Energy and force analysis of Ti-6Al-4V linear friction welds for computational modeling input and validation data

The linear friction welding (LFW) process is finding increasing use as a manufacturing technology for the production of titanium alloy Ti-6Al-4V aerospace components. Computational models give an insight into the process, however, there is limited experimental data that can be used for either modeling inputs or validation. To address this problem, a design of experiments approach was used to investigate the influence of the LFW process inputs on various outputs for experimental Ti-6Al-4V welds. The finite element analysis software DEFORM was also used in conjunction with the experimental findings to investigate the heating of the workpieces. Key findings showed that the average interface force and coefficient of friction during each phase of the process were insensitive to the rubbing velocity; the coefficient of friction was not coulombic and varied between 0.3 and 1.3 depending on the process conditions; and the interface of the workpieces reached a temperature of approximately approximately 1273 K (1000 °C) at the end of phase 1. This work has enabled a greater insight into the underlying process physics and will aid future modeling investigations.EPSRC, Boeing Company, Welding Institut

Maximizing the integrity of linear friction welded Waspaloy

The Ni-base superalloy, Waspaloy, was linear friction welded (LFWed) under various processing parameters and then subjected to a post weld heat treatment (PWHT). Tensile testing integrated with the optical image correlation Aramis\uae system indicated that there is a critical axial shortening value (2 mm) below which LFWed and post weld heat treated (PWHTed) Waspaloy exhibited weak integrity. At and above this critical shortening, the yield strength and ultimate tensile stress (UTS) values were more or less the same as for the parent material. However, total elongation continued to increase with axial shortening even above the critical value due to decrease in width of thermo-mechanically affected zone (TMAZ). The sample with the highest axial shortening (4.9 mm) exhibited an elongation 91% of the parent material elongation. According to Aramis\uae data, the mixture rule can be used reliably to determine the contribution of TMAZ to the tensile elongation of PWHTed Waspaloy. Microstructure characterization across the weld in the as-LFWed and PWHTed conditions was carried out to correlate the process parameters and microstructural changes that affect the tensile properties. Weak integrity at axial shortening below 2 mm was mainly due to lack of bonding and/or presence of oxides at the weld interface. In the as-welded condition, a loss in hardness was observed, and related to the extensive dissolution of strengthening phase (\u3b3\u2032) in the weld area. The applied PWHT restored the hardness in the weld region.Peer reviewed: YesNRC publication: Ye

Evolution of flow stress and microstructure during isothermal compression of Waspaloy

The evolution of the flow stress and microstructure for Waspaloy was studied in the 950-1140\ub0C temperature range under constant true strain rate conditions of 0.001-1s-1 up to a true strain of 0.83 using isothermal hot compression testing. The impact of friction at the sample/anvil interface and adiabatic heating during deformation on the flow stress evolution was also examined. Mathematical models relating the flow stress to the deformation temperature and strain rate were derived using a power-law relationship. The strain rate sensitivity and the activation energy for hot deformation of Waspaloy were found to be considerably different for deformation in the subsolvus and supersolvus temperature ranges. According to the microstructural investigations, at 950\ub0C dynamic recovery (DRV) was the main softening mechanism. By contrast, dynamic recrystallization (DRX), partial or complete, occurred at temperatures above 950\ub0C and resulted in flow softening.Peer reviewed: YesNRC publication: Ye

Analysis of integrity and microstructure of linear friction welded Waspaloy

Nickel-base superalloy, Waspaloy, was linear friction welded (LFWed) under different axial shortening conditions of 2.0, 3.4, and 4.6 mm. The tensile properties and microhardness of the weldments were investigated in the as-LFWed condition and compared with those in the post-weld heat treated (PWHTed) condition. Mechanical properties were related to microstructures following examination by optical microscopy, high resolution scanning electron microscopy, and electron backscatter diffraction (EBSD). Analyses of the EBSD results in terms of the misorientation angle distribution, which represents the stored energy, were performed. In the as-LFWed condition, the yield strength (YS) and ultimate tensile strength (UTS) increased with axial shortening due to greater expulsion of the softened interfacial material toward the periphery as flash. In contrast, with increasing axial shortening the total elongation initially remained constant and then decreased. This was also related to the expulsion of the softened interfacial material into the bifurcated flash. Extensive dissolution of the strengthening phase (\u3b3\u2032) in the weld area during linear friction welding (LFW) contributed to the lower YS and UTS in the as-welded condition compared to the PWHTed condition where the \u3b3\u2032 particles were recovered. After performing post-weld heat treatment (PWHT), the total elongation improved due to the relaxation of stored energy and grain growth in the thermomechanically affected zone (TMAZ).Peer reviewed: YesNRC publication: Ye

Modeling grain size and strain rate in linear friction welded Waspaloy

The high-temperature deformation behavior of the Ni-base superalloy, Waspaloy, using uniaxial isothermal compression testing was investigated at temperatures above the \u3b3\u2032 solvus, 1333 K, 1373 K, 1413 K (1060 C, 1100 C, 1140 C) for constant true strain rates of 0.001, 0.01, 0.1, 1 s -1 and up to a true strain of 0.83. Flow softening and microstructural investigation indicated that dynamic recrystallization took place during deformation. For the investigated conditions, the strain rate sensitivity factor and the activation energy of hot deformation were 0.199 and 462 kJ/mol, respectively. Constitutive equations relating the dynamic recrystallized grain size to the deformation temperature and strain rate were developed and used to predict the grain size and strain rate in linear friction-welded (LFWed) Waspaloy. The predictions were validated against experimental findings and data reported in the literature. It was found that the equations can reliably predict the grain size of LFWed Waspaloy. Furthermore, the estimated strain rate was in agreement with finite element modeling data reported in the literature. \ua9 2013 The Minerals, Metals & Materials Society and ASM International.Peer reviewed: YesNRC publication: Ye

Suppressed liquation and microcracking in linear friction welded WASPALOY

Fusion welding of nickel-base superalloys is often associated with fusion zone solidification cracking and/or liquation induced heat affected zone (HAZ) cracking. As an alternative joining technology, linear friction welding (LFW) was used in the current study to join the nickel-base superalloy, WASPALOY. Under the experimental conditions used in the present investigation, the temperature data recorded by inserting thermocouples at different locations from the weld interface indicated that the temperature in the weld area reached up to 1280 \ub0C, which is at least 50 \ub0C below the melting point of the bulk alloy. However, this temperature is well above the liquation temperature of the low melting point components in the alloy (1245 \ub0C). As a result, liquation may occur in linear friction welded (LFWed) WASPALOY. The occurrence of liquation and/or microcracking was investigated using optical microscopy (OM), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) X-ray mapping. The high pressure applied during the oscillation and forge phases of the LFW process and the resulting \u3b3 grain refinement contributed in preventing liquation and microcracking in the weldments. Furthermore, according to the SEM and X-ray mapping results, LFW altered the chemical composition, morphology and size of the \u3b3\u2032 precipitates at a location of 2 mm from the weld interface. It was determined that \u3b3\u2032 coalescence at 2 mm from the weld interface played a role in decreasing the microhardness (by 30%) relative to the base metal.Peer reviewed: YesNRC publication: Ye

Microstructure and Mechanical Properties of Laser Welded Ti–10V–2Fe–3Al (Ti1023) Titanium Alloy

The microstructure, microhardness, tensile properties, and fracture characteristics of the laser welded Ti–10V–2Fe–3Al (Ti1023) titanium alloy in the as-welded condition were examined. The mechanical properties were related to the microstructure development across the weld. In the base material (BM), the primary α phase with spherical and lath morphologies was dispersed in the β matrix. The volume fraction of the α phase in the heat affected zone (HAZ) decreased to some extent compared to the BM as a result of its partial dissolution and/or transformation to the β phase. In the fusion zone (FZ), primary α phase was completely transformed to the β phase. The BM exhibited a higher hardness than HAZ and FZ due to a higher volume fraction of the primary α phase, which is harder than β phase. The yield strength (YS) and ultimate tensile strength (UTS) of the weldments were somewhat lower than those of the BM due to the presence of a softer phase in the FZ and a lower volume fraction of the α phase in the HAZ. Also, the presence of porosity, undercut, concavity, and coarse columnar β grains in the FZ contributed to lower YS, UTS, and total elongation in the weldments in comparison to the unwelded material. Examination of the fracture surface in the weldment tensile samples indicated a mixed brittle and ductile fracture mode

Linear friction welding behavior of Waspaloy

Over conventional joining methods, linear friction welding (LFW) exhibits a high weld quality and economical benefits for the aerospace industry. In particular, LFW enables the removal of the fir-tree in conventional blade to disk assembly, which results in weight reduction and improvement in engine efficiency. Considering these advantages, the LFW behavior of Waspaloy, used in jet engines as blade and disc material, has been investigated at different processing conditions. This study specifically highlights the influence of frequency, amplitude, and pressure during oscillation on the microstructure and mechanical properties of the welded samples. Microstructure and mechanical characteristics of the thermo-mechanically affected zones (TMAZ) were investigated by optical microscopy, EBSD, SEM, and microhardness. The LFW operating window resulting in welds free from defects was determined from these data. Microstructure examination revealed that dynamic recrystallization (DRX) occurred in the 0.9 mm narrow band of the TMAZ resulting in up to 50% reduction in the grain size. Furthermore, at the weld interface a considerable volume fraction of the ?' precipitates dissolved, contributing to a drop in hardness.Peer reviewed: YesNRC publication: Ye