Microhardness Of Stir Zone Research Articles

Effect of tool tilt angle on weld joint strength and microstructural characterization of double-sided friction stir welding of AZ31B magnesium alloy

Mechanical and metallurgical behavior of double-sided friction stir welded (DS-FSW) joints of 6 mm thick magnesium alloy (AZ31B) plates, under 0° (Joint W1) and 2° (Joint W2) tool tilt angle, using scrolled shoulder and screwed pin tool were investigated for the first time. Joint fabrication using tool rotational speed of 800 rpm and welding speed of 50 mm/min resulted in tunnel defect in W1 joint and defect free W2 joint. The tensile test results show the joint efficiency of 74% for W1 joint and 87% for W2 joint. The defect free micro tensile samples extracted from first pass (FP), second pass (SP) and overlap region (OZ) of both the welds shows higher degree of grain refinement. Also, microhardness in stir zone (SZ), thermo-mechanically affected zone (TMAZ) and heat-affected zone (HAZ) of W2 joint is high compared to W1 joint. These metallurgical enhancements in each zone of 2° tilt angle joint led to the remarkable improvement in joint efficiency of FP-2 (80%), OZ-2 (71%) and SP-2 (91%) tensile specimens compared to FP-1 (76%), OZ-1 (63%) and SP-1 (67%) for W1 welded joint. Significant improvement by 26%, 19% and 32% and in percentage elongation was observed in the first pass, overlap region and second pass respectively for W2 joint over W1 joint. Scanning electron microscopy (SEM) characterization revealed ductile and shear fracture modes, wherein, larger area fraction of dimpled morphology and tearing ridges was observed for the macro and micro tensile specimens of W2 joint.

Strengthening mechanism of friction stir welded alpha titanium alloy specially designed T-joints

In this study, the Ti-4Al-0.005B (TA5, Chinese brand) titanium alloy was used in a specially designed T-joint joined by friction stir welding, where two plates served as the skins and one plate served as the stringer with two separate welds. The base material (BM) has a typical rolling microstructure, consisting of α grains deformed along the rolling direction. The grains in the heat-affected zone (HAZ) are coarser than those in the BM, while the grains in the thermo-mechanically affected zone are deformed along the shear direction. When the temperature is higher than the β transus in the welding process, there are lamellar α phase structures in the stir zone (SZ). When the temperature is lower than the β transus in the welding process, there are fine equiaxed grains in the SZ due to sufficient dynamic recrystallization, while the grains in the first weld are slightly larger than those in the second weld due to their longer exposure to the thermal cycle of the second weld. The microhardness of the first weld decreases following the second weld, and the area of lowest microhardness occurs in the HAZ on the advancing side near the first weld. The microhardness of SZ at the position of 0.5 mm from the upper surface is slightly higher in the thickness direction. In the tensile tests, the initial position of the fracture occurs in the area where the skin and the stringer are joined by FSW. With the increase of tool rotation speed from 450 rpm to 850 rpm, the tensile strength increases first and then decreases along the direction of skin and stringer. In addition, a coupled Eulerian-Lagrangian (CEL) numerical model is developed to investigate the temperature distribution and material flow during FSW, showing stronger correlation with the experiments.

Investigating on microstructural and mechanical properties of Al6061/nano–SiC composites fabricated via friction stir processing

In this investigation, metal matrix composite was fabricated on the surface of Al6061 plates using means of 50 nm SiC particles via friction stir processing(FSP). X-ray diffraction(XRD), electron backscattering diffraction (EBSD) and transmission electron microscopy(TEM) were employed to detect and analyze the morphology and characteristics of precipitated phase and composite strengthening phase as well as the grain microstructure in samples. The microhardness and tensile properties of the samples were also tested. The results show that the ratio of low angle grain boundaries(LAGBs) decreased from 56% to 39% of the base metal owing to severe plastic deformation and the addition of nano-SiC particles, and that of the high angle grain boundaries (HAGBs) increased from 44% to 61%, which significantly refines the grain structure in the stir zone(SZ). The hardness profile of Al/SiC metal matrix composite sample is an inverted ‘U’ curve, which is different from the case that the hardness distribution of FSPed Al6061 alloy is a typical ‘W’ shape. Grain refinement and SiC dispersion strengthening compensate the defects of hardness reduction and uneven distribution caused by the dissolution in stir zone or coarsening in thermo-mechanically affected zone of Mg2Si precipitates due to the increase of temperature. Microhardness in stir zone increased from 78 HV to 100 ± 5 HV, which was improved up to about 28% comparing with base metal Al6061 alloy.

Effects of nano-SiC particles on the FSSW welded AZ31 magnesium alloy joints

The microstructure of nano-SiC enhancing friction stir spot welding (FSSW) joint with dwell time of 3 s was characterised by an onion ring structure which consisted of alternate SiC-free zones and SiC-rich zones where SiC particles refined the grains. However, onion ring structure disappeared and SiC particles dispersed homogeneously when dwell time was 5 s. The microhardness of stir zone (SZ) and tensile shear load of SiC enhancing FSSW joint were higher than those of conventional FSSW joints. After heat treatment at 200°C for an hour, grains of the SZ grew substantially and coursed reduction in mechanical properties of joints, while grain size of SZ and tensile shear load of SiC enhancing joint was invariant but the microhardness of SZ increased.

Microstructure, microhardness and tensile properties of FSWed DP 800 steel

Purpose: Dual phase (DP) steels are widely used in the automotive industry due to their properties of a high balance of strength and formability. However, it is known that conventional welding of high strength steel leads to some undesirable results such as hardness decrease in the heat affected zone. Friction stir welding (FSW) is a new solid state joining method, which is used to join these steels due to its advantage of low heat input. The aim of this study is to evaluate the microstructural change and mechanical properties of friction stir welded DP800 steel. Design/methodology/approach: DP 800 steels with 1.5 mm thickness were subjected to friction stir welding, by using a tungsten carbide (WC) tool. The tool was tilted 2°, and downforce of the tool was kept constant at 6 kN. During processing, the tool rotation and traverse speed were fixed at 1600 rpm and 170 mm∙min-1, respectively. Findings: The friction stir welded region comprises martensite, bainite, refined ferrite. The average microhardness of stir zone has increased from 260 HV0.2 to about 450 HV0.2. The tensile sample shows a decrease in the ultimate tensile strength (σUTS) about 3%, from 827 MPa to 806 MPa for the joint. The yield strength (YS) of the joint is about 566 MPa and the value is near that of DP800. Research limitations/implications: The tungsten carbide tool used for the friction stir welding has suffered deterioration in the pin profile after 1 meter welding operation. It may be advisable to drill a pre-hole in the specimens for a longer tool life. Practical implications: Tool wear for industrial applications will be a major problem. Therefore, the use of tools with high wear resistance such as polycrystalline cubic boron nitride may be recommended. Originality/value: Works on friction stir welding of dual phase steels are limited and they mostly focus on spot welding. Also, this study systematically investigates the microstructure and mechanical properties of dual-phase 800 steels after the friction stir welding.

Effect of Enhanced Cooling on the Microstructure and Mechanical Properties of Friction Stir Spot-Welded AZ31 Magnesium Alloy

AZ31 magnesium alloy was friction stir spot welded in air and cooling in water. The effect of the enhanced cooling rate on the microstructure and mechanical properties of the joint was analyzed. The results showed that flowing water had obvious cooling effect instantaneously, which significantly restrained the growth of dynamic recrystallized grains. The average grain size in stir zone was 1.3μm in cooling water condition, which is far smaller than that of the joint prepared in air cooling condition. Under the condition of enhanced cooling, the microhardness in stir zone significantly increased, the ultimate tensile load (~ 3.99kN) increased by 15.7%, and the tensile deformation value (~ 3.65 mm) increased by 62.2%. Dimples in SEM fracture morphologies indicated the better plastic deformation capacity of joints prepared by cooling water, which failed through a mixture mode of ductile and brittle fracture.

Development of novel heating tool friction stir spot welding (HT-FSSW) for AZ31 magnesium alloy

In this work, a new heating tool friction stir spot welding (HT-FSSW) process was developed, and its impacts on the microstructure and mechanical properties of the welded AZ31 magnesium alloy joints were investigated by microstructure observation, tensile tests and microhardness tests. An increase in the heating tool temperature resulted in a decrease in the grain size of the stir zone (SZ) and an increase in the grain size of the thermomechanically affected zone (TMAZ). The rising heating tool temperature also aggrandised the bonded zone width and enhanced the tensile shear load strength per unit area of the HT-FSSW welded joints. With an increase in the heating tool temperature, the microhardness of SZ increased while that of the TMAZ decreased. Moreover, the slope of the Hall–Petch relationship between microhardness and grain size of the TMAZ is larger than that of the SZ.

The effect of SiC nanoparticles on the friction stir processing of severely deformed aluminum

The 1050 aluminum sheets are severely deformed by two passes of the constrained groove pressing (CGP) process to obtain the strain of 2.32. Friction stir processing (FSP) is then performed on these specimens at two conditions of with and without SiC nanoparticles. Microhardness measurements indicate that in the state of FSP without any particle, the microhardness of stir zone is decreased due to the recrystallization and grain growth occurrence because of high stored strain energy in the CGPed specimens. In order to enhance the mechanical properties of the stir zone, SiC nanoparticles are used during FSP. Also, the effect of FSP pass number on the distribution of nanoparticles is investigated. Microstructure investigations reveal that after 1 and 2 passes of FSP with SiC nanoparticles, specimens experience brittle fracture at the retreating side of the stir zone during transverse tensile test due to the presence of clustered nanoparticles in this region. However, it is found that after 3 passes of FSP, SiC nanoparticles are uniformly distributed leading to the prohibition of grain growth in the stir zone. This can be responsible for the improvement of stir zone microhardness by about 118.8% with respect to the specimen FSPed without nanoparticles.

숄더 지름과 회전 속도에 따른 AZ31 마그네슘합금의 마찰교반접합 특성

Friction stir welding (FSW) is a relatively new joining technique particularly for magnesium and aluminum alloys that are difficult to fusion weld. In this study, AZ31 Mg alloys were joined by FSW with shoulder diameter 11, 19 mm and rotating speed 900, 1200, 1500, 1800 rpm. The shoulder diameter and welding speed depended on the heat input during FSW process. As a result, the microstructures of stir zone were a fine grain by dynamic recrystallization. According to the larger shoulder diameter and the higher rotating speed, refined grain sizes of stir zone were grown by higher heat input, and the microhardness of stir zone was lower. The tensile strength at the shoulder diameter 19 mm, rotating speed 900 rpm was obtained maximum value. This value compared with the base metal was over 93%.

Effects of pin diameter on microstructures and mechanical properties of friction stir spot welded AZ31B magnesium alloy joints

The effects of pin diameter on the microstructure and mechanical properties of friction stir spot welded AZ31B magnesium alloy joints were investigated using microstructural observations, tensile tests and microhardness tests. The results showed that with an increase in the pin diameter, the height and width of the curved interface in the friction stir spot welded AZ31B magnesium alloy joints increased because of the strong effect of stirring and high temperature obtained when large sized pins were adopted. An increase in the pin diameter led to the coarsening of α-Mg grains in the stir zone, thermomechanical affected zone and heat affected zone because of heat generation, resulting in the decrease in microhardness of stir zone, thermomechanical affected zone and heat affected zone. The tensile shear force of the friction stir spot welded AZ31B magnesium alloy joints increased with the increase in pin diameter because the height and width of the curved interface dominated the failure of the specimens.

Effect of III-Pass on Microstructure, Micro Hardness and Static Immersion Corrosion Resistance of AA6061-T6/SiCp Surface Composite Fabricated by Friction Stir Processing

In this current investigation, SiC particles 20μm in average size were incorporated into the commercially AA6061-T6 to prepare surface composite by using Friction stir processing (FSP). The morphology of the reinforcement inside the Al matrix has been evaluated from scanning electron microscope (SEM) and the corrosion characteristics of the resulted composite were evaluated using static immersion corrosion (SIC) behavior in 3.5% NaCl aqueous solution at various regimes. From the results, it observed that the SiC particles were distributed uniformly inside the stir zone (SZ) in both first and third-pass of FSP. The micro hardness of stir zone with SiC particles of I-pass was higher compared to III-pass and as-received Al alloy. In static immersion corrosion test the FSP AA6061-T6/SiCp exhibited significantly greater corrosion resistance in I-pass than compared to the III-pass and as-received Al alloy.