Showing papers on "Heat-affected zone published in 2011"

Microstructural characteristics and toughness of the simulated coarse grained heat affected zone of high strength low carbon bainitic steel

Abstract: The correlation of microstructural characteristics and toughness of the simulated coarse grained heat affected zone (CGHAZ) of low carbon bainitic steel was investigated in this study. The toughness of simulated specimens was examined by using an instrumented Charpy impact tester after the simulation welding test was conducted with different cooling times. Microstructure observation and crystallographic feature analysis were conducted by means of optical microscope and scanning electron microscope equipped with electron back scattered diffraction (EBSD) system, respectively. The main microstructure of simulated specimen changes from lath martensite to coarse bainite with the increase in cooling time. The deterioration of its toughness occurs when the cooling time ranges from 10 to 50 s compared with base metal toughness, and the toughness becomes even worse when the cooling time increases to 90 s or more. The MA (martensite–austenite) constituent is primary responsible for the low toughness of simulated CGHAZ with high values of cooling time because the large MA constituent reduces the crack initiation energy significantly. For crack propagation energy, the small effective grain size of lath martensite plays an important role in improving the crack propagation energy. By contrast, high misorientation packet boundary in coarse bainite seems to have few contributions to the improvement of the toughness because cleavage fracture micromechanism of coarse bainite is mainly controlled by crack initiation.

Morphology investigation on direct current pulsed gas tungsten arc welded additive layer manufactured Ti6Al4V alloy.

Abstract: The effects of pulsed gas tungsten arc welding parameters on the morphology of additive layer manufactured Ti6Al4V has been investigated in this study. The peak/base current ratio and pulse frequency are found to have no significant effect on the refinement of prior beta grain size. However, it is found that the wire feed rate has a considerable effect on the prior beta grain refinement at a given heat input. This is due to the extra wire input being able to supply many heterogeneous nucleation sites and also results in a negative temperature gradient in the front of the liquidus which blocks the columnar growth and changes the columnar growth to equiaixal growth.

Fibre laser welding of dissimilar alloys of Ti-6Al-4V and Inconel 718 for aerospace applications

Abstract: Challenges in dissimilar materials welding are the differences of physical and chemical properties between welding materials and the formation of intermetallic brittle phases resulting in the degradation of mechanical properties of welds. However, dissimilar materials welding is increasingly demanded from the industry as it can effectively reduce material costs and improve the design. In aerospace applications, Ti-6Al-4V titanium alloy and Inconel 718 nickel alloy have been widely used because of their superior corrosion resistance and mechanical properties. In this study, a single-mode continuous-wave fibre laser was used in butt welding of Ti-6Al-4V to Inconel 718. Investigations including metallurgical and mechanical examinations were carried out by means of varying processing parameters, such as laser power, welding speed and the laser beam offset position from the interface of the metals. Simple analytical modelling analysis was undertaken to explain the phenomena that occurred in this process. Results showed that the formation of intermetallic brittle phases and welding defects could be effectively restricted at welding conditions produced by the combination of higher laser power, higher welding speed and shifting the laser beam from the interface to the Inconel 718 alloy side. The amount of heat input and position of laser beam to improve the Ti-6Al-4V/Inconel 718 weld quality are suggested.

Analysis of transient temperature and residual thermal stresses in friction stir welding of aluminum alloy 6061-T6 via numerical simulation

Abstract: Residual stress is lower in friction stir welding (FSW) compared with other melting weldment processes. This is due to being solid-state process in its nature. There are several advantages in utilizing stir welding process. Lower fluctuation and shrinkage in weldment metal-enhanced mechanical characteristics, less defects, and ability to weld certain metals otherwise impractical by other welding processes are to name just a few of these advantages. These have caused an ever increasing attention by the concerned to the process of FSW. In this investigation, three-dimensional numerical simulation of friction stir welding was concerned to study the impact of tool moving speed in relation with heat distribution as well as residual stress. Simulation was composed of two stages. Firstly, thermal behavior of the piece while undergoing the welding process was studied. Heat is generated due to the friction between tool and the piece being welded. In the second stage, attained thermal behavior of the piece from previous stage is considered as inlet heat of an elasto-plastic, thermo-mechanical model for the prediction of residual stress. Also, in the second stage, tool is eliminated and residual stress distribution is found after complete cooling of the piece and disassembly of the clamp. Material characteristic are introduced into the proposed model as temperature-dependent parameters. Obtained residual indicate that heat distribution along thickness varies and is asymmetrical enormously. Moreover, longitudinal residual stress in the weld which increases as speed of process and tool movement ascends. In the prediction of results of residual stress, only heat impact was studied. This was recognized as the main element causing minor difference in results obtained for simulation in comparison with that of actual experiment.

Spatter in laser welding

Abstract: Spatter, the ejection of melt from a weld pool, is a major problem whenever it occurs in a welding process. The ejection of droplets from the weld metal results in a weld with underfill, undercuts, craters, blowholes, or blowouts—all of which can have a detrimental effect on the mechanical properties of the weld. This paper presents a systematic description of the different types of spatter phenomena which occur during laser welding. A categorization system is proposed to facilitate the comparison and combination of research findings on spatter. This should allow researchers in this area to act as a more effective team in future.

Formation and distribution of brittle structures in friction stir welding of aluminium and copper: influence of process parameters

Abstract: Morphological, metallographic and structural analyses of aluminium–copper dissimilar welds produced under different friction stir welding conditions were conducted in order to analyse the mechanisms of intermetallic phases formation, its relation with welding conditions and its consequences in the weld structure and morphology. Under lower heat input conditions, only a thin intermetallic layer distributed along the aluminium/copper interface was depicted inside the nugget. Increasing the heat input promoted material mixing and formation of increasing amounts of intermetallic rich structures. The intermetallic phase content and the homogeneity of the mixed area increased with increasing heat input, evolving from structures containing Al, Cu, CuAl2 and Cu9Al4 to structures predominantly composed of Cu9Al4 and Cu(Al). In order to explain these results, the mechanisms of intermetallic phases formation are discussed, taking into account the process parameters and material flow mechanisms in friction st...

Material interactions in a novel pinless tool approach to friction stir spot welding thin aluminum sheet

Abstract: The requirement for a probe, or pin, in friction stir spot welding (FSSW) leads to an undesirable keyhole and “hooking,” which can influence the fracture path and weld strength. Furthermore, the full weld cycle for FSSW is typically longer than ideal for the automotive industry, being 2 to 5 seconds. Here, it is shown that using a novel pinless tool design it is possible to achieve high lap shear strength (~3.4 kN) in thin aluminum sheet (~1 mm thick), with short weld cycle times (<1 second). Several techniques have been exploited to study the material flow and mechanisms of weld formation in pinless FSSW, including high-resolution X-ray tomography, to understand the role of the tool design and weld parameters. Despite the “simple” nature of a pinless tool, material flow in the weld zone was found to be surprisingly complex and strongly influenced by surface features on the tool, which greatly increased the penetration of the plastic zone into the bottom sheet. Because of the rapid thermal cycle and high level of grain refinement, the weld zone was found to develop a higher strength than the parent material with little evidence of a heat affected zone (HAZ) after postweld natural aging.

Ultrasonic spot welding of aluminium to steel for automotive applications—microstructure and optimisation

Abstract: Energy efficient methods for joining aluminium to steel have potential for major applications in the automobile industry. Results are reported where 1 mm gauge 6111 aluminium and DC04 steel automot...

Measuring the process efficiency of controlled gas metal arc welding processes

Abstract: The thermal or process efficiency in gas metal arc welding (GMAW) is a crucial input to numerical models of the process and requires the use of an accurate welding calorimeter. In this paper, the authors compare a liquid nitrogen calorimeter with an insulated box calorimeter for measuring the process efficiency of Fronius cold metal transfer, Lincoln surface tension transfer and RapidArc, Kemppi FastRoot and standard pulsed GMAW. All of the controlled dip transfer processes had a process efficiency of ∼85% when measured with the liquid nitrogen calorimeter. This value was slightly higher when welding in a groove and slightly lower for the RapidArc and pulsed GMAW. The efficiency measured with the insulated box calorimeter was slightly lower, but it had the advantage of a much smaller random error.

Effect of Pulse Parameters on Weld Quality in Pulsed Gas Metal Arc Welding: A Review

Abstract: The weld quality comprises bead geometry and its microstructure, which influence the mechanical properties of the weld. The coarse-grained weld microstructure, higher heat-affected zone, and lower penetration together with higher reinforcement reduce the weld service life in continuous mode gas metal arc welding (GMAW). Pulsed GMAW (P-GMAW) is an alternative method providing a better way for overcoming these afore mentioned problems. It uses a higher peak current to allow one molten droplet per pulse, and a lower background current to maintain the arc stability. Current pulsing refines the grains in weld fusion zone with increasing depth of penetration due to arc oscillations. Optimum weld joint characteristics can be achieved by controlling the pulse parameters. The process is versatile and easily automated. This brief review illustrates the effect of pulse parameters on weld quality.