Weld Nugget Grain Research Articles

Selection of optimal process parameters and assessment of its effect in micro-friction stir welding of AA6061-T6 sheets

Selection of appropriate welding parameters in micro-friction stir welding (micro-FSW) is challenging and a practical problem. In this regard, an assessment of variation in the favourable process parameters due to downscaling of sheet thickness can prove beneficial. In this work, FSW in 1- and 0.5-mm-thick AA6061-T6 sheets was compared for suitable process parameters based on the tensile strength of the weld. Comparatively high tool rotational speed combined with a high speed of tool travel and low plunge depth was found to be suitable welding parameters in micro-FSW (0.5-mm sheet). The effect of using this favourable set of process parameter was also assessed on frictional heat input requirement per unit weld volume, deformation conditions during welding (predicted using FEM simulation) and weld’s mechanical properties and microstructural characteristics in each sheet thickness. A high frictional heat input per unit weld volume was found in micro-FSW. A reduced weld zone temperature, an increased value of strain rate and a higher cooling rate were found in micro-FSW which led to a smaller weld nugget grain size, an improvement in weld’s ultimate tensile strength (UTS) but a drop in its ductility in this case.

The study of thermomechanical and microstructural issues in dissimilar FSW of AA6061 wrought and A390 cast alloys

Dissimilar friction stir welding (FSW) was successfully performed using 6-mm thick plates of AA6061 wrought and A390 cast alloys. The effects of material location and welding process parameters were considered, and thermomechanical and microstructural issues in the welding zone were investigated using a three-dimensional finite element method (FEM) and microstructural examination. The rotational to linear speed ratios leading to the sound weld were 120–250 and 200–250 rev/cm by placing the A390 alloy on the advancing and retreating sides, respectively. Also, when the A390 alloy was located on the advancing side, the size of the grain was larger than that of when the alloy was located on the retreating side. When the AA6061 alloy was placed on the advancing side, the effect of strain rate on weld nugget grain size was much higher than that of the temperature. However, with the A390 alloy on the advancing side, particle stimulated nucleation and temperature mainly contributed to an increase in the grain size.

Recrystallisation mechanism during friction stir welding of ultrafine- and coarse-grained AISI 304L stainless steel

Ultrafine-grained (UFG) and a coarse-grained (CG) 304L stainless steels were produced by a combination of cold rolling and annealing. Friction stir welding (FSW) was applied at the same welding conditions to study the effects of the grain size of the base metal on the grain structure of the weld nugget and its recrystallisation mechanism. The results showed that the recrystallisation mechanism changes with the grain size of the base metal. Continuous dynamic recrystallisation (CDRX) was found to be dominant at the FSW of the UFG sample while discontinuous dynamic recrystallisation is the main recrystallisation mechanism in the FSW of the CG sample. Although the weld nugget grain size of the UFG welded sample is slightly higher than that of the CG welded sample, its nanohardness was found to be higher. This behaviour is attributed to the higher dislocation densities of the austenite grains in the UFG welded sample produced through CDRX.

Microstructure–property characteristics of a novel non-weld-thinning friction stir welding process of aluminum alloys

Abstract The weld thinning phenomenon occurring in conventional friction stir welding (C-FSW) has negative effects on the formation quality and serviceable reliability of FSW joints. The existing methods for solving the weld thinning problem in FSW exhibit some universal deficiencies, such as low efficiency, low quality and narrow applicable range. Motivated by this challenge, a novel non-weld-thinning (NWT) FSW process, during which a zero shoulder plunge depth is applied to the welding tool, is proposed and investigated in the present paper. The results indicate that the high quality NWT-FSW joints can be efficiently produced by using this novel process. Microstructural analysis implies that the grains in weld nugget of NWT-FSW joints present smaller size and a more uniformly distributed feature than those of C-FSW joints. Compared with the C-FSW joints, the NWT-FSW joints are characterized by narrower softening regions and higher minimum hardness values, leading to an increase in tensile strength during tensile test. The temperature history analysis suggests that the thermal effect of tool shoulder is effectively controlled during NWT-FSW due to the application of zero shoulder plunge depth, which is the intrinsic reason for the grain refinement and strength improvement of NWT-FSW joints.

Heterogeneity of Microstructure and Mechanical Properties of Friction Stir Welded Joints of Al-Zn-Mg Alloy AA7039

Al-Zn-Mg aluminium alloy AA7039 was friction stir welded in order to investigate the influence of structural heterogeneity on mechanical properties of welded joints. Quality and mechanical properties of the developed weld joints were assessed by conducting bend, tensile and microhardness tests. Optical and scanning electron microscopy was used to analyze the microstructure and fracture surfaces respectively. It was found that joints were heterogeneous as different layers of joint exhibited significant heterogeneity in respect of microstructure and mechanical properties. Weld nugget grain size was found to decrease from top to bottom. Top layer of joint was strongest while middle layer was weakest. Weight % of Zn and Mg decreased from top to bottom layer of the joint. Mode of fracture was also found to be different for different layers of joint.

Corrosion Resistance of Friction Stir Welded AA6061 Aluminium Alloy

Present work pertains to microstructure and the corrosion behaviour of the nugget zone (NZ), thermo mechanically affected zone (TMAZ) and heat affected zone (HAZ) of friction stir welded AA6061 alloy. An attempt was made to find the influence of prior thermal temper of the alloy. Potentio-dynamic polarisation testing was used to determine the pitting corrosion resistance of the welds. Optical microscopy (OM) and transmission electron microscopy (TEM) with Energy dispersive X-ray spectroscopy studies were carried out to find the mechanism of formation of TMAZ and corrosion. Friction stir welding of this alloy resulted in fine recrystallized grains in weld nugget which has been attributed to frictional heating and plastic flow. The process also produced a softened region in the weld nugget, which may be due to the dissolution and growth of possible precipitates. Corrosion resistance of nugget zone has been found to be higher than that of TMAZ and base metal. Corrosion resistance of naturally aged (T4) alloy exhibit higher corrosion resistance than that of artificial aged (T6) alloy.

Correlation between weld nugget grain size, weld nugget hardness and tensile strength of friction stir welded commercial grade aluminium alloy joints

Friction stir welding process (FSW) is an emerging solid state joining process in which the material that is being welded does not melt and recast. The FSW process and tool parameters play a major role in deciding the joint strength. Joint strength is influenced by the grain size and hardness of the weld nugget region. Hence, in this investigation an attempt was made to develop empirical relationships to predict the grain size and hardness of weld nugget of the friction stir welded AA1100 aluminium alloy joints. Linear regression relationships are established between grain size and hardness of the weld nugget and weld nugget hardness and tensile strength of FSW joints. The developed relationships can be effectively used to predict the weld nugget grain size and tensile strength of the joint non-destructively by measuring the weld nugget hardness.

Evolution of microstructures and mechanical properties in similar and dissimilar friction stir welding of AA5086 and AA6061

In this work, thermo-mechanical behavior and microstructural evolution in similar and dissimilar friction stir welding of AA6061-T6 and AA5086-O have been investigated. Firstly, the thermo-mechanical behaviors of materials during similar and dissimilar FSW operations have been predicted using three-dimensional finite element software, ABAQUS, then, the mechanical properties and the developed microstructures within the welded samples have been studied with the aid of experimental observations and model predictions. It is found that different strengthening mechanisms in AA5086 and AA6061 result in complex behaviors in hardness of the welded cross section where the hardness variation in similar AA5086-O joints mainly depends on recrystallization and generation of fine grains in weld nugget, however, the hardness variations in the weld zone of AA6061/AA6061 and AA6061/AA5086 joints are affected by subsequent aging phenomenon. Also, both experimental and predicted data illustrate that the peak temperature in FSW of AA6061/AA6061 is the highest compared to the other joints employing the same welding parameters.

Developing Empirical Relationships to Predict Grain Size and Hardness of the Weld Nugget of Friction Stir Welded AA7075-T6 Aluminium Alloy Joints

High-strength, precipitation-hardening AA7075-T6 alloy is used extensively in aircraft primary structures. Friction stir welding process is an emerging solid state joining process in which the material that is being welded does not melt and recast. The FSW process and tool parameters play a major role in deciding the joint strength. Joint strength is influenced by the grain size and hardness of the weld nugget region. Hence, in this investigation an attempt has been made to develop empirical relationships to predict the grain size and hardness of the weld nugget of the friction stir welded AA7075-T6 aluminium alloy joints. Six factors, five levels, central composite, rotatable design matrix is used to optimize the experimental conditions. The empirical relationships are developed by response surface methodology incorporating tool and process parameters. A linear regression relationship is also established between grain size and hardness of the weld nugget of friction stir welded joints. The weld nugget grain size is related with hardness of the joint. The developed relationships can be effectively used to predict the weld nugget grain size of the joint non-destructively by measuring the weld nugget hardness.

Effect of process parameters on friction stir welding of aluminum alloy 2219-T87

In this work, successful friction stir welding of aluminum alloy 2219 using an adapted milling machine is reported. The downward or forging force was found to be dependent upon shoulder diameter and rotational speed whereas longitudinal or welding force on welding speed and pin diameter. Tensile strength of welds was significantly affected by welding speed and shoulder diameter whereas welding speed strongly affected percentage elongation. Metallographic studies revealed fine equiaxed grains in weld nugget and microstructural changes in thermo-mechanically affected zone were found to be the result of combined and interactive influences of frictional heat and deformation. A maximum joining efficiency of 75% was obtained for welds with reasonably good percentage elongation. TEM studies indicated coarsening and/or dissolving of precipitates in nugget. For the gas metal arc weld, SEM investigations revealed segregation of copper at grain boundaries in partially melted zone.

Microstructural and Mechanical Characterization of Friction Stir Welded- 1050 Aluminium Alloy

Friction stir welding (FSW) is a fairly recent technique that utilizes a non-consumable rotating welding tool to generate frictional heat and plastic deformation at the welding location in the continuously-fed work piece. In the present investigation this welding process is applied to join 1050 cold-rolled aluminium plates. The effects of varying the welding parameters namely welding speed [56, 90 and 140 mm/min] and tool rotational speed [850 and 1070 rpm] on the mechanical and microstructural properties were studied. Vickers micro hardness results across the weldment showed that the weld nugget hardness is dependant upon the welding speed and the tool rotational speed. Increasing the welding speed at 850 rpm reduced the hardness at the weld nugget, whereas, at 1070 rpm the weld nugget hardness merely did not change. However, the hardness achieved at 850 rpm was constantly higher than that achieved with 1070 rpm irrespective to welding speeds. In the same fashion, the yield and ultimate strengths of the joints were influenced by varying the welding parameters. Increasing the welding speed at 850 rpm reduced both strengths whereas; at 1070 rpm they were almost unchanged. Microstructural study showed that the weld region is composed of unaffected base metal and the stir zoned [weld nugget] which is characterised by a fine equiaxed grain structure. Increasing the welding speed at constant tool rotational speed has caused a slight refinement in the weld nugget's grain size, whereas, decreasing the rotational speed has also led to weld nugget grain refinement.

Friction stir welding parameters: a tool for controlling abnormal grain growth during subsequent heat treatment

Recent investigations showed that the fine weld nugget grain structure of friction stir welds (FSW) undergoes abnormal grain growth (AGG) in the post-weld solution heat-treated condition. This significantly deteriorates the strength of FSW of high-strength heat-treatable Al-alloys. In the current research, various welds of AA2095 sheets were prepared by manipulating the FSW parameters, and subsequently heat-treated to investigate the influence of the process parameters (feed rate and tool rotation speed) on the extent of AGG. Characterization of the microstructure and tensile properties revealed that the tool rotational speed and the feed rate appreciably affect the extent of AGG. The higher the rotation speed the lower the AGG at low-to-intermediate feed rates.