P. Wanjara

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 Al–Cu: Part 1 – Process evaluation

Abstract Aluminium–copper assemblies are used as power connectors in various industrial processes. Fusion welding of aluminium to copper faces two main challenges that are related to the high thermal conductivity of the materials and the aggressive reaction between the materials that forms brittle intermetallic phases at the interface. Though high energy density techniques such as electron beam welding can overcome the thermal conductivity issue, only solid state joining techniques can viably manufacture the aluminium–copper assemblies while minimising the formation of the intermetallic phases that are problematic for the electrical conductivity and efficiency of the connector. In this work, an alternative approach for manufacturing the aluminium–copper assemblies has been developed using linear friction welding, an emergent solid state joining technology. The influence of process conditions on the joint integrity and the characteristics of the interface were studied using optical microscopy and electrical conductivity measurements. Under optimum processing conditions, integral joints with limited intermetallic phases at the interface could be repeatedly produced. In comparison with aluminium–copper assemblies manufactured by explosive welding, the fraction and size of the intermetallic phases located at the interface, as well as the extent of the interfacial region over which changes in the properties occur, were determined to be considerably reduced when employing linear friction welding. Des assemblages d’aluminium-cuivre sont utilisés comme connecteurs d’alimentation dans divers processus industriels. Le soudage par friction de l’aluminium au cuivre fait face à deux obstacles principaux qui sont reliés (1) à la conductivité thermique élevée des matériaux et (2) à la réaction agressive entre les matériaux, ce qui forme des phases intermétalliques fragiles à l’interface. Bien que des techniques à haute densité d’énergie, comme le soudage par faisceau d’électrons, puissent surmonter le problème de la conductivité thermique, seules les techniques d’assemblage par diffusion peuvent fabriquer de façon viable les assemblages d’aluminium-cuivre tout en minimisant la formation des phases intermétalliques qui sont problématiques pour la conductivité électrique et pour le rendement du connecteur. Dans ce travail, on a développé une approche de rechange pour la fabrication d’assemblages d’aluminium-cuivre en utilisant le soudage par friction linéaire (LFW), une technologie émergente d’assemblage par diffusion. On a étudié l’influence des conditions du processus sur l’intégrité de l’assemblage et sur les caractéristiques de l’interface en utilisant la microscopie optique et des mesures de conductivité électrique. Sous des conditions optimales de traitement, on pouvait produire à répétition des joints intégraux avec des phases intermétalliques limitées à l’interface. Par rapport aux assemblages d’aluminium-cuivre fabriqués par soudage par explosion (EW), on a déterminé que la fraction et la taille des phases intermétalliques situées à l’interface, ainsi que l’étendue de la région interfaciale où il se produit des changements de propriétés, étaient considérablement réduites lorsqu’on utilisait le LFW.

Thin Gauge Titanium Manufacturing Using Multiple-Pass Electron Beam Welding

The optimum processing conditions for joining thin sheet materials (1–3.2 mm) of commercial purity titanium and Ti-6%Al-4%V by electron beam welding were determined by using single- and double-pass melting sequences. For commercial purity titanium weldments, the tensile properties were observed to be similar to the parent material and independent of the sheet thickness and the welding procedure. However, for Ti-6%Al-4%V, the processing conditions during welding were observed to change the characteristics of the alpha lamellae spacing and plate thickness, which in turn influenced the properties. Hence, for joining alpha-beta titanium alloys with use of electron beam welding, the definition of critical processing conditions enables optimization of the structure-property characteristics of the joint.

A Study of Linear Friction Weld Microstructure in Single Crystal CMSX-486 Superalloy

The microstructure of linear friction welds in single crystal (SX) CMSX 486 superalloy was studied. Gleeble thermomechanical simulation of the welding process was also performed in order to understand the microstructural changes induced in the alloy during the joining process. Microstructural analysis of the welded and Gleeble-simulated specimens showed that extensive liquation occurred in the alloy during joining, which is in contrast to the general assumption that linear friction welding (LFW) occurs exclusively in the solid state. The study revealed the application of the compressive load during the forging stage of LFW induced rapid solidification of the resultant metastable liquid phase. Nevertheless, part of the liquid resulted in a continuous Hf-base oxide phase along the weld line. Possible ways of preventing the formation of the potentially deleterious oxide film and, thus, improve the prospect of applying LFW for the joining of CMSX-486 superalloy are suggested.

Linear friction welding of Al–Cu Part 2 – Interfacial characteristics

Abstract Metallurgically bonded aluminium–copper assemblies are used as electrical transition pieces in high direct current bus systems to transmit electricity. The brittle, high electrical resistance intermetallics that tend to form at the interface between these two materials are problematic for the electrical conductivity and efficiency of the connector. The manufacture of such assemblies therefore requires low heat input during joining so as to minimise the extent of formation of the intermetallic phases. The application of linear friction welding (LFW), an emerging high energy density solid state joining technology, for this purpose limits the formation of the intermetallic phases and produces more promising interfacial characteristics. In this work, aluminium–copper assemblies with two different geometries, namely, laboratory scale and full scale (customised to actual industrial requirements) were produced by LFW. These were characterised using scanning electron microscopy (SEM), electron probe microanalysis (EPMA) and energy dispersive spectrometry (EDS) to identify the phases at the interface. In order to simulate the in-service behaviour of the aluminium–copper assemblies manufactured by LFW, the laboratory scale samples were heat treated at 300°C. The evolution in the interfacial region was compared with that of post-service connectors produced by explosive welding (EW) and LFW. Les assemblages d’aluminium–cuivre à joint métallurgique sont utilisés comme pièces de transition électrique dans les systèmes de barre omnibus à courant continu élevé pour transmettre l’électricité. Les intermétalliques fragiles, à haute résistance électrique, qui tendent à se former à l’interface entre ces deux matériaux sont problématiques pour la conductivité électrique et pour le rendement du connecteur. La fabrication de tels assemblages requiert donc un faible apport de chaleur lors de l’assemblage afin de minimiser le degré de formation des phases intermétalliques. L’application du soudage linéaire par friction (LFW), une technologie émergeante d’assemblage par diffusion à haute densité d’énergie, pour cet objectif, limite la formation des phases intermétalliques et produit des caractéristiques interfaciales plus prometteuses. Dans ce travail, des assemblages d’aluminium–cuivre à deux géométries différentes, soit à l’échelle du laboratoire et à pleine échelle (adaptés pour des besoins industriels réels), ont été produits par LFW. On les a caractérisés en utilisant la microscopie électronique à balayage (SEM), la microanalyse par électrons (EPMA) et la spectroscopie à dispersion d’énergie (EDS) afin d’identifier les phases interfaciales. Afin de simuler le comportement en service des assemblages d’aluminium–cuivre fabriqués par LFW, on a traité thermiquement à 300°C les échantillons à l’échelle du laboratoire. On a comparé l’évolution de la région interfaciale à celle de connecteurs d’après service produits par soudage par explosion (EW) et par LFW.

EFFECT OF SPECIMEN THICKNESS AND PUNCH DIAMETER IN SHEAR PUNCH TESTING

Abstract A good method to directly measure the mechanical properties of a material deformed in compression or torsion is by using the shear punch test. Unlike tensile testing, the shear punch test is not limited by specimen size or shape. However, it is unclear whether there is a limit to the allowable specimen thickness and punch diameter in order to attain (through conversion) tensile properties with reliability and accuracy. According to this study, it was found that the shear punch test was not sensitive to varying specimen thicknesses or punch diameters over a range of 200 to 400 μm and 1.5 to 5 mm, respectively. L’essai de cisaillement par poinçonnage constitue une bonne méthode de mesure directe des propriétés mécaniques d’un matériau déformé en compression ou en torsion. Contrairement à l’essai de traction, l’essai de cisaillement par poinçonnage n’est pas limité par la taille ou la géométrie de l’échantillon. Cependant, la gamme d’épaisseurs de l’échantillon ainsi que de diamètres du poinçon qui permettent d’obtenir (après conversion) des propriétés de traction fiables n’est pas clairement définie. Dans cette étude, nous montrons que l’essai de cisaillement par poinçonnage est indépendant de l’épaisseur de l’échantillon pour des épaisseurs allant de 200 à 400 μm ainsi que du diamètre du poinçon s’il est entre 1.5 et 5 mm.

Enhanced resistance to weld cracking by strain-induced rapid solidification during linear friction welding

A study of weld cracking resistance during linear friction joining of a difficult-to-weld nickel-based superalloy was performed by Gleeble thermomechanical simulation coupled with an extensive microstructural analysis. It was found that crack-free welds produced by the supposedly solid-state joining technique of linear friction welding is not due to preclusion of grain boundary liquation as has been commonly assumed and reported. Instead, resistance to cracking can be related to a reduction in liquid stability by the imposed compressive strain during welding.

Tolerances of joint gaps in Nd:YAG laser welded Ti-6Al-4V alloy with the addition of filler wire

The effect of joint gap on the butt joint quality of Ti-6Al-4V alloy welded using a 4 kW Nd:yttrium aluminum garnet laser was evaluated in terms of the welding defects, microstructure, hardness, and tensile properties. The joint gap was proportionally filled using the filler wire with the compositions of the parent alloy. Fully penetrated welds without cracking were obtained up to a joint gap of 0.5 mm. The main defects observed in the welds were porosity and underfill. Specifically, the porosity area increased with increasing joint gap but remained less than 1% of the fusion zone area. Large underfill defects appeared in the weldments in the absence of a joint gap, but filler wire addition was observed to reduce this defect in the presence of a joint gap. The weld hardness decreased slightly with increasing joint gap, but the tensile properties were optimized at an intermediary gap size, probably due to the compromise between the low underfill (after the use of a filler wire) and a limited amount of poro...

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.

Laser welding of Ti–5Al–5V–5Mo–3Cr

Abstract Ti–5Al–5V–5Mo–3Cr butt joints were welded using a 4 kW continuous wave Nd:YAG laser. The effect of welding speed and defocusing distance on the weld quality was investigated. Welds with full penetration were achieved at a defocusing distance ranging from −1 to 0 mm and welding speeds from 2·25 to 6·0 m min−1. Underfill and porosity were the two main defects most frequently observed; however, within the optimum process window, these defects could be maintained to meet aerospace specification tolerances. The fusion zone consisted entirely of retained β with a refined dendritic morphology. Compared with the bimodal α+β microstructure of the base metal, dissolution of the α phase in the heat affected zone and the presence of entirely metastable/retained β phase in the fusion zone were observed, which led to a significant decrease in the HAZ and FZ hardness. To determine the tensile properties of the welds, an automated three-dimensional deformation measurement system was used to measure the local strain in the weld region. The yield strength and ultimate tensile strength increased with welding speed, achieving a maximum joint efficiency of 75%, albeit with a concomitant reduction in the ductility. On a soudé des joints de Ti-5553 bout à bout en utilisant un laser continu Nd:YAG de 4 kW. On a étudié l’effet de la vitesse de soudage et de la distance de défocalisation sur la qualité de la soudure. On a obtenu des soudures avec pénétration complète à une distance de défocalisation allant de −1 mm à 0 mm et à des vitesses de soudage de 2·25 à 6·0 m/min. Le manque de métal et la porosité étaient les deux principaux défauts les plus souvent observés, mais dans le cadre de la fenêtre optimale du procédé, on pourrait contenir ces défauts afin de satisfaire les tolérances de la spécification aérospatiale. La zone de fusion consistait entièrement de bêta résiduel à morphologie dendritique raffinée. Comparée à la microstructure bimodale a+b du métal de base, on a observé la dissolution de la phase a dans la zone affectée par la chaleur (HAZ) et la présence de la phase b entièrement métastable/résiduelle dans la zone de fusion (FZ), ce qui menait à une diminution significative de la dureté de la HAZ et de la FZ. Afin de déterminer les propriétés de traction des soudures, on a utilisé un système 3D automatisé de mesure de la déformation pour mesurer l’allongement local dans la région de la soudure. La limite d’élasticité et l’UTS augmentaient avec la vitesse de soudage, obtenant un rendement maximum du joint de 75%, quoique avec une réduction concomitante de la ductilité.