Unusual fatigue behavior of friction-stir welded Al-Mg-Si alloy

In this work, high-cycle fatigue behavior of friction-stir welded AA6061-T6 was studied. An emphasis has been made on the inter-relationship between microstructure, residual stress and fatigue resistance. The welds were produced under optimized conditions, including a combination of relatively high welding temperature and rapid cooling rate, and subsequently undergone a standard post-weld aging heat treatment. The optimized welds exhibited excellent fatigue performance that was comparable (or even superior) to that of the base material. This result was attributed to a considerable grain refinement in the stir zone, subtle material softening in the heat-affected zone as well as to significant residual stress generated during the optimized FSW

Stress relaxation behaviour in IN718 nickel based superalloy during ageing heat treatments

Designing microstructure of components made from Inconel 718 nickel based superalloy (IN718) with tailored mechanical properties for high temperature applications, require sequential thermo-mechanical processing. This often includes straining and annealing at solution annealing temperature (i.e. ≈980℃) followed by water quenching and subsequent aging heat treatments at lower temperatures. In addition to the microstructure development (i.e. precipitation) the aging heat treatment partially relieve the residual stresses generated at previous stages of forging and water quenching, however the stress field will not be completely relaxed. In this study, a series of experiments were conducted on round tensile specimens made from IN718 bar to investigate tensile stress relaxation behaviours at elevated temperatures used for aging heat treatments. The stress relaxation curves obtained can be described by a hyperbolic function with a non-zero asymptotic stress (σ∞), which seems to be proportional to the initially applied stress (σ0) for a fixed temperature. This behaviour is investigated at temperatures between 620℃ and 770℃ that is a temperature range used in industry to perform the aging heat treatments to obtain microstructures with tailored mechanical properties. It has been shown that the σ∞/ σ0 ratio has decreased rapidly with increasing temperature at this range. The relaxation behaviour has been assessed numerically and an empirical relationship has been defined for each temperature that can be used for modelling purposes

Residual stress distributions in dissimilar titanium alloy diffusion bonds produced from powder using field-assisted sintering technology (FAST-DB)

The conventional approach when engineering components manufactured from titanium is to design the thermomechanical processing to develop an optimal microstructure in a single alloy. However, this conventional approach can lead to unnecessary over-engineering of components, particularly when only a specific subcomponent region is under demanding service stresses and environments. One approach being developed to join multiple alloys in a single component and enhance engineering performance and efficiency is FAST-DB - whereby multiple alloys in powder form are diffusion bonded (DB) using field-assisted sintering technology (FAST). But as the joining of multiple alloys using conventional welding and joining techniques can generate high residual stress in the bond region that can affect the mechanical performance of the components. In this study, the residual stress distribution across dissimilar titanium alloy diffusion bonds, processed from powder using FAST, were measured using X-Ray diffraction and the Contour method. The measurements show low residual stress in the bulk material processed with FAST as well as in the diffusion bond region. In addition, FAST-DB preforms subsequently hot forged into different near-net shapes were also analyzed to understand how the residual stress in the bond region is affected by a subsequent processing. Overall, no sharp transitions in residual stress was observed between the dissimilar alloys. This study reinforces confidence in the solid-state FAST process for manufacturing next generation components from multiple titanium alloy powders

Continuous drive friction welding of AISI 8630 low-alloy steel : experimental investigations on microstructure evolution and mechanical properties

Continuous drive friction welding (CDW) is a state-of-the-art solid-state welding technology for joining metallic components used in aerospace, oil and gas, and power generation industries. This study summarizes the results of mechanical and microstructural investigations on a modified AISI-8630 steel subjected to CDW. The effects of welding process parameters, including rotational speed, friction, and forge forces, during CDW were explored to determine an optimum welding condition. The mechanical properties of the weld, and microstructural characteristics across different regions of the weld were measured and examined. The microstructure characterization results suggest that the weld zone (WZ) experiences temperatures above the Ac3 and the thermo-mechanically affected zone (TMAZ) experiences temperatures between Ac1 and Ac3 of the material. Investigations with electron backscatter diffraction (EBSD) demonstrated the occurrence of strain-induced dynamic recrystallization in the weld. The weld demonstrated higher yield and ultimate tensile strengths at the expense of ductility and hardening capacity compared to the base metal (BM). The strain-hardening profiles of the welds exhibited a dual-slope characteristic, an indication of different levels of plastic deformation experienced by the constituent phases (i.e., martensite, bainite and ferrite) present in the microstructure. The maximum strength-to-ductility combination and static toughness values were obtained for the weld produced under the highest rotational speed, maximum friction force and an intermediate forge force of 1200-1400 rpm, 375-425 kN, and 600-650 kN, respectively

Behaviour of Short Intergranular Stress Corrosion Crack in Type 304 Austenitic Stainless Steel

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Influence of microstructure and stress on short intergranular stress corrosion crack growth in austenitic stainless steel type 304

Intergranular stress corrosion cracking (IGSCC) causes failures in austenitic stainless steels when the appropriate electrochemical, metallurgical and mechanical conditions exist. In this study, the effects of time, applied stress, residual stress and microstructure on population of short crack nuclei has been investigated in sensitised type 304 austenitic stainless steel, tested under static load in an acidified potassium tetrathionate (K2S4O6) environment. Statistical analysis, using the Gumbel distribution method, enables analysis of the growth rate of short crack nuclei. This methodology is being developed, in order to quantitatively evaluate the influence of grain boundary engineering and surface finishing on crack nucleation

Characterisation of the sensitisation behaviour of thermo-mechanically processed type 304 austenitic stainless steel using DL-EPR testing and image analysis methods

Standard test methods such as the Electrochemical Potentiokinetic Reactivation Test (EPR–ASTM G108) and the Double-Loop EPR test (DL-EPR–ISO12732) are commonly used to characterise sensitisation behaviour in austenitic stainless steels. These tests provide a quantitative assessment of microstructure susceptibility. Factors such as different grain size may be accounted for, but additional information on the network of sensitised boundaries is neglected. This paper reports a new approach to characterise the development of sensitisation, applied to a Type 304 austenitic stainless steel subjected to thermo-mechanical processing. DL-EPR testing is augmented by large area Image Analysis (IA) assessments of optical images to measure the dimensions and connectivity of the attacked grain boundary network. Comparison is made with the standard assessment methods, and a new method is proposed, based on normalisation by a cluster parameter to describe the network of susceptible grain boundaries. This parameter can be estimated by electron backscatter diffraction (EBSD) methods in the non-sensitised condition. The proposed method allows a simple quantitative assessment of the degree of sensitisation of different microstructures and heats of austenitic stainless steels

A new approach for DL-EPR testing of thermo-mechanically processed austenitic stainless steel

Standard test methods such as the electrochemical potentiokinetic reactivation test (EPR) and double- loop EPR test (DL-EPR) are commonly used to characterise sensitisation behaviour in austenitic stainless steels and nickel-based alloys. In this study, the DL-EPR test is augmented by large-area image analysis (IA) to characterise and quantify the networks of attacked grain boundaries. A new analysis approach that is based on a grain boundary cluster parameter is proposed to describe the network of corrosion suscep- tible grain boundaries, which may be estimated from electron backscatter diffraction (EBSD) data. This method may provide a better assessment of the relative DOS of different heats of austenitic stainless steels

Characterisation of grain boundary cluster compactness in austenitic stainless steel

The distribution of grain boundaries of particular crystallographic character can provide descriptive information on the properties of engineering materials. For example, the fraction and connectivity of corrosion susceptible grain boundaries typically correlates with the extent of intergranular corrosion and stress corrosion cracking resistance in sensitised austenitic stainless steels. A parameter defining the cluster compactness is proposed to describe the breakup of the network of corrosion susceptible grain boundaries. It may therefore provide a measure of intergranular stress corrosion cracking resistance. The cluster compactness of the network of random grain boundaries (.S29) in electron backscatter diffraction assessments of micro-structure is shown to decrease with increasing fraction of S3 boundaries. However, the cluster compactness of the network of corroded grain boundaries identified after electrochemical testing is less sensitive to changes in microstructure obtained by thermomechanical processing

Evolution of microstructure in MLX®19 maraging steel during rotary friction welding and finite element modelling of the process

Inertia friction welding (IFW) is a solid-state welding process for joining engineering materials. In this paper, a 2.5D finite element (FE) model was developed to simulate IFW of MLX®19 maraging steel. The predicted results showed a non-uniform temperature distribution, with a decrease in temperature from the periphery to the centre of weld interface. Higher temperature and lower stress distributions were predicted in the weld zone (WZ) and the adjacent regions in the vicinity of the WZ. The von-Mises effective stress, effective strain and strain-rate were investigated at different time steps of the FE simulation. The effective stress was minimum at the weld interface, and the effective strain and strain-rate attained a quasi-steady state status with the on-going IFW after a threshold time (~ 6.5 s). The simulated results were validated by comparing the predicted flash morphology with an actual IFW weld, and temperature profiles measured at specific locations using embedded thermo-couples. The difference between experimental and the simulated results was ~ 4.7%, implying a good convergence of the model. Microstructural characterisations were performed across different regions and the observed features were found to be in agreement with the expected microstructure based on the simulated thermal profiles, which included almost complete (~90%) and partial transformation of martensite to austenite in the WZ and thermo-mechanically affected zone (TMAZ), respectively. Analyses of crystallographic texture showed that the material (i.e., both transformed austenite and martensite) underwent pure shear deformation during IFW