Showing papers on "Welding published in 2010"

Cold welding of ultrathin gold nanowires

Abstract: The welding of metals at the nanoscale is likely to have an important role in the bottom-up fabrication of electrical and mechanical nanodevices. Existing welding techniques use local heating, requiring precise control of the heating mechanism and introducing the possibility of damage. The welding of metals without heating (or cold welding) has been demonstrated, but only at macroscopic length scales and under large applied pressures. Here, we demonstrate that single-crystalline gold nanowires with diameters between 3 and 10 nm can be cold-welded together within seconds by mechanical contact alone, and under relatively low applied pressures. High-resolution transmission electron microscopy and in situ measurements reveal that the welds are nearly perfect, with the same crystal orientation, strength and electrical conductivity as the rest of the nanowire. The high quality of the welds is attributed to the nanoscale sample dimensions, oriented-attachment mechanisms and mechanically assisted fast surface-atom diffusion. Welds are also demonstrated between gold and silver, and silver and silver, indicating that the technique may be generally applicable.

Friction stir welding of dissimilar alloys – a perspective

Abstract: Friction stir welding does not involve bulk melting of the components that are joined. This has inspired attempts to exploit it for joining materials which differ in properties, chemical composition or structure, and where fusion can lead to detrimental reactions. The purpose of this special issue of Science and Technology of Welding and Joining was to assess the status of friction stir welding of dissimilar alloys and to identify the opportunities and challenges for the future.

Melt pool and keyhole behaviour analysis for deep penetration laser welding

Abstract: One usually defines the main characteristic of the welding performances of a given laser system by its 'penetration curve' that corresponds to the welding depth as a function of the welding speed Vw for a given set of operating parameters. Analysis of a penetration curve is interesting and gives very fruitful results. Coupled with high-speed video imaging of melt pool surface and ejected plume behaviour, the analysis of this penetration curve on a very large range of welding speeds, typically from 0 to 50 m min−1, has allowed us to observe very different and characteristic regimes. These regimes are mainly characterized by the physical processes by which they impede the laser beam penetration inside the material. We show that it is only at rather high welding speeds that these limiting processes are reduced. Consequently, the scaling law of welding depth with welding speed is in agreement with adapted modelling of this process. On the other hand, as the welding speed is reduced, different effects depending on the weld pool dynamics and plume interaction strongly disturb the keyhole stability and are responsible for the deviation of the penetration curve from the previous modelling that agrees with a 1/Vw scaling law. A corresponding criterion for the occurrence of this effect is defined.

Mechanisms of joint and microstructure formation in high power ultrasonic spot welding 6111 aluminium automotive sheet

Abstract: Resistance spot welding (RSW) is difficult to apply to aluminium automotive alloys. High power ultrasonic spot welding (HP-USW) is a new alternative method which is extremely efficient, using ∼2% of the energy of RSW. However, to date there have been few studies of the mechanisms of bond formation and the material interactions that take place with this process. Here, we report on a detailed investigation where we have used X-ray tomography, high resolution SEM, and EBSD, and dissimilar alloy welds, to track the interface position and characterise the stages of weld formation, and microstructure evolution, as a function of welding energy. Under optimum conditions high quality welds are produced, showing few defects. Welding proceeds by the development and spread of microwelds, until extensive plastic deformation occurs within the weld zone, where the temperature reaches ∼380 °C. The origin of the weld interface ‘flow features’ characteristic of HP-USW are discussed.

Friction model for friction stir welding process simulation: Calibrations from welding experiments

Abstract: The accurate 3D finite element simulation of the Friction Stir Welding (FSW) process requires a proper knowledge of both material and interface behaviors, but friction, the key phenomenon of this process, is quite difficult to model and identify. According to the extreme encountered conditions and the highly coupled nature of the material flow, simple tribological tests are not representative enough, so the welding process itself has been utilized in most analyses of the literature, although its complexity has led to use simplified numerical models and approaches. The recent development of more accurate 3D simulation software, which allows modeling the entire complexity of the FSW process, makes it possible to follow a much more rigorous inverse analysis (or calibration) approach. FSW trials are conducted on an Al 6061 aluminum plate with an unthreaded concave tool. Forces and tool temperatures are accurately recorded at steady welding state, for different welding speeds. The numerical simulations are based on an Arbitrary Lagrangian Eulerian (ALE) formulation that has been implemented in the Forge3® F.E. software. The main feature of the numerical approach is to accurately compute the contact and frictional surface between the plate and the tool. A first study using Norton's friction model show the great sensitivity of welding forces and tool temperatures to friction coefficients, the need to take into account the changes brought to the contact surface by slight friction variations (thanks to the ALE formulation), the possibility to get very accurate calibrations on forces, and the impossibility to properly render the tool temperature profile. On the other hand, the use of Coulomb's friction model allows obtaining realistic temperature profiles and so calibrating a friction coefficient that offers an excellent agreement with experiments, on forces as much as on tool temperatures, for various welding speeds.

Measurement and Defect Detection of the Weld Bead Based on Online Vision Inspection

Abstract: Weld bead inspection is important for high-quality welding. This paper summarizes our work on weld bead profile measurement, monitoring, and defect detection using a structured light-based vision inspection system. The configuration of the sensor is described and analyzed. In this configuration, the system presented in this paper can easily be calibrated. The image processing and extraction algorithms for laser profiles and feature points are presented. The dimensional parameters of the weld bead are measured, and the weld defects are detected during multilayer welding processes. Experiments using the vision inspection system were conducted with satisfactory results for online inspection.

Intermetallic compounds in Al 6016/IF-steel friction stir spot welds

Abstract: The joining of a 1.2 mm thickness Al 6016 to a 2 mm thickness IF-steel has been performed by friction stir spot welding (FSSW). The intermetallic compounds (IMC) have been identified at the interface Al 6016/IF-steel and quantified as a function of the rotational speed and tool penetration. TEM observations indicated the presence of tangles of elliptical intermetallic compounds. FeAl 3 , Fe 2 Al 5 and FeAl 2 were identified depending on welding conditions. The influence of IMC on tensile shear strength has been established. An IMC layer seems necessary to improve the weld strength, but if the layer is too thick, cracks initiate and propagate easily through the hard IMC tangles.

Advances in friction welding process: a review

Abstract: Friction welding is now well established as one of the most economical and highly productive methods in joining similar and dissimilar metals. It is widely used in automotive and aerospace industrial applications. Friction welding is often the only viable alternative in this field to overcome the difficulties encountered in joining the materials with widely varying physical characteristics. This process employs a machine that is designed to convert mechanical energy into heat at the joint to weld using relative movement between workpieces, without the use of electrical energy or heat from other sources. This review deals with the fundamental understanding of the process. The focus is on the mechanism of friction welding, types of relative motions of the process, influence of parameters, heat generation in the process, understanding the deformation, microstructure and the properties of similar and dissimilar welded materials.

Friction stir welding : from basics to applications

Abstract: Introduction. Part 1 General issues: The friction stir welding process: An overview Material deformation and joint formation in friction stir welding Friction stir welding equipment Industrial applications of friction stir welding The future of friction stir welding Inspection and quality control in friction stir welding. Part 2 Variables in friction stir welding: Residual stresses in friction stir welding Effects and defects of friction stir welds Modeling thermal properties in friction stir welding Metallurgy and weld performance in friction stir welding.

Al-to-Mg Friction Stir Welding: Effect of Material Position, Travel Speed, and Rotation Speed

Abstract: Because joining dissimilar metals is often difficult by fusion joining, interest has been growing rapidly in using friction stir welding (FSW), which is considered a revolutionary solid-state welding process, as a new way to join dissimilar metals such as Al alloys to Mg alloys, Cu, and steels. Butt FSW of Al to Mg alloys has been studied frequently recently, but the basic issue of how the welding conditions affect the resultant joint strength still is not well understood. Using the widely used alloys 6061 Al and AZ31 Mg, the current study investigated the effect of the welding conditions, including the positions of Al and Mg with respect to the welding tool, the tool travel speed, and the tool rotation speed on the weld strength. Unlike previous studies, the current study (1) determined the heat input by both torque and temperature measurements during FSW, (2) used color metallography with Al, Mg, Al3Mg2, and Al12Mg17 all shown in different colors to reveal clearly the formation of intermetallic compounds and material flow in the stir zone, which are known to affect the joint strength significantly, and (3) determined the windows for travel and rotation speeds to optimize the joint strength for various material positions. The current study demonstrated clearly that the welding conditions affect the heat input, which in turn affects (1) the formation of intermetallics and even liquid and (2) material flow. Thus, the effect of welding conditions in Al-to-Mg butt FSW on the joint strength now can be explained.

Microstructure and properties of magnesium AZ31B–aluminum 7075 explosively welded composite plate

Abstract: A composite plate of Mg alloy (AZ31B) and an Al alloy (7075) was fabricated by explosive welding. The microstructure and properties of the bonding interface after explosive cladding were investigated. The results show that the bonding interface had a wavy appearance with solidified melts in a regularly spaced pattern of discrete regions. Adiabatic shear bands and twin structure were found on the AZ31B Mg alloy side. “Metallurgical bonding” of the explosive welding interface was achieved by local diffusion with an approximate 3.5 μm thick diffusion layer. No intermetallics were formed. Shear strength across the bonding interface of AZ31B/7075 composite was ca. 70 MPa. The maximum bending stress reached 670 MPa.

Microstructure and mechanical properties of laser welded DP600 steel joints

Abstract: To reduce fuel consumption and greenhouse gas emissions, dual phase (DP) steels have been considered for automotive applications due to their higher tensile strength, better initial work hardening along with larger elongation compared to conventional grade of steels. In such applications welding and joining have to be involved, which would lead to a localized alteration of materials and create potential safety and reliability issues under cyclic loading. The aim of this investigation was to evaluate microstructural change after laser welding and its effect on the tensile and fatigue properties in DP600 steel. The welding resulted in a significant increase of hardness in the fusion zone, but also the formation of a soft zone in the outer heat-affected zone (HAZ). While the ductility decreased after welding, the yield strength increased and the ultimate tensile strength remained almost unchanged. Fatigue life at higher stress amplitudes was almost the same between the base metal and welded joints despite slightly lower fatigue limit after welding. Tensile fracture and fatigue failure at higher stress amplitudes occurred at the outer HAZ. Fatigue crack initiation was observed to occur from the specimen surface and crack propagation was characterized by the characteristic mechanism of striation formation. Dimples and deformation bands were observed in the fast propagation area.

Material flow during friction stir spot welding

Abstract: Material flow during friction stir spot welding (FSSW) is investigated using tracer material technique. Three distinct regions are developed in a weld after the rotating shoulder comes in close contact with the upper sheet. They are called flow transition zone, stir zone, and torsion zone which are evolved due to the combination of rotational, horizontal and vertical motions of the plasticized material. An incorporation of the upper and lower sheet materials takes place in the flow transition zone, and the intermingled materials flowing from the flow transition zone contribute primarily to the formation of the stir zone. A new model of material flow during FSSW is developed on the basis of experimental observations. It is believed that the intrinsic driving force for the downward motion of the plasticized material is originated from the material release from the rotating pin through an outward-spinning motion.

Effect of morphology of martensite–austenite phase on fracture of weld heat affected zone in vanadium and niobium microalloyed steels

Abstract: In multipass welding, the intercritically reheated coarse grained heat affected zone (HAZ) demonstrates the worst toughness in the welded joint, since it contains a high carbon martensite w...

Tensile properties of a friction stir welded magnesium alloy: Effect of pin tool thread orientation and weld pitch

Abstract: Microstructures, tensile properties and strain hardening behavior of a friction stir welded (FSWed) AZ31B-H24 magnesium alloy were studied at varying welding speeds, rotational rates and pin tool thread orientations. After friction stir welding (FSW) both yield strength (YS) and ultimate tensile strength (UTS) were observed to be lower but strain hardening exponent became much higher due to the presence of recrystallized grains in the stirred zone (SZ) and thermomechanically affected zone (TMAZ). The left-hand thread pin tool rotating clockwise generated good FSWed joints and mechanical properties due to the downward material flow close to the pin surface, while the right-hand thread pin tool turning clockwise caused an upward material flow and resulted in inferior joints. The YS and UTS increased and strain hardening exponent decreased with increasing welding speed. The YS as a function of grain sizes obeyed the Hall-Petch relationship well, and it also increased with decreasing rotational rate. Both YS and UTS were observed to increase linearly with increasing weld pitch (a ratio of welding speed to rotational rate). A significantly higher YS of ∼170 MPa was achieved at a high weld pitch of 1.2 mm/rev, in comparison with that (∼110 MPa) using a weld pitch ranging from 0.0039 to 0.24 mm/rev. All the FSWed AZ31B-H24 joints failed in-between the SZ and TMAZ. Dimple-like ductile fracture characteristics appeared in the base metal, while some cleavage-like flat facets together with dimples and river marking were observed in the FSWed samples.