Finite element method simulation of residual stresses in Al2O3–AISI 304 steel joints

Abstract

Thermal residual stresses are very detrimental to the mechanical resistance of metal–ceramic joints and thin metallic foils acting as stress relieving interlayers have been used to reduce their effect. The present work presents finite element method simulations of the residual stress field in Al₂O₃–AISI 304 steel joints using interlayers. Different interlayer materials (Ti, Ni, Mo, and Cu) were considered, either separately or in combination. Calculations show that among the different interlayer materials considered, Cu and Ti/Cu are most effective in reducing the thermal stresses and that this role is determined mainly by the ductility of the interlayer material. The calculated results were validated by shear tests performed on real joints obtained by diffusion bonding and it was concluded that residual stresses control the mechanical resistance of the joints.     

中间层Ni对TC4/2A14异种金属搅拌摩擦焊接头组织和性能的影响

目的 添加0.05 mm厚的Ni箔作为中间层,对3 mm厚的TC4钛合金和2A14铝合金进行搅拌摩擦焊,分析Ni对接头力学性能的影响。方法 采用扫描电镜、EDS能谱及XRD衍射等微观表征分析方法,对焊接接头的断口形貌、成分进行分析,探究Ni箔对焊接接头力学性能的影响。结果 由于钛合金和铝合金存在较大的物理化学性能差异,Ti/Al异种金属焊接性较差,界面容易产生TiAl3、TiAl、Ti3Al等金属间化合物,其中脆性相TiAl3对接头性能的影响最大,会导致综合力学性能下降。当加入中间层材料Ni后,由于Ni与Al晶体结构均属于面心立方,因此Ni与Al的扩散系数大于Ti与Al的扩散系数,Ni和Al之间优先形成金属间化合物且弥散分布于焊缝中,从而缩短了Ti与Al之间的相互扩散时间,减少了TiAl3相的生成。结论 在未添加中间层材料时,接头平均抗拉强度为237.3 MPa,约为2A14铝合金母材抗拉强度的56.7%;当添加中间层Ni后,对焊缝中金属间化合物的种类和数量进行了调控,减少了对性能影响最大的TiAl3相的生成,接头平均抗拉强度达到285.3 MPa,为2A14铝合金母材抗拉强度的68%。     

高熵合金中间层对TiNi/TC4电子束焊接头组织及性能影响

目的 解决TiNi/TC4异种金属的焊接问题,扩大此2种金属的结合应用.方法 选用3 mm×40 mm×0.5 mm的FeCoCrNiMn高熵合金薄片作为中间层,采用电子束焊接方法焊接TiNi/TC4.对形成的接头进行宏观分析,采用电子显微镜以及X射线衍射仪对其微观组织进行表征;通过拉伸和硬度测试,分析该接头的力学性能.结果 接头宏观成形存在裂纹缺陷,观察微观组织发现,在焊接接头中只形成了AlTi3相和Ni0.35Al0.30Ti0.35相,没有形成Ti2Ni相.接头的抗拉强度为81.8 MPa,伸长率为4.38％,平均显微硬度为HV661,断口呈脆性断裂特征.结论 将高熵合金作为中间层焊接TiNi/TC4是一种有效的方法,可以成功实现异种金属的焊接,中间层的存在可以有效调控焊缝中的析出相种类.     

Diffusion bonding of TiAl based alloy to Ti–6Al–4V alloy using amorphous interlayer

Powder metallurgical TiAl based alloy and Ti–6Al–4V (TC4) alloy were diffusion bonded at 915 °C for 1 h under a pressure of 80 MPa. Single Ti‐based amorphous or Ni‐based amorphous foil was utilized as an interlayer during diffusion bonding process. The tensile mechanical properties of the diffusion bonded joints between TiAl based alloy and TC4 alloy were tested. The fracture surface and microstructure characteristic of these joints were also observed carefully by scanning electron microscope. The TiAl based alloy with a fully lamellar microstructure is more suitable for diffusion bonding to TC4 alloy. Adding a Ti‐based amorphous interlayer is more conducive to the element diffusion, giving rise to the improvement in the mechanical properties of the diffusion bonded joints. Results also show that the diffusion bonded joints form a metallurgical bond and fracture in a brittle manner.     

Joining Si3N4 to type 405 steel with soft metal interlayers

Abstract

The suitability of four pure metals, Ni, Fe, Ti, and Nb, as interlayers for bonding silicon nitride to type 405 ferritic stainless steel has been examined. Bonding was carried out by hot isostatic pressing at temperatures between 1473 and 1673 K under a pressure of 100 MPa. Of the metals tested, Fe and Nb formed the strongest interfacial bonds with Si₃N₄. To reduce the influence of residual stress in joints, Fe/W and Nb/W laminated interlayers were also examined. The use of W restricted the contraction of Fe or Nb, and the Si₃N₄/steel joints with these interlayers had a tensile strength of more than 50 MN m^?2. A low-pressure (10 MPa) bonding process was also used for the Fe/W interlayer.

MST/470     

Structure‐property investigations on a laser beam welded dissimilar joint of aluminium AA6056 and titanium Ti6Al4V for aeronautical applications. Part II: Resistance to fatigue crack propagation and fracture

Investigations were continued on the dissimilar laser beam welds of AA6056 and Ti6Al4V, fabricated by inserting Ti‐sheet into the profiled Al‐sheet and melting AA6056 alone. By using microstructure, hardness and strength as the criteria, sites exhibiting non‐uniform microstructure and localized plastic deformation due to strength mismatch were investigated in two orientations: ? crack parallel to the weld and ? crack perpendicular to the weld for fatigue crack propagation and fracture toughness at room temperature. Effect of temper of AA6056 on these properties was studied for two conditions; welding in T4 followed by post weld heat treatment T6, and welding in T6 and naturally aged for a defined period. The orientation “crack parallel to the weld” was investigated in 3 locations on the side of AA6056: the interface and the two changeovers on the Al‐side. Firstly, between the fusion zone and the heat affected zone (3 mm from the interface) and secondly, between (primary) heat affected zone and towards the base material (7 mm from the interface). Although brittle intermetallic TiAl₃ had been formed at the interface, uncontrolled separation or debonding at the interface was not observed. Insofar the bond quality of the weld was good. However, the ranking of interface was the lowest since fatigue crack propagation was relatively faster than that in the fusion zone and heat affected zone, and fracture toughness was low. Therefore, unstable fatigue crack propagation is observed when the crack propagates perpendicular to the weld from AA6056 towards Ti6Al4V. The results have shown that the dissimilar joints exhibit improved performance when laser beam welded in the T6 condition.     

TiNi形状记忆合金/不锈钢异种材料激光焊研究

采用纯镍中间夹层和激光焊技术,连接TiNi形状记忆合金丝和不锈钢丝异种材料。对比分析了加镍夹层与未加镍夹层的激光焊接头的组织和性能。结果表明,与未加镍夹层的接头相比,加镍夹层的接头组织和性能得到明显改善,接头组织中γ-Fe相含量增加,TiFe2,TiCr2等金属间化合物相含量减少,焊缝区硬度明显降低,接头抗拉强度提高。未加镍夹层的接头的抗拉强度仅为187MPa,断口呈现典型的脆性断裂特征。加镍夹层后,接头抗拉强度提高至372MPa,断口具有脆-韧混合型断裂特征。     

Comparative study on characteristics of hybrid laser-TIG welded AZ61/Q235 lap joints with and without interlayers

AZ61 Mg alloy to Q235 mild steel were lap joined using hybrid laser-TIG welding technique. At the joint interface and fusion zone (FZ), microstructure was revealed by scanning electron microscopy equipped with energy dispersive X-ray spectroscopy; element distribution was analyzed by electron probe micro-analyzer; intermediate phases were identified using X-ray diffraction test. Comparing with interlayer-free joints, the new intermediate phases Mg₂Ni and Mg₂Cu were generated in the FZ and at the Mg alloy/interlayer interface, and the solid solution of Ni or Cu in Fe was found along the edge of weld pool on steel side. It was found that direct joint without any interlayer was mechanical bonding, while Ni- and Cu-added joints were semi-metallurgical bonding. The joint shear strength was not only related to the penetration depth, but also related to the bonding mode. The strengthening effect on Cu-added joint was higher than that on Ni-added joint.     

Infrared brazing of high-strength titanium alloys by Ti–15Cu–15Ni and Ti–15Cu–25Ni filler foils

Microstructures and fracture behaviors of infrared heated, vacuum brazed Ti–6Al–4V and Ti-15-3 alloys using two Ti–Cu–Ni braze fillers have been characterized to establish the effects of brazing process parameter and chemical composition on the strength of brazed joints. The brazed joint initially contains two prominent phases; a Ti alloy matrix alloyed with V, Cr, Ni, Cu and Al and a Cu–Ni-rich Ti phase. Brazing temperature and soak time control the amount of Cu–Ni-rich Ti phase in the brazed joints. The fracture mode changes from brittle cleavage to quasi-cleavage to ductile dimple as the amount of Cu–Ni-rich Ti phase is reduced in the brazed joint. Both brazing temperature and soak time are critical to eliminate the Cu–Ni-rich Ti phase for optimal shear strength and ductile fracture of brazed joints. A post-brazing annealing at lower temperature is also shown to be an effective way to homogenize the microstructure of brazed joint for improved joint strength.     

Laser welding of TiNi shape memory alloy and stainless steel using Ni interlayer

Laser welding of TiNi shape memory alloy wire to stainless steel wire using Ni interlayer was investigated. The results indicated that the Ni interlayer thickness had great effects on the chemical composition, microstructure, gas-pore susceptibility and mechanical properties of laser-welded joints. With an increase of Ni interlayer thickness, the weld Ni content increased and the joint properties increased due to decreasing brittle intermetallic compounds (TiFe₂ and TiCr₂). The joint fracture occurred in the fusion zone with a brittle intermetallic compound layer. The tensile strength and elongation of the joints reached the maximum values (372 MPa and 4.4%) when weld Ni content was 47.25 wt.%. Further increasing weld Ni content resulted in decreasing the joint properties because of forming more TiNi₃ phase, gas-pores and shrinkage cavities in the weld metals. It is necessary to select suitable Ni interlayer thickness (weld composition) for improving the mechanical properties of laser-welded joints.     

梯度结构Al_2O_3-(W,Ti)C-TiN-Mo-Ni纳米金属陶瓷刀具材料的设计及制备

基于对高速硬切削时刀具应力和温度分布,以及刀具内部疲劳裂纹扩展仿真分析,提出一个组分含量分布和微观结构具有梯度特征的设计模型。通过韧性相的添加和梯度结构的引入,实现疲劳裂纹扩展速率的减缓,从而提高刀具寿命。采用二阶段热压烧结工艺制备出具有梯度结构的Al_2O_3-(W,Ti)C-TiN-Mo-Ni纳米复合刀具材料,并对其微观结构和力学性能进行研究。结果表明:所制备的梯度结构金属陶瓷材料表层硬度、内层的断裂韧度和抗弯强度分别达到19.258GPa,10.015MPa·m~(1/2)和1017.475MPa,满足高速硬切削刀具的性能要求。材料的断口出现韧窝和黏结相撕裂形成的断裂棱,有利于断裂韧度和抗弯强度的增强,从而提高刀具抗疲劳裂纹扩展能力。     

层状Ti3SiC2陶瓷的组织结构及力学性能

利用热压烧结TiH₂,Si和C粉获得了致密度大于98%的层状Ti₃SiC₂陶瓷。利用压痕法,在不同的载荷下测定了材料的维氏硬度, 发现其硬度值随载荷的增加而降低,在最大载荷30kg时,硬度值为4GPa。压痕对角线没有发现径向裂纹的出现。 这归因于多重能量吸收机制——颗粒的层裂、裂纹的扩展、颗粒的变形等。利用三点弯曲法和单边切口梁法测定了材料的强度和韧性分别为270MPa和6.8MPa·m1/2。Ti₃SiC₂材料的断口表现出明显的层状性质,大颗粒易于发生层裂和穿晶断裂,小颗粒易被拔出。当裂纹沿平行于Ti₃SiC₂基面的方向扩展造成颗粒的层裂,当裂纹沿垂直于基面的方向扩展时,裂纹穿过颗粒的同时,在颗粒内部发生偏转,使裂纹的扩展路径增加。裂纹的扩展路径类似人们根据仿生结构设计的层状复合材料。裂纹在颗粒内的多次偏转、裂纹钉扎以及颗粒的层裂和拔出等是材料韧性提高的主要原因。此外,在室温下得到的荷载-位移曲线,说明Ti₃SiC₂材料不象其它陶瓷材料的脆性断裂,而是具有金属一样的塑性。     

中间层M o 网对Al2O3/Nb 钎焊接头热震抗力的影响

首次研究了在Al₂O₃/Nb 的钎焊界面之间加入一层金属Mo 网, 可大幅度提高接头的热震抗力, 经用N i51T i49 钎料焊接后, 700℃淬水后的剩余强度达到了不加网的焊后强度, 强度提高了180% 以上, 用Cu52Ni18Ti30 钎料焊接并加Mo 网后, 700℃淬水后的剩余强度也提高了130%以上。接头断裂特征明显不同, 分析认为金属网的加入显著降低了钎焊界面的残余应力。      

Ti/Nb作中间层脉冲加压扩散连接TiC金属陶瓷与不锈钢

用Ti/Nb作中间层,在温度890℃、时间4~12min、脉冲压力2~10MPa、频率f=0.5Hz、恒压10MPa下,对TiC金属陶瓷和304不锈钢(304SS)进行脉冲加压与恒压扩散焊,获得了牢固的固相扩散焊接头。通过扫描电镜SEM、能谱EDS、X射线衍射XRD与剪切性能测试,对接头的显微组织、界面产物与强度进行分析。结果显示:两种接头的界面物相相似,主要有σ相,(β-Ti,Nb)与α+β-Ti固溶体。连接时间10min时,恒压下的TiC/304SS接头抗剪强度为55.6MPa,而脉冲加压下的接头抗剪强度达110MPa。恒压下接头断裂方式为TiC陶瓷断裂,而脉冲压力下接头断裂方式为TiC陶瓷与界面产物间交替进行的混合断裂。     

Influence of Ni Interlayers on the Mechanical Properties of Ti6Al4V/(WC-Co) Friction Welds

Ni interlayers were introduced prior to dissimilar friction welding of Ti6Al4V base material to three cemented carbide substrates. The fracture strength of Ti6Al4V/(WC-6 wt% Co) welds were poor and were markedly improved when 20-µm thick Ni interlayers were introduced prior to dissimilar friction welding. These results were only produced when the (WC-6 wt% Co) cermet was electroplated prior to friction welding. When the Ti6Al4V alloy was electroplated prior to friction welding, fractured WC particles and cracking were observed in the (WC-Co) carbide substrate. The fracture strengths of Ti6Al4V/(WC-11 wt% Co) and Ti6Al4V/(WC-24 wt% Co) welds were not improved when 20-µm thick Ni interlayers were introduced prior to friction welding. During mechanical testing, the Ni layer retained at the dissimilar joint interface created a region of weakness.     

Fatigue crack growth behaviour of gas tungsten arc,electron beam and laser beam welded Ti–6Al–4V alloy

The present investigation is aimed to evaluate fatigue crack growth parameters of gas tungsten arc, electron beam and laser beam welded Ti–6Al–4V titanium alloy for assessing the remaining service lives of existing structure by fracture mechanics approach. Center cracked tensile specimens were tested using a 100 kN servo hydraulic controlled fatigue testing machine under constant amplitude uniaxial tensile load. Crack growth curves were plotted and crack growth parameters (exponent and intercept) were evaluated. Fatigue crack growth behavior of welds was correlated with mechanical properties and microstructural characteristics of welds. Of the three joints, the joint fabricated by laser beam welding exhibited higher fatigue crack growth resistance due to the presence of fine lamellar microstructure in the weld metal.     

The use of interlayers in modeling interface crack propagation

Delaminations are a common mode of failure at interfaces between two material layers which have dissimilar elastic constants. There is a well-known oscillatory nature to the singularity in the stress fields at the crack tips in these bimaterial delaminations, which creates a lack of convergence in the modewise energy release rates. This makes constructing fracture criteria somewhat difficult. An approach used to overcome this is to artificially insert a thin, homogeneous, isotropic layer (the interlayer) at the interface. The crack is positioned in the middle of this homogeneous interlayer, thus modifying the original ‘bare’ interface crack problem into a companion ‘interlayer’ crack problem. Individual modes I and II energy release rates are convergent and calculable for the companion problem and can be used in the construction of a fracture criterion or locus. However, the choices of interlayer elastic and geometric properties are not obvious. Moreover, a sound, consistent, and comprehensive methodology does not exist for utilizing interlayers in the construction and application of mixed-mode fracture criteria in interface fracture mechanics. These issues are addressed here. The role of interlayer elastic modulus and thickness is examined in the context of a standard interface fracture test specimen. With the help of a previously published analytical relation that relates the bare interface crack stress intensity factor to the corresponding interlayer crack stress intensity factor, a suitable thickness and elastic modulus are identified for the interlayer in a bimaterial four-point bend test specimen geometry. Interlayer properties are chosen to make the interlayer fracture problem equivalent to the bare interface fracture problem. A suitable mixed-mode phase angle and a form for the fracture criterion for interlayer-based interface fracture are defined. A scheme is outlined for the use of interlayers for predicting interface fracture in bimaterial systems such as laminated composites. Finally, a simple procedure is presented for converting existing bare interface crack fracture loci/criteria into corresponding interlayer crack fracture loci.     

The microstructure and mechanical properties of prolonged and lower temperature aged Fe–Ni–Mn–Mo–Ti–Cr maraging steel

In this study, the microstructure and mechanical properties of Fe–Ni–Mn–Mo–Ti–Cr maraging steel at low temperature and prolonged aging condition were investigated. Optical and scanning electron microscopy examinations, tensile and hardness tests were conducted to study the microstructure, aging behavior and mechanical properties of the cold‐rolled steel. The results showed that aging of cold rolled Fe–Ni–Mn–Mo–Ti–Cr maraging steel resulted in the formation of Mo rich and Ti rich Lave phase precipitates. Existence of many dislocation cores due to cold rolling and subsequently, low temperature aging caused to formation of uniform distribution of very fine precipitates. The presence of these precipitates increased the yield and ultimate tensile strengths but couldn't improve the uniform tensile ductility. This alloy showed ultra‐high fracture stress of about 1950 MPa with a negligible tensile elongation (about 2 %) at the peak aged condition. The fractographic studies indicated this alloy shows semi‐brittle fracture in the subsequent aging treatment.     

发动机热端部件电子束焊修复接头的高温疲劳裂纹扩展行为

研究了力学不均匀性对电子束焊接修复接头疲劳裂纹扩展行为的影响规律，结果表明，由于力学不均匀性的影响，位于修复法接头焊缝和热潮影响区的疲劳裂纹垧母材偏转扩展，力学不均匀性与裂纹偏转导致裂纹扩展的驱动力下降，使得焊接区的表现疲劳裂纹扩展速率低于母材。     

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