scroll grooved tool without probe (scroll tool). Tensile‐shear fatigue tests were performed using lap‐shear specimens at a stress ratio R = 0.1, and the fatigue behaviour of dissimilar welds was discussed. Tensile‐shear force of the dissimilar welds was higher than that of the A6061 similar ones. Furthermore, the dissimilar welds exhibited nearly the same fatigue strengths as the A6061 similar ones, indicating FSSW by a scroll tool was effective technique for joining aluminium to steel sheet. Fatigue fracture modes of the dissimilar welds were dependent on load levels, where shear fracture through the interface between A6061 and steel occurred at high load levels, while crack grew through A6061 sheet at low load level.     

Effect of intermetallic particles and grain boundaries on short fatigue crack growth behaviour in a cast Al–4Cu–3Ni–0.7Si piston alloy

The short fatigue crack growth behaviour in a model cast aluminium piston alloy has been investigated. This has been achieved using a combination of fatigue crack replication methods at various intervals during fatigue testing and post‐mortem analysis of crack profiles. Crack–microstructure interactions have been clearly delineated using a combination of optical microscopy, scanning electron microscopy and electron backscatter diffraction. Results show that intermetallic particles play a significant role in determining the crack path and growth rate of short fatigue cracks. It is observed that the growth of short cracks is often retarded or even arrested at intermetallic particles and grain boundaries. Crack deflection at intermetallics and grain boundaries is also frequently observed. These results have been compared with the long crack growth behaviour of the alloy.     

An investigation on friction spot welding of AA2198‐T8 thin sheets

The feasibility of joining 1.6‐mm‐thick sheets of AA2198‐T8 by the novel friction spot welding technology and the resulting microstructural features on the welds cross sections were assessed, with further evaluation of the process parameters on the weld performance. Besides the intrinsic discontinuity related to the interface between the sheets, the hook feature was found to be inherent to the welding process, and its morphology was determinant to the weld performance and fracture mode. A beneficial response on the shear strength was achieved with the minimization of the hook feature because of the absence of a potential site for crack nucleation, although the generation of other defects, depending on the combination of parameters, could erase this benefit. Through statistical analysis, the most influent parameters on the weld performance were plunge depth and welding time. In the present study, regardless of the weld discontinuities, the optimum shear strength revealed a satisfactory performance in mechanical terms for aerospace applications.     

Effect of boron content and welding current on the mechanical properties of electrical resistance spot welds in complex-phase steels

When complex phase steel where tensile strength is more than 1 GPa grade is joined by resistance spot welding (RSW) optimum boron (B) content should be chosen to satisfy weldability and mechanical properties. Therefore, in this study, the effect of the B content (0–40 ppm) on the tensile-shear strength of the RSW were investigated. As the resistivity of the base metal was independent on the B content it did not affect to nugget diameter. Regardless of the B content the specimens under 5t^1/2 (t = sheet thickness) were fractured at interfacial failure mode. In the low welding current condition (lower than 6.4 kA), measured nugget diameters were smaller than calculated critical nugget diameter regardless of the amount of B addition so that fracture mode was interfacial failure. Pull out failure occurred at the softened zone which was boundary between the base metal and the heat affected zone. Tensile-shear load of the specimen failure at the pull-out mode was increased as the fractured diameter and hardness of the softened zone were increased. Shear load was only dependent on the fractured diameter. The equations to calculate the shear and tensile-shear load were suggested for the specimens fractured at interfacial and pull-out failure modes respectively. Correlation coefficients between measured and calculated values of shear and tensile-shear load were 0.98 and 0.97, respectively. Therefore, shear and tensile-shear load of advanced high strength steel joined by RSW could be predicted successfully using the suggested equation.     

Effect of explosive characteristics on the explosive welding of stainless steel to carbon steel in cylindrical configuration

The aim of this research is to study the influence of explosive characteristics on the weld interfaces of stainless steel AISI 304L to low alloy steel 51CrV4 in a cylindrical configuration. The effect of ammonium nitrate-based emulsion, sensitized with different quantities and types of sensitizing agents (hollow glass microballoons or expanded polystyrene spheres) and Ammonium Nitrate Fuel Oil (ANFO) explosives on the interface characteristics is analyzed. Research showed that the type of explosive and the type and proportion of explosive sensitizers affect the main welding parameters, particularly collision point velocity. The morphology of the wavy weld interfaces, chiefly the amplitude and length of the waves, is affected both by the impact velocity and the type and particle size of the explosive sensitizers, and increases with particle size. All the weld interfaces, except welds done with ANFO, displayed localized melted and solidified regions, whose chemical composition resulted from the contribution of both flyer and base metal.     

Ni中间层对QBe2.5/钢激光点焊接头力学性能的影响

流量阀的执行机构常应用铍青铜/钢复合元件,本文采用0.05 mm纯Ni作为中间层对QBe2.5铍青铜薄片与20#钢异种金属激光点焊搭接接头力学性能进行优化.通过光学显微镜、拉剪试验、显微硬度计和能谱测试仪对比分析添加Ni与不添加Ni层时焊接接头的成形、力学性能以及元素分布.研究表明:与未加镍层接头相比,加镍中间层的焊缝熔深较小,且基本不出现下塌现象,加镍中间层比未加中间层的接头抗拉剪力提高了61.5%;加入镍中间层后的焊缝中Fe元素含量增加,而Cu元素含量降低,接头韧性提高;加镍中间层的接头焊缝硬度值沿焊点深度方向逐渐增大,而未加镍层的基本不变,且加镍层比未加镍层的接头界面显微硬度值低.镍中间层材料可以显著改善铍青铜/钢异种金属接头焊缝熔合比,提高接头的抗拉剪强度、塑性和韧性     

Effect of Zr on the microstructure, mechanical properties and corrosion resistance of Mg–10Gd–3Y magnesium alloy

The influence of Zr on the microstructure, mechanical properties and corrosion resistance of Mg–10Gd–3Y (wt.%) magnesium alloy was investigated. The grain size of alloys decreased with Zr content from 0% to 0.93% (wt.%). The addition of Zr greatly improved the ultimate tensile strength (UTS) and the elongation (EL), while slightly improved the tensile yield strength (TYS). The UTS and the EL of the alloy containing 0.93% Zr increased by 125.8 MPa and 6.96% compared with base alloy, respectively. The corrosion resistances were found to decrease with Zr content from 0% to 0.42% and then increase from 0.42% to 0.93%. The differences in the sizes and distributions of the Zr-rich particles have significant effects on the corrosion behaviors. The alloy with 0.42% Zr addition revealed the optimum combination of mechanical properties and corrosion resistance.     

Effect of Heat Treatment on Acoustic Emission in 7075-T651 Aluminum Alloy

The longitudinal(L),long-transverse(T)and short-transverse(S)orientated samplesof precipitation-strengthened 7075-T651 aluminumalloy were examined.The acoustic emission(AE)characteristicsmeasured during tensile deformation in the alloywere related to prior aging treatment and orienta-tions of samples.A rms AE peak occurred in all the samplesat plastic strain greater than one percent.The heights of peaks have been measured as afunction of prior heat treatment.The shiftof the peak location was dependent on heat treat-ment in samples with L- and T-orientation.The pre-yield,burst-type emission(PYBE)wereobserved in S-orientated samples.The behavioursof PYBE were as a function of prior ageing treat-ment too.The stress and plastic strains at whichthe AE peak stress were generated were consistentwith the peaks arising from the fracture anddebonding of inclusions.Waveform frequencyanalysis of AE signals and fractographic observa-tions using SEM as well as data of microhardnessand charpy test were also consistent with be-haviours of AE,that is,high stress requiredfor the rms peak or the initiation of the PYBE,was higher for the heat treatment,at whichthe interface between inclusion and matrix wasweaker.     

Effect of temperature on the fracture behaviour of heat‐treated Al–Cu–Li alloy laser welds under low‐cycle fatigue loading

The paper presents the results of the studies of the effect of temperature on the fracture behaviour of Al–Cu–Li alloy laser welds under low‐cycle fatigue loading. The mechanical properties and the microstructure of the welded joints without and after postweld heat treatment (PWHT) were investigated. The tensile strength and the low‐cycle fatigue resistance of the welded joints were studied at various test temperatures (20°C, 85°C and ? 60°C). It was been found that heating up to 85°C and cooling down to ?60°C reduced the maximum number of loading cycles of the welded joints after PWHT by 1.5–2.0 times compared with that at a test temperature of 20°C.     

贝氏体区等温时间对TRIP钢残奥及力学性能影响

为研究贝氏体区等温时间对热轧TRIP钢残余奥氏体和力学性能的影响,采用金相显微镜、X射线衍射、拉伸实验等方法对3种不同贝氏体区等温时间下制备的热轧TRIP钢进行分析.结果表明:随着贝氏体等温时间的延长,残余奥氏体量减少而残余奥氏体碳含量增加,残余奥氏体晶粒尺寸及残余奥氏体形貌变化不大;热轧TRIP钢的力学性能随着贝氏体...     

Influence of heat input on microstructure and fracture toughness property in different zones of X80 pipeline steel weldments

In this paper, microstructure observations and mechanical behaviour of fusion line and offsetting positions from fusion line by 1, 2 and 3 mm were analysed. For the welding of X80 pipeline steel plates, different magnitudes of heat inputs such as high heat input (HHI) 25 kJ/cm, medium heat input (MHI) 20 kJ/cm and low heat input (LHI) 15 kJ/cm were employed. Critical values of J‐integral (J_0.2) and crack tip opening displacement (CTOD_0.2) for predetermined regions in the X80 weldment were determined as per ASTM‐E1820a. M‐A constituents of different sizes such as small (1–2 μm), large >2 μm and slender (>4 μm) were observed in the microstructure of subzones of weldments for different heat inputs. Formation of granular bainite, M‐A constituents and inclusions of Ti, Si, Mo in the microstructure impaired fracture toughness property. In the X80 weldment, the fusion line (FL) for HHI was found weakest in terms of fracture resistance, which subsequently increases the risk of fracture.     

Effect of post-weld heat treatment on fracture toughness and fatigue crack growth behaviour of electron beam welds of a titanium (α+β) alloy

The effects of a post-weld heat treatment on the fracture toughness and fatigue crack growth behaviour of electron beam welds of an α + β titanium alloy, Ti–6.5Al–1.9Zr–0.25Si have been studied. Welds in the stress-relieved condition exhibited poor fracture toughness due to poor energy absorbing capacity of the thin α and α' phases. Post-weld heat treatment which resulted in the decomposition of α' to α + β and the coarsening of intragranular and intergranular α resulted in improved toughness. This improvement in the toughness is related to improved ductility leading to crack blunting, crack path deviation at the thick intragranular and intergranular α phase. Fatigue crack growth resistance of welds was superior to the base metal in the α + β heat-treated condition. The superior crack growth resistance of the welds is due to the acicular α microstructure which results in a tortuous crack path and possible crack closure arising from crack path tortuosity.     

珠光体对ZG120Mn13钢拉伸断裂过程的影响

为探究珠光体降低高碳高锰钢机械性能的原因,本文采用金相组织分析、机械性能测试和断口微观形貌分析等实验方法,研究了奥氏体基体上含体积分数23%珠光体的ZG120Mn13高碳高锰钢的拉伸性能及其裂纹形核和扩展过程.结果表明:通过时效处理,在奥氏体基体上析出的条状、颗粒状以及沿晶界连续分布的珠光体将使ZG120Mn13钢的强度和塑性大幅度下降.机械性能的降低与其力学行为有关,当基体为单一奥氏体时,裂纹将在大量孪生变形后,在孪晶界、孪晶与晶界交界处形核,并沿孪晶界长大而相互连接、扩展.而奥氏体基体上存在珠光体时,裂纹主要在珠光体团内形核,并通过相邻珠光体间奥氏体的塑性耗竭、切断而得以扩展.     

Influence of Solution Temperature on Microstructure and Mechanical Properties of Two Cast Al-Si Alloys

In the present paper the influence of solution temperature 450-550°C on microstructure and mechanical properties of cast Al-12%-0.3% Mg and Al-16%-0.3% Mg alloys has been reported. It was observed that an increase in solution temperature increased the tensile strength of all alloys under investigation. Ductility was adversely affected. Higher solution temperature produced better refinement and distribution of eutectic silicon crystals than a low temperature. Heat treatment of all alloys showed spheroidization of eutectic silicon crystals. Scanning electron microscopy (SEM) of tensile-fractured surfaces was carried out to investigate the influence of solution temperature on the mode of fracture.     

Al-V含量对TiH2粉制备钛基合金性能的影响

为了揭示Al-V含量对钛基合金性能的影响,以TiH₂粉、Ti粉为原料,制备不同含量铝、钒(质量分数:0,2.5%,5%,7.5%,10%)的钛合金试样,通过对样品的力学性能、物相、显微组织、断口形貌的分析,研究了铝钒含量对合金的微观组织和力学性能的影响。结果表明:两种原料对应烧结试样的相主要由密排六方结构α-Ti和少量的体心立方结构β-Ti构成,铝钒含量的增加对烧结样品物相的相对含量无明显影响,但XRD衍射峰稍向右偏移;纯TiH₂和纯Ti烧结试样为等轴组织,加入铝、钒合金元素后纯TiH₂粉对应的组织为片状组织,纯Ti粉对应的组织以等轴为主,随铝、钒的增加,对应组织形貌变化较小;纯TiH₂和纯Ti烧结样品的抗拉强度分别为562.88和513.44 MPa,强度较低,但延伸率高,分别为28.15%和29.09%;随铝、钒含量的增加,TiH₂和Ti对应样品的强度、硬度增加,延伸率下降,最大强度值分别为914.10和937.23 MPa,对应的延伸率分别为7.60%和10.89%。随铝钒含量的增加两类合金的脆性增加,塑性降低,加入铝钒后纯TiH₂粉对应试样断口形貌主要呈条形花样和少量韧窝,纯Ti粉对应试样断口以韧窝为主。     

Effect of carnauba wax as a part of feedstock on the mechanical behavior of a part made of 4605 low alloy steel powder using metal injection molding

Metal injection molding is a growing technology for producing complex metallic components. Preparation of feedstock for metal injection molding is a very crucial step during this process. This is because the deficiencies in quality of the feedstock once made, cannot be corrected by subsequent processing steps. One of the challenges in producing the feedstock is its formulation. In this study, the effect of the percentage of carnauba wax along with other binder constituents of paraffin wax, polypropylene and stearic acid is investigated on the density, strength, hardness and rheological behavior of a part, made of 4605 low alloy steel powder using metal injection molding process. For this reason, six binder systems including paraffin wax, polypropylene and stearic acid having different percentages of carnauba wax have been produced. After preparation of the samples, tensile testing, Vickers hardness test, density and rheological behavior of the samples have been measured and compared. The results show that carnauba wax as a part of feedstock, has a positive effect on the strength and density while negative effect on the hardness of the final sintered part. Also with the increase in the percentage of the carnauba wax inside the polymer binder, the viscosity of the feedstock was reduced significantly.