Weld Pitch Research Articles

Friction stir extrusion of thin AA 7075-T6 into low-carbon steel: Effects of low weld pitch value

Friction stir extrusion, a derivative process of friction stir welding, forms a mechanical joint between workpieces by forging one into a prefabricated geometry within the other. Thin sheet AA7075-T651 extrusion joints were made into an O-ring dovetail channel within a low-carbon steel workpiece. The effects of low weld pitch process parameters on joint characteristics between the extruded thin sheet AA7075-T651 and low-carbon steel were observed. The weld pitch was varied from 0.02 to 0.05 mm rev−1. No material mixing was observed. Tensile-shear test results were normalised by sample width. The 0.04 mm rev−1 weld pitch group yielded the highest average joint strength at 493 N mm−1. The weld pitch parameters reduced machine in-process forces without a large decrease in joint strength.

Experimental and numerical analysis of intermetallics in Al–Mg friction stir welds

In this research work, it was aimed to analyse the thermal behaviour during FSW in order to understand the diffusion behaviour of Al (AA6061)-Mg (AZ31B) dissimilar joints. Three heat input levels at different weld pitch ratios (WPR) of 0.087, 0.068 and 0.051 are accounted for the analysis. Finite element modelling (FEM) is employed to predict temperature evolutions. From the FEM results and fundamental diffusion equations, the intermetallic thickness and the diffusion behaviour between the Al and Mg material were analyzed and found that the Al-rich intermetallic phases Al3Mg2 grow faster and wider than the Mg-rich phase Al12Mg17. Tensile test demonstrates that a lower welding pitch ratio (WPR) leads to the formation of thicker intermetallic layers, resulting in reduced tensile strength and joint efficiency. In contrast, a higher WPR (0.087) minimizes intermetallic thickness, yielding superior tensile properties (138mpa). Microhardness measurements at the stir zone reveal a broad range from 70 to 164 HV, signifying mechanical heterogeneity. Microstructural reveals that a complex interplay between Al and Mg materials, resulting in fine equiaxed grains, intermetallic compounds, and distinct flow patterns in the stir zone.

Friction stir processing: An effective thermo-mechanical processing technique for carbon steels

This review paper deals with the detailed study of published research work on FSW/FSP of different types of steels and post processing such as heat treatment effects on additively manufactured steel beads. In this context, it comprises of post processing effects on steel beads fabricated by additive manufacturing technique that includes different techniques like selective laser melting (SLM), Laser-TIG (Tungsten inert gas), Metal inert gas welding, etc. along with post processing techniques such as hot iso-static pressing (HIP), conventional heat treatment (HT), post weld heat treatment (PWHT), etc. Also, it consists of the pros and cons of FSW/ FSP, especially on low and medium carbon steels. Moreover, this paper is mainly related to the effects of friction stir welding/ processing on the evolution of microstructure, weld morphology, and temperature gradients in the processed region. In addition, there are detailed analyses of tool selection, effect of weld pitch ratio, effect of carbon content (Wt. %), and hard phase volume fraction on yield strength i.e. carbide distribution in the FSP low and medium carbon steels, and their correlation with the mechanical responses as well as crystallographic orientation relationship. Finally, based on published research, gaps are highlighted and presented as future scope of research work.

Insight into the effect of weld pitch on the microstructure-properties relationships of St 37/AISI 316 steels dissimilar welds processed by friction stir welding

This research aims to evaluate the effect of weld pitch on the microstructure and properties of St 37/AISI 316 steels friction stir welds. The microstructure and microtexture of the samples were studied by means of electron backscatter diffraction analysis. The mechanical behaviors of the weldments were also investigated by microhardness, shear punch testing, and uniaxial tensile testing. In addition, the corrosion behavior of the welds was also evaluated. It was observed that the weld pitch of 0.0830 or 0.0625 mm/rotation resulted in producing defect-free welds with acceptable tensile strength level. Moreover, hardness and shear strength of the weld zone were improved up to 57% and 74%, respectively as compared to the base metals. It was due to the grain refinement which occurred within the stir zones by activating the dynamic recrystallization mechanism. Initial texture components of the base metals were replaced by recrystallized texture components in the stir zones. Orientation of grains in the stir zones was found to be highly dependent on the weld pitch. Regarding the corrosion resistance, the obtained results revealed that stir zone of St 37 steel was the weakest part of the weldments. Overall, the weldment employing the weld pitch of 0.0625 mm/rotation was found to achieve superior microstructural features and optimum properties.

Effects of welds distribution and high-low temperature humidity alternating aging on sealing performance of weld-bonded stainless steel structures

A sealant is introduced in the resistance spot welding (RSW) process to solve the poor sealing performance of stainless steel metro vehicles. The effects of welds distribution and high-low temperature humidity alternating aging on sealing performance are systematically studied by specially-designed pillow specimens and sealing tests apparatus. Study shows that the distributed RSW welds act as pinning points play a decisive role in sealant spreading and corresponding sealing performance. The sealing performance exhibits a maximal value when RSW spots centerline coincides with the gumming centerline and is inversely proportional to the weld pitch. Substrate deformation, sealant burning around the welds, and sealant thickness together contribute to the continuous invading of the high-pressure gas, and the initial failure prefers to happen adjacent to RSW welds. On the other hand, the sealing performance of the specimens has an inflection point at 240 h, which corresponds to the morphology evolution of the sealant, and should be attributed to the sealant state changes from non-drying state to set to touch state due to the degradation of the sealant. Hence, the sealant used in the current study is technically feasible to improve the sealing performance of metro stainless steel bodies.

Influence of process parameters on the microstructural evolution and mechanical characterisations of friction stir welded Al-Mg-Si alloy

Heat generation and plastic deformation during Friction Stir Welding (FSW) produce profound changes in the microstructure and structural properties of welded joints. Strengthening precipitate, grain size and crystallographic texture evolution are the most important microstructural changes in the case of welding aluminium alloys. An interaction relationship has been developed in this study to understand the evolution of microstructure during FSW of Al-Mg-Si alloy for a wide range of welding temperatures and plastic deformations by controlling two important process parameters (tool rotation and welding speeds). Electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) have been used for microstructural characterisation. The mechanical properties were evaluated using microhardness, tensile and low cycle fatigue tests. In all cases, fine recrystallised equiaxed grains with a partial <111> fibre texture was evolved in the nugget zone. Microstructural development was found to be significantly influenced by the weld pitch (welding speed/rotation speed), as it is controlled the heat input, cooling rate, exposure time and plastic deformations. The strength of the FSW joints is improved by increasing the welding speed because of grain refinement, incomplete dissolution or re-precipitate of strengthening precipitates Mg5Si6(β)´´ and the introduction of edge dislocations. The fatigue performance of FSW joints is dependent on grain size with a remarkable improvement in the fatigue life observed for smaller grain sizes.

Gap-tolerance control for friction stir butt welding of 2A14 aluminium alloy

Herein, we report the effects of gap variation upon the joint quality and thermomechanical behaviour of 2A14-T6 aluminium alloy friction stir butt welding. Based on the optimal weld pitch, gap-free and variable gap (from 0 to 2 mm in the weld) friction stir welding (FSW) experiments were conducted. The results indicate that welds without metallurgical defects were obtained up to a butt gap of 0.8 mm; however, voids and tunnel defect occurred at gap widths of 1.6 and 2 mm, respectively; moreover, the results showed that joint efficiency began to sharply decrease when the gap exceeded 1.6 mm. Furthermore, the thermomechanical model of varied gap FSW showed when the gap exceeded 0.8 mm, the material transferred by the tool would not completely fill the cavity formed by the pin’s forward motion. Finally, the gap-tolerance window for FSW was evaluated in terms of welding defects, joint efficiency, and thermomechanical behaviour.

Influence of Cu/Li ratio on the microstructure evolution of bobbin-tool friction stir welded Al–Cu–Li alloys

Two modern aluminum lithium alloys were welded by semi-stationary bobbin tool friction stir welding. The influence of the Cu/Li ratio on precipitation phenomena under process heat impact was investigated by comparing the response of low Cu/Li alloy 2196-T8 and high Cu/Li alloy 2060-T8. Identical process parameters with a weld pitch of one rotation per mm were used to conduct flawless weldments. The thermal history and microstructural features were studied and correlated to the resulting mechanical properties of the welds. Analysis of microstructure using differential scanning calorimetry and high energy X-ray diffraction technique showed significant differences in the precipitation sequence of the base metal and in the welded samples of the two alloys of interest. A low Cu/Li ratio led to a higher softening resulting in a reduction of 43% of base metal yield strength while the high Cu/Li ratio alloy AA 2060 could demonstrate more thermal stability (38% reduction). Severe dissolution of the T1 precipitate and presence of equilibrium phases were confirmed for the stirred zone of both alloys. The heat affected zone suffered dissolution and overaging reactions leading to a mechanically unfavorable microstructure. The low Cu/Li alloy 2196 developed a higher process temperatures and exhibited a more evolved precipitation sequence.

The effect of the Weld Pitch on Shunting in Robotic Resistance Welding

The effect of the Weld Pitch on Shunting in Robotic Resistance Welding

Microstructure and mechanical properties of friction stir welded AA6092/SiC metal matrix composite

There is a need for improved understanding on the effects of friction stir welding (FSW) on the metallurgical and mechanical properties of aluminium matrix composite (AMC). In this study, AA6092/SiC/17.5p-T6 AMC joints were produced by using FSW with varying tool rotation and traverse speeds. The microstructural characterisation by scanning electron microscopy equipped with electron backscattered diffraction (EBSD) system revealed a substantial grain refinement and a homogeneous distribution of reinforcement particles in the nugget zone. The grain size of the nugget zone was greatly influenced by weld pitch, as a key indicator to control the amount of heat input, exposure time and cooling rate. Vickers microhardness profile across the welding zone revealed a significant difference in microhardness among the base metal, heat affected zone, thermo-mechanically affected zone and nugget zone. The tensile strength of the cross-weld specimens showed a high joint efficiency of about 75% of the base metal combined with relatively high ductility. Low-cycle fatigue properties were investigated in the axial total strain-amplitude control mode (from 0.3% to 0.5%) with R=εmin/εmax=−1. The results indicate that the fatigue life of the cross-welded joints varies with grain size in the nugget zone and it is lower than that of the base metal. A significant improvement of fatigue life is found to be related to the finer equiaxed grains dominated by high angle grain boundaries in the nugget zone.

Adaptive Resistance Spot Welding Process that Reduces the Shunting Effect for Automotive High-Strength Steels

Although short-pitch resistance spot welding (RSW) significantly increases vehicle body strength, its application has been limited because of the associated shunting effect. In this study, a reference-based adaptive RSW process intended to minimize the shunting effect was proposed, and a related RSW system that controls welding current and power was developed to implement the adaptive method. The proposed RSW method compensates for the heat input loss caused by the shunting effect based on the estimated weld pitch and reference data obtained under suitable welding conditions. An exponential model was developed using a unique indicator (i.e., the ratio of the adaptive welding heat input to the reference welding heat input until the reference welding power curve peak) to estimate the weld pitch. A logistic growth model was next developed based on the relationship of the nugget diameter, heat input, and weld pitch to estimate the heat input compensation. A unique strategy using power control with a modified reference power curve was applied to supply the calculated heat input compensation. The experimental results intended to validate the proposed adaptive RSW process indicated that the proposed process effectively reduced the shunting effect and produced an improved nugget shape relative to the conventional RSW process.

Adaptive resistance spot welding method that reduces the shunting effect

A reference-based adaptive resistance spot welding (RSW) method intended to reduce the shunting effect in short-pitch (≤40 mm) RSW is proposed in this study. As the weld pitch decreases, the nugget diameter and dynamic resistance level concurrently decrease by an amount equivalent to the increased shunting effect. Based on this fundamental relationship, an exponential model capable of predicting weld pitch as a function of dynamic resistance was estimated. Next, the relationship between the nugget diameter and the heat input as a function of weld pitch was investigated, and a logistic growth model capable of predicting heat input compensation was established. These exponential and logistic growth models form the basis of the proposed RSW method’s control algorithm. The proposed RSW method compensates for the heat input loss caused by the shunting effect by adjusting welding time in real time under a constant current control until the predicted heat input compensation is obtained. Experimental results indicated that the RSW method proposed in this study concurrently increased the nugget diameter and decreased the shunting effect in short-pitch RSW compared with conventional welding method results. This finding suggests that the proposed reference-based adaptive RSW method is effective in reducing the shunting effect in short-pitch RSW.

Semi-stationary shoulder bobbin tool friction stir welding of AA2198-T851

The aluminum lithium alloy AA2198-T851 has been bobbin tool friction stir welded using a tool concept with one stationary and one rotating shoulder. Defect free welds in 3mm thick sheet have been produced featuring a high quality surface finish on the stationary side. The macrostructure forms an asymmetrical shape with microstructural characteristics known from standard friction stir welding. Because of only one rotating side a material flow direction towards the stationary shoulder has been observed. A parameter survey shows that a weld pitch of one mm per rotation combined with high pressure between the shoulders lead to good results. Mechanical performance of 82% of base metal ultimate tensile strength and 77% of base metal hardness have been achieved. The fracture analysis indicates two competing fracture modes, one being in the heat affected zone and the other at the borderline of the stirred zone on the advancing side. The first mode forms due to thermal cycle influence, whereas the second location suffers from weak bonding as a result of the thermal cycle and experienced deformation.

Effects of the shoulder diameter and weld pitch on the tensile shear load in friction-stir welding of AA6111/AA5023 aluminum alloys

The heights of deformed interface and surface affected the tensile shear load of lap joints fabricated by friction stir welding. The influences of shoulder diameter, weld pitch and evolution of mechanical properties has been investigated experimentally. The heights of the softened surface decreased and the bonded area widened with increasing the SD. The interface transformed from a hooked shape to a flat interface with decreasing the WP. The peak load was obtained by increasing the SD and WP. Fractured positions were divided in two types, shear and tensile fracture. Shear fracture occurred at the interface which contains stir zone. Tensile fracture was observed at the thermo-mechanically affected zone of the top plate. The fracture mode indicating the peak load was the shear fracture rather than tensile fracture. This is because the exact position of shear fracture was where the base material penetrated the bottom plate. The larger SD and higher WP produced higher peak load because of widened bonded area and little deformation. But the peak load decreased inversely in case of the higher rotation speed over than 1500rpm which means higher heat input conditions.

Formation mechanisms of periodic longitudinal microstructure and texture patterns in friction stir welded magnesium AZ80

Many studies of friction stir welding have shown that periodicity of metal flow around the tool pin may result in the formation of periodic differences in microstructure and texture in the weld nugget area correlated with the weld pitch. The current work investigates the periodicity of magnesium weld microtexture in the nugget region and its association with material flow using optical and electron microscopy. Two welds created in AZ80 at different processing conditions are presented in detail, one illustrating periodic longitudinal texture change, and one showing for the first time that periodic variations in texture, grain size, or composition are not defining features of periodic nugget flow. While nugget texture is dominated by shear deformation, it was found here to be affected to a lesser degree by compaction of material behind the welding tool, which led to reduction in intensity of the shear texture fiber. The decreased tendency for magnesium based alloys to form periodic patterns as compared to aluminum based alloys is explained with reference to the shear textures.

Experimental investigation on material flow and mechanical properties in friction stir welding of copper sheets

This paper focuses on the mechanism of onion rings emergence by analyzing optical macrographs and micrographs of friction stir welds on the copper alloy produced by a non-threaded tool pin. Also tensile tests, hardness measurements, and microstructural evaluations were performed on the welded specimens. The microstructural study shows that the weld has lamellar structure and the bands are sections of weld layers, which are produced by forward and rotational movement of the tool. The weld layers with same form, similar to the pin form, reside into each other with tool rotation and form a quasi-spherical shape. On the transverse cross section of the weld, the boundary of the layers is emerged like concentric quasi-circles, which are called onion rings. Each layer is emerged with one tool rotation, and its distance with the next consecutive layer is equal to the weld pitch. The formation of onion ring structures is due to the cutting of the weld layers with the same recesses and dimples. Hardness measurements show that the core of onion-ring-shaped structure at two points near the weld surface and weld root possesses maximum hardness. The average hardness of weld decreases from upper surface toward bottom until mid-depth, where then increases. HAZ has lower hardness on RS compared to that of AS. The scanning microhardness evaluation technique can be used as a complement method to the conventional hardness measurement procedures.

Development of High Welding Speed in Friction Stir Welded 5182-H111, and the Resulting Influence on down Force

In this manuscript the development of the process parameters to friction stir weld 5.15mm thick AA5182-H111 at a feed rate of 1500mm/min is discussed. Compared to a weld made at 200mm/min the weld-pitch had to be reduced from 2.0rev/mm to 0.3rev/mm and down force increased from 27kN to 59kN to create a weld with tensile-and yield strength exceeding that of the parent material, whilst elongation was only marginally reduced. The low weld pitch coupled with the high feed rate resulted in a reduction in the weld temperature and an increase in the process reaction forces. A lower down force was sufficient to consolidate the stir zone and result in ultimate tensile strength yield strength exceeding that of the parent material. However, elongation was reduced since the low weld pitch also reduced the effectiveness of the flutes on the pin to break up and disperse the oxide layer, rendering a pseudo bond along the ‘Lazy S’. This pseudo bond was eliminated through an increase in the down force.

Process operational windows and industrialization scenarios for assembly of large aluminium structures by robotic friction stir welding

In this work, friction stir welding (FSW) of 3.18 mm thick AA6061-T6 sheets in the butt- and lap-joint configuration was investigated with the objective of industrializing the process using low-cost serial industrial robots. The influence of weld pitch on the welding defects, microstructure, hardness, and bend performance of butt and lap welds was examined to identify process operational windows for both joint types. In parallel with these trials, a methodology based on kinetostatic analysis was developed to identify and evaluate viable robotized scenarios for FSW. On the basis of the experimental FSW process development results, this methodology was then applied to identify optimized FSW scenarios for the fabrication of large integrated AA6061 structural components with stringer-to-skin and skin-to-skin joints. Candidate workcell layouts are also presented.

The Effects of Spot Weld Pitch to the Axial Crushing Characteristics of Top-Hat Crash Box

Numerical study of the effect of spot weld pitch with respect to top-hat crash box crushing characteristics are presented in this paper. Belytschko-Lin-Tsay shell element was used for modeled columns wall with Piecewise Linear Plasticity material model. The impactor was modeled using hexahedral solid elements and assumed as a rigid body. Spot weld joints used to connect mild steel St37 plates of the columns were modeled using beam element and solid element. Impact characteristics related to the spot weld pitch and models were evaluated from simulation results in the form of crushing force vs axial deformation of the column. The results show that spot weld pitch does not significantly affect the crushing characteristics for top-hat crash box with beam element spot weld model, while solid element spot weld model show otherwise. The difference between beam element spot weld model and solid element spot weld model is larger at spot weld pitch 0.50H – H, and tend to close at higher spot weld pitch. Top-hat crash box model becomes stiffer with solid element applied as spot weld model.

Material adhesion and stresses on friction stir welding tool pins

During friction stir welding, polygonal tool pins experience severe stresses and, under certain conditions, loss of functionality due to adhesion of plasticised material on their surfaces. The extent of adhesion is analysed for various pin geometry and welding conditions based on the theory of machining. The effective stresses on the polygonal pins are evaluated following the principles of mechanics. The results show that the polygonal pins with fewer sides can avoid permanent adhesion of plasticised material at higher weld pitch, which is defined as a ratio of welding speed and tool rotational speed. The computed pin geometries for minimum adhesion are compared with the pin profiles recommended by various investigators based on independent experiments. The computed stresses show that pins with larger number of sides will experience lower stresses for any given set of welding variables.