Through-Thickness Residual Stresses, Microstructure, and Mechanical Properties of Electron Beam-Welded CA6NM Martensitic Stainless Steel after Postweld Heat Treatment

In this study, the integrity of electron beam- (EB-) welded CA6NM—a grade of 13% Cr-4% Ni martensitic stainless steel—was assessed through the entire joint thickness of 90 mm after postweld heat treatment (PWHT). The joints were characterized by examining the microstructure, residual stresses, global mechanical properties (static tensile, Charpy impact, and bend), and local properties (yield strength and strain at fracture) in the metallurgically modified regions of the EB welds. The applied PWHT tempered the "fresh" martensite present in the microstructure after welding, which reduced sufficiently the hardness (<280 HV) and residual stresses (<100 MPa) to meet the requirements for hydroelectric turbine assemblies. Also, the properties of the EB joints after PWHT passed the minimum acceptance criteria specified in ASME sections VIII and IX. Specifically, measurement of the global tensile properties indicated that the tensile strengths of the EB welds in the transverse and longitudinal directions were on the same order as that of the base metal (BM). Evaluation of the local tensile properties using a digital image correlation (DIC) methodology showed higher local yield strengths in the fusion zone (FZ) and heat-affected zone (HAZ) of 727 MPa and 740 MPa, respectively, relative to the BM value of 663 MPa. Also, the average impact energies for the FZ and HAZ were 63 J and 148 J, respectively, and attributed to the different failure mechanisms in the HAZ (dimples) versus the FZ (quasi-cleavage consisting of facets and dimples). This study shows that the application of PWHT plays an important role in improving the weld quality and performance of EB-welded CA6NM and provides the essential data for validating the design and manufacturing process for next-generation hydroelectric turbine products

Evaluation of Key Spatiotemporal Learners for Print Track Anomaly Classification Using Melt Pool Image Streams

Recent applications of machine learning in metal additive manufacturing (MAM) have demonstrated significant potential in addressing critical barriers to the widespread adoption of MAM technology. Recent research in this field emphasizes the importance of utilizing melt pool signatures for real-time defect prediction. While high-quality melt pool image data holds the promise of enabling precise predictions, there has been limited exploration into the utilization of cutting-edge spatiotemporal models that can harness the inherent transient and sequential characteristics of the additive manufacturing process. This research introduces and puts into practice some of the leading deep spatiotemporal learning models that can be adapted for the classification of melt pool image streams originating from various materials, systems, and applications. Specifically, it investigates two-stream networks comprising spatial and temporal streams, a recurrent spatial network, and a factorized 3D convolutional neural network. The capacity of these models to generalize when exposed to perturbations in melt pool image data is examined using data perturbation techniques grounded in real-world process scenarios. The implemented architectures demonstrate the ability to capture the spatiotemporal features of melt pool image sequences. However, among these models, only the Kinetics400 pre-trained SlowFast network, categorized as a two-stream network, exhibits robust generalization capabilities in the presence of data perturbations.Comment: This work has been accepted to IFAC for publication under a Creative Commons Licence CC-BY-NC-N

Joining of dissimilar alloys Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo-0.1Si using linear friction welding

Dissimilar joints between Ti-6Al-4V (Ti-64) and Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti-6242) were manufactured using linear friction welding. The weld quality, in terms of the microstructure and mechanical properties, was investigated after stress relief annealing (SRA) at 750 °C for 2 h and compared with the as-welded (AWed) results. The central weld zone (CWZ) microstructure in the AWed condition consisted of recrystallized prior-β grains with α' martensite, which transformed into an acicular α+β structure after SRA. The hardness in the AWed condition was highest in the CWZ and decreased sharply through the thermomechanically affected zones (TMAZ) to the parent materials (PMs). After SRA, the hardness of the CWZ decreased, mainly due to tempering of the α' martensite microstructure. Static tensile testing of the dissimilar welds in both the AWed and stress relief annealed (SRAed) conditions resulted in ductile fracture occurring exclusively in the Ti-6Al-4V side of the joint. The promising results on joining of Ti-64 to Ti-6242 provide valuable insight for tailoring performance of next-generation aero-engine products

Characterization of Ti-6%A1-4%VTiC particulate reinforced metal matrix composites consolidated by sintering and thermomechanical processing

TiC reinforcement particles were incorporated into a Ti-6 %Al-4%V matrix and processed by two powder metallurgy techniques, namely elevated temperature pressureless sintering and hot deformation-assisted sintering (also known as hot pressing). For these composites, processing by sintering alone necessitated high temperatures (>1500°C) for near-complete density consolidation, whilst the conditions for temperature and hold time were reduced (i.e. 1000°C and 1/2 hour) through deformation-assisted sintering. During high temperature processing in the absence of deformation, considerable coarsening of the lamellar matrix microstructure occurred. The interfacial reaction between the reinforcement and matrix was characterized by in situ neutron diffraction sintering studies at temperatures between 1100°C and 1350°C. Initial reaction occurred by carbon diffusion from the TiC particle to the titanium alloy, as evidenced through the increase in the lattice parameter of the matrix phase with holding time at the various sintering temperatures. Beyond the carbon solubility limit of the matrix phase, a stable stoichiometric phase formed as shown by the appearance of distinct peaks in the neutron diffraction patterns. Room temperature lattice parameter measurement gave a value of 4.290 A with a fractional occupancy of carbon of 0.45 +/- 0.04, which corresponds to a stoichiometry of Ti2C. For the various isothermal sintering temperatures, change in the Ti2C volume fraction with hold time was determined and growth of this interfacial phase was reasoned to occur by carbon diffusion from the TiC particles, through the reaction zone and to the Ti-6%Al-4%V alloy. Transformation of the entire TiC particle to Ti2C occurred in the composites sintered at 1500°C. For the composites processed by sintering only, the mechanical properties determined by shear punch testing indicated that the strength and ductility are limited at low temperature sintering because of high po

Hybrid laser-arc welding of AA6061-T6 butt joints

Hybrid laser-arc welding (HLAW) of butt joints in 3.18 mm thick aluminum alloy (AA) 6061-T6 sheets was investigated in the present study. Under optimized process conditions, high integrity welds with approximately 2% shrinkage and gas porosity were obtained. The weld bead geometry was determined to conform to the crown and root reinforcement specifications for welded aluminum construction - CSA W 59.2 M1191. Softening in the heat-affected zone (HAZ) and fusion zone (FZ) of the welds was observed, the former due to grain coarsening and the latter due to the dissolution of the hardening precipitates as well as the dilution from the application of the ER 5356 filler wire. Under optimized process conditions, the gap tolerance was determined to be 0.5 mm, beyond which the performance of the joints during bend testing was compromised.Peer reviewed: YesNRC publication: Ye

Special issue on advances in high temperature joining of materials

Peer reviewed: YesNRC publication: Ye

Fiber Laser Deposition of INCONEL 718 Using Powders

Additive manufacturing is an emerging manufacturing and remanufacturing (repair) technique for value-added materials such as Inconel 718, particularly using a laser beam as the heating source. In this research work additive manufacturing of Inconel 718 using a continuous wave fiber laser and filler wire feed was studied. To simulate industrial conditions, the deposits were built on parent metal substrates that were extracted from an after-service Inconel 718 aerospace component. The evolution of the macrostructure, defects, microstructure and hardness of the deposits was evaluated in the as-deposited and post-clad heat treated conditions. The hardness of the laser deposits can be fully recovered to the level of the after-serviced parent metal after solution heat treatment and aging. The high integrity deposits had no visible pores but only some minor weld metal liquation cracking, which indicates the great potential of the laser deposition process to manufacture and repair superalloy components for aerospace applications.A CD-only volumePeer reviewed: YesNRC publication: Ye

Evidence for stable stoichiometric Ti2C at the interface in TiC particulate reinforced Ti alloy composites

Ti-6%Al-4%V composites containing 20?vol.% TiC particles were sintered at temperatures between 1273 and 1773?K for holding times of up to 20?h. Neutron diffraction and low voltage field emission gun scanning electron microscopy were used to investigate the development of the interfacial reaction region between the reinforcement and matrix. It has been observed that there is an interaction zone surrounding each particle caused by the diffusion of carbon from the reinforcement to the titanium alloy matrix. The extent of this reaction increases with increasing processing temperature and holding time. The single phase formed at the interfacial boundary between the particles and the matrix was determined from lattice parameter measurements to be stoichiometric Ti2C. The significance of these findings are discussed in terms of previous work on interfacial characterization of TiC particulate reinforced Ti-6%Al-4%V composites.NRC publication: Ye

Microstructure, Tensile Properties, and Fatigue Behavior of Linear Friction-Welded Ti-6Al-2Sn-4Zr-2Mo-0.1Si

This paper presents the microstructural characteristics and mechanical properties of linear friction-welded (LFWed) Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti-6242) in as-welded (AWed) and stress relief-annealed (SRAed) conditions. The weld center (WC) of the AWed Ti-6242 consisted of recrystallized prior-&beta; grains with &alpha;&rsquo; martensite that were tempered during SRA at 800 &deg;C for 2 h and transformed into an acicular &alpha; + &beta; microstructure. The peak hardness values, obtained in the AWed joints at the WC, sharply decreased through the thermomechanically affected zones (TMAZs) to the heat-affected zone (HAZ) of the Ti-6242 parent metal (PM). The SRA lowered the peak hardness values at the WC slightly and fully recovered the observed softening in the HAZ. The tensile mechanical properties of the welds in the AWed and SRAed conditions surpassed the minimum requirements in the AMS specifications for the Ti-6242 alloy. Fatigue tests, performed on the SRAed welds, indicated a fatigue limit of 468 MPa at 107 cycles, just slightly higher than that of the Ti-6242 PM (434 MPa). During tensile and fatigue testing, the welds failed in the PM region, which confirms the high mechanical integrity of the joints. Both the tensile and fatigue fracture surfaces exhibited characteristic features of ductile Ti-6242 PM