J. Gholipour

Effect of Postweld Heat Treatment on Microstructure, Hardness, and Tensile Properties of Laser-Welded Ti-6Al-4V

The effects of postweld heat treatment (PWHT) on 3.2-mm- and 5.1-mm-thick Ti-6Al-4V butt joints welded using a continuous wave (CW) 4-kW Nd:YAG laser welding machine were investigated in terms of microstructural transformations, welding defects, and hardness, as well as global and local tensile properties. Two postweld heat treatments, i.e., stress-relief annealing (SRA) and solution heat treatment followed by aging (STA), were performed and the weld qualities were compared with the as-welded condition. A digital image correlation technique was used to determine the global tensile behavior for the transverse welding samples. The local tensile properties including yield strength and maximum strain were determined, for the first time, for the laser-welded Ti-6Al-4V. The mechanical properties, including hardness and the global and local tensile properties, were correlated to the microstructure and defects in the as-welded, SRA, and STA conditions.

Linear Friction Welding of a Single Crystal Superalloy

The effect of forging pressure on linear friction welding (LFW) behaviour of a single crystal Ni-based superalloy was investigated. Crystal orientations of the specimens were controlled and results indicated that welding success is dependent on proximity of the oscillation direction to the <011> direction. Characterization of the welds included optical examination of the microstructural features of the weld and thermomechanically affected zones (TMAZ) in relation to the parent material. Mechanical properties of the welded material examined via microhardness testing showed an increase in strength in the weld zone (WZ). Microstructural examination indicated that some recrystallization occurred in the WZ, as well as a small amount of distortion of the dendrites in the TMAZ. With increased forge pressure, recrystallized grains remaining in the weld were minimized.

Distortion and residual stresses in electron beam-welded hydroelectric turbine materials

Heavy-section assembly of hydroelectric turbine runner materials using single-pass, autogenous EBW was demonstrated to penetrate a 90-mm-thick butt joint. The welding-induced distortions and residual stresses were characterised to understand the impact of the materials and process conditions (e.g. preheating and/or PWHT). Using 3D optical measurements, the angular distortions of EB-welded UNS S41500 and CA6NM steels were determined to be 0.13° and 0.38°, respectively. The longitudinal residual stresses, measured through the contour method, had a M-shaped distribution throughout the thickness with minimum (∼−500 MPa) compressive stresses in the FZ and maximum (∼600 MPa) tensile stresses in the HAZ. After PWHT, the tensile and compressive stresses reduced to ∼100 MPa.

Evolution of microstructure, microtexture and mechanical properties of linear friction welded IMI 834

Abstract Titanium alloys have been of great interest in the aerospace industry for many years. Recently, linear friction welding has also been making strides in conquering a part of the aerospace manufacturing market, with its clear advantages over fusion welding and mechanical fastening methods for integrated bladed rotors. High tech near-α alloy IMI834 (Ti–5·8Al–4Sn–3·5Zr–0·7Nb–0·5Mo–0·35Si) was designed to have improved creep resistance and retains its mechanical properties at temperatures up to 600°C. It balances creep resistance and fatigue strength, making it an excellent material for compressor discs and blades. IMI834 with an initial bimodal α+β microstructure was welded using varying axial pressures during welding and then characterised using both microstructural examination and mechanical testing. Electron backscatter diffraction (EBSD) was used to characterise the texture and phase fraction of the welded IMI834 samples in the weld zone (WZ) and thermomechanically affected zones. The EBSD analysis revealed fine recrystallised grains at the weld centres. The microhardness evaluation of the weldments showed that the recrystallised WZ was slightly harder than the parent material (PM). The local and global tensile properties of the welds, investigated using a tensile testing rig with integrated digital image correlation, revealed higher strength in the WZ and failure in the PM. Les alliages de titane ont suscité un grand intérêt dans l’industrie aérospatiale pendant plusieurs années. Récemment, le soudage par friction linéaire a également fait de grands progrès dans la conquête d’une partie du marché de la fabrication aérospatiale, avec ses avantages clairs par rapport aux méthodes de soudage par fusion et d’assemblage mécanique pour les rotors à lames intégrées. L’alliage de haute technologie quasi-alpha, IMI834 (Ti–5·8Al–4Sn–3·5Zr–0·7Nb–0·5Mo–0·35Si) a été conçu pour avoir une résistance améliorée au fluage et il retient ses propriétés mécaniques à des températures jusqu’à 600°C. Il balance la résistance au fluage et la résistance à la fatigue, ce qui en fait un excellent matériau pour les disques et les lames de compresseur. IMI834, avec une microstructure initiale bimodale α+β, a été soudé en utilisant des pressions axiales variables lors du soudage. On l’a ensuite caractérisé en utilisant tant l’examen de la microstructure que les essais mécaniques. On a utilisé la diffraction d’électrons rétrodiffusés (EBSD) pour caractériser la texture et la fraction des phases des échantillons soudés d’IMI834 dans la zone de soudure (WZ) et dans les zones affectées thermomécaniquement. L’analyse par EBSD a révélé des grains fins recristallisés au centre des soudures. L’évaluation de la microdureté des ensembles soudés a montré que la zone de soudure recristallisée était légèrement plus dure que le matériau de base (PM). Les propriétés de traction locales et globales des soudures, examinées au moyen d’un assemblage d’essai de traction avec corrélation intégrée d’image digitale, ont révélé une résistance plus élevée dans la zone de soudure et la défaillance du matériau de base.

Linear Friction Welding of a Forged Near-α Titanium Alloy

IMI834 (Ti-5.8Al-4Sn-3.5Zr-0.7Nb-0.5Mo-0.35Si) is a high-tech near-α titanium alloy with improved creep resistance and mechanical property retention at temperatures up to 600°C [1]. It is used in the aerospace industry for compressor disks and blades due to its excellent balance between creep resistance and fatigue strength [2]. The linear friction welding (LFW) behaviour of IMI834 displaying an initial bimodal α+β microstructure was investigated using varying axial pressures during welding. Electron backscatter diffraction (EBSD) was used to characterize the texture and phase fraction of the welded IMI834 samples in the weld zone (WZ) and thermomechanically affected zones (TMAZ) in relation to the base material. Based on microhardness evaluation of the weldments, the WZ was determined to be slightly harder than the base material.

Linear Friction Welding of IN718 to Ti6Al4V

Linear friction welding (LFW), an emerging automated technology, has potential for solid-state joining of dissimilar materials (bi-metals) to enable tailoring of the mechanical performance, whilst limiting the assembly weight for increased fuel efficiency. However, bi-metallic welds are quite difficult to manufacture, especially when the material combinations can lead to the formation of intermetallic (brittle) phases at the interface, such as the case with assembly of Ti base alloys with Ni base superalloys. The intermetallic phase, once formed, lowers the performance of the as-manufactured properties and its growth during elevated temperature service can lead to unreliable performance. In this project, it was demonstrated that linear friction welding can be applied to join Ti-6%Al-4%V (workhorse Ti alloy) to INCONEL® 718 (workhorse Ni-base superalloy) with minimized interaction at the interface. Of particular merit is that no intermediate layer (between the Ti alloy and Ni-base superalloy) was needed for bonding. Characterization of the bi-metallic weld included macro-and microstructural examination of the flash and interface regions and evaluation of the hardness.

Mechanical and metallurgical evolution of stainless steel 321 in a multi-step forming process

This paper examines the metallurgical evolution of AISI Stainless Steel 321 (SS 321) during multi-step forming, a process that involves cycles of deformation with intermediate heat treatment steps. The multi-step forming process was simulated by implementing interrupted uniaxial tensile testing experiments. Evolution of the mechanical properties as well as the microstructural features, such as twins and textures of the austenite and martensite phases, was studied as a function of the multi-step forming process. The characteristics of the Strain-Induced Martensite (SIM) were also documented for each deformation step and intermediate stress relief heat treatment. The results indicated that the intermediate heat treatments considerably increased the formability of SS 321. Texture analysis showed that the effect of the intermediate heat treatment on the austenite was minor and led to partial recrystallization, while deformation was observed to reinforce the crystallographic texture of austenite. For the SIM, an Olson-Cohen equation type was identified to analytically predict its formation during the multi-step forming process. The generated SIM was textured and weakened with increasing deformation.

Effect of Material Model on Finite Element Modeling of Aerospace Alloys

Increasing acceptance and use of hydroforming technology within the aerospace industry requires a comprehensive understanding of critical issues such as the material characteristics, friction condition and hydroformability of the material. Moreover, the cost of experiments that can be reduced by accurate finite element modeling (FEM), which entails the application of adapted constitutive laws for reproducing with confidence the material behavior. In this paper, the effect of different constitutive laws on FEM of tubular shapes is presented. The free expansion process was considered for developing the FEM. Bulge height, thickness reduction and strains were determined at the maximum bulge height using different constitutive models, including Hollomon, Ludwik, Swift, Voce, Ludwigson. In order to minimize the effect of friction, the free expansion experiments were performed with no end feeding. The simulation results were compared with the experimental data to find the appropriate constitutive law for the free expansion process.

Prediction of Burst Pressure in Multistage Tube Hydroforming of Aerospace Alloys

Bursting, an irreversible failure in tube hydroforming (THF), results mainly from the local plastic instabilities that occur when the biaxial stresses imparted during the process exceed the forming limit strains of the material. To predict the burst pressure, Oyans and Brozzos decoupled ductile fracture criteria (DFC) were implemented as user material models in a dynamic nonlinear commercial 3D finite-element (FE) software, ls-dyna. THF of a round to V-shape was selected as a generic representative of an aerospace component for the FE simulations and experimental trials. To validate the simulation results, THF experiments up to bursting were carried out using Inconel 718 (IN 718) tubes with a thickness of 0.9 mm to measure the internal pressures during the process. When comparing the experimental and simulation results, the burst pressure predicated based on Oyanes decoupled damage criterion was found to agree better with the measured data for IN 718 than Brozzos fracture criterion.

Piecewise Fifth Order Spline Interpolation for Line Heating Forming Process

A fifth order piecewise spline interpolation model has been developed for computing the evolving geometry of a plate deformed by line heating thermal gradients. 3D formulations are presented and applied to continuously derivable geometries to demonstrate the capability of the methodology. Then the developed formulation is used to form gradually, with a sequence of heating lines, a 3D shape from an initially flat plate. The geometric results obtained from finite element simulations with three heating lines are used to illustrate where heating lines should be applied on a flat plate to achieve the intended geometry of a workpiece. Furthermore, it is shown that applying the developed piecewise fifth order spline interpolation model to the same flat plate produces results very close to the ones obtained from the thermal structural FE simulations.Copyright