Corrosion behavior of aluminum 6061 alloy joined by friction stir welding and gas tungsten arc welding methods

Wrought aluminum sheets with thickness of 13 mm were square butt-welded by friction stir welding (FSW) and gas tungsten arc welding (GTAW) methods. Corrosion behavior of the welding zone was probed by Tafel polarization curve. Optical metallography (OM) and scanning electron microscopy together with energy dispersive spectroscopy (SEM-EDS) were used to determine morphology and semi-quantitative analysis of the welded zone. FSW resulted in equiaxed grains of about 1–2 μm, while GTAW caused dendritic structure of the welded region. Resistance to corrosion was greater for the FSW grains than the GTAW structure. In both cases, susceptibility to corrosion attack was greater in the welded region than the base metal section. T6 heat treatment resulted in shifting of the corrosion potential towards bigger positive values. This effect was stronger in the welded regions than the base metal section.     

Spot friction stir welding of low carbon steel plates preheated by high frequency induction

In this study, high frequency induction heating assisted spot friction stir welding was applied to 1.6 mm thick S12C low carbon steel plates. With the same welding parameter including an applied load of 2500 kg, rotation speed of 800 rpm and dwell time of 2 s, the average grain size in the stir zone slightly increased from 12.9 μm for the welds without preheating to 14.8 μm when 10 s preheating was used. However, larger joint interface was formed within the stir zone of the welds with preheating and therefore the bonding strength can be significantly increased. As a result, the shear tensile load of the joint increased from 8 kN to12.4 kN with preheating and the joint fractured through the plug failure mode rather than interfacial failure mode. It was revealed that the frictional heat generated between the rotating tool and the work-piece can be reduced to obtain sound welds by means of high frequency induction preheating.     

Evaluation of microstructures and mechanical properties of friction stir welded lap joints of Inconel 600/SS 400

The microstructures and mechanical properties of friction stir welded Inconel 600 and SS 400 lap joints were evaluated in this study. Friction stir welding was carried out at a tool rotation speed of 200 rpm and a welding speed of 100 mm/min. Application of friction stir welding was notably effective in reducing the grain size of the stir zone, as a result, the average grain size of Inconel 600 was reduced from 20 μm in the base material to 8.5 μm in the stir zone. The joint interface between Inconel 600 and SS 400 was soundly welded without voids and cracks, and MC carbides with a size of 50 nm were partially formed in the region of the lap joint interface in Inconel 600. In addition, a hook from SS 400 was formed on the advancing side of the Inconel 600 alloy, which directly affected an increase in the peel strength of the weld. In this study, we systematically discussed the effect of friction stir welding on the evolution of the microstructures and mechanical properties of friction stir lap jointed Inconel 600 and SS 400.     

Microstructure and surface mechanical property of AZ31 Mg/SiCp surface composite fabricated by Direct Friction Stir Processing

In this paper, Direct Friction Stir Processing (DFSP) to produce surface composite was proposed. A hollow and pinless DFSP tool was designed to fabricate the surface composite on the AZ31 plate of rolling state. The reinforcement particles flowed out through the through-hole of DFSP tool and entered the enclosed space between shoulder and base metal. They were then pressed into the workpiece as the rotating tool advanced along it just like a ‘planter’. Microstructure observed by optical microscope and scanning electron microscope showed that the reinforcement particles were homogeneous and dispersed in the stir zone. The thickness of the composite layer reached 150 μm. The grain size decreased from 16.57 μm to 1.24 μm in the composite layer. The micro-hardness increased from 57.77 Hv to 115.51 Hv. The plow width of the AZ31 with SiC decreased from 620 μm to 410 μm for the wear test.     

Investigation on the effect of friction stir processing on the superplastic forming of AZ31B alloy

Superplastic forming has now become conventional for forming complex parts from sheet metals. In many superplastically formed aerospace components, only a selective region undergoes superplastic forming. In those cases, instead of selecting a material exhibiting superplastic properties, a light weight and low cost material can be chosen and its microstructure can be modified locally by the Friction Stir Processing (FSP) technique. In this work, AZ31B magnesium alloy is chosen, and friction stir processing is performed by varying the process parameters, such as tool axial force, tool traversing speed and tool rotational speed. The process parameter that produced equiaxed grains in the stirred zone with a grain size less than 10 μm is selected. With this parameter, single pass FSP, multiple pass FSP without overlapping and multiple pass FSP with overlapping are performed on the AZ31B magnesium alloy sheets and their superplastic behaviour was examined. Also the theoretical modelling was carried out to determine the strain rate sensitivity for the friction stir processed AZ31B magnesium alloy and for the nonprocessed AZ31B magnesium alloy. It is found that the strain rate sensitivity for the friction stir processed component has increased, when compared to the base metal.     

Microstructure,mechanical and corrosion properties of friction stir welded high nitrogen nickel-free austenitic stainless steel

Friction stir welding (FSW) was applied to a 2.4 mm thick high nitrogen nickel-free austenitic stainless steel plate using tungsten–rhenium (W–Re) tool. The high-quality weld was successfully produced at a tool rotational speed of 400 rpm and a traveling speed of 100 mm/min. The microstructure, mechanical and corrosion properties of the weld were studied. The nitrogen content of the weld was almost identical to that of base metal (BM). FSW refined grains in the stir zone (SZ) through dynamic recrystallization and led to increase in hardness and tensile strength within the SZ, while the ductility was slightly decreased. The failure of tensile specimens occurred in the BM. TEM results revealed precipitates of Cr₂₃C₆ of size ~ 1 μm in the SZ, although their content was small. The precipitation of Cr₂₃C₆ and increase in δ-ferrite in the SZ led to small decrease in both pitting and intergranular corrosion resistance.     

Investigating effects of process parameters on microstructural and mechanical properties of Al5052/SiC metal matrix composite fabricated via friction stir processing

Friction stir processing (FSP) is a novel process for refinement of microstructure, improvement of material’s mechanical properties and production of surface layer composites. In this investigation via friction stir processing, metal matrix composite (MMC) was fabricated on surface of 5052 aluminum sheets by means of 5 μm and 50 nm SiC particles. Influence of tool rotational speed, traverse speed, number of FSP passes, shift of rotational direction between passes and particle size was studied on distribution of SiC particles in metal matrix, microstructure, microhardness and wear properties of specimens. Optimum of tool rotational and traverse speed for achieving desired powder dispersion in MMC was found. Results show that change of tool rotational direction between FSP passes, increase in number of passes and decrease of SiC particles size enhance hardness and wear properties.     

Effect of friction stir processed microstructure on tensile properties of an Al-Zn-Mg-Sc alloy upon subsequent aging heat treatment

An as-cast Al-Zn-Mg-Sc alloy was friction stir processed varying tool related parameters, yielding microstructures with different grain sizes (0.68, 1.8 and 5.5 μm). Significant increases in room temperature ductility were obtained in these materials with reasonable enhancement in strength. It is demonstrated that the type of microstructure produced by friction stir processing (FSP) has a significant influence on the choice of post-FSP heat treatment design for achieving improved tensile properties. It is also found that the ultrafine grained FSP material could not achieve the desired high strength during the post-FSP heat treatment without grain coarsening, whereas the micro-grained FSP materials could reach such strength levels (>560 MPa) under conventional age hardening heat treatment conditions.     

Microstructure and mechanical properties of friction stir welded 6082 AA in as welded and post weld heat treated conditions

Friction stir welding of 6082 AA-T651 was performed using three different combinations of feed rates (90, 140 and 224 mm/min) and tool rotational speeds (850, 1070 and 1350 rpm). Mechanical properties of the weldments were evaluated by hardness measurements on the transverse section and tensile testing, while microstructure evaluation was done by optical microscopy and electron back scattered diffraction (EBSD). Irrespective to welding parameters the dynamically recrystallized grains in the stir zone were measured to be in the range of 2–3 μm for different feeds rates and rotational speeds. A considerable loss in hardness in the stir zone and more severely in the thermo-mechanically affected zone was noted due to dissolution of β′ and β″ second phase particles. A post weld heat treatment (PWHT) of 175 °C for 5 and 12 h was given to the weldments for all welding conditions and the mechanical properties and microstructure were re-evaluated. The hardness and strength were partially recovered and this was attributed to the possible re-precipitation of the β″ precipitates. The grain size barely exhibited a change, whereas the texture displayed a significant diminish in the Goss orientation after PWHT.     

Gradient micro-structured surface layer on aluminum alloy fabricated by in situ rolling friction stir welding

A gradient micro-structure was formed in the surface layer of 2219 aluminum alloy joint by means of in situ rolling friction stir welding (IRFSW). The micro-structured surface layer is about 200 μm deep, corresponding to a gradient change in microhardness from 86.8 to 59.4 HV in the coarse-grained weld nugget zone (WNZ). Compared with those of the base material, the friction coefficient values are evidently decreased and the wear resistance is obviously enhanced on the surface layer. The corrosion current was relatively low and corrosion potential value was positive with respect to that of the base material. The second-phase particles in the upper surface layer were much more and smaller than those of the base material.     

Microstructural characteristics and mechanical properties of non-combustive Mg–9Al–Zn–Ca magnesium alloy friction stir welded joints

Non-combustive Mg–9Al–Zn–Ca magnesium alloy was friction stir welded with rotation speeds ranging from 500 to 1250 rpm at a constant welding speed of 200 mm/min. Defect-free joints were successfully produced at rotation speeds of 750 and 1000 rpm. The as-received hot extruded material consisted of equiaxed α-Mg grains with β-Mg₁₇Al₁₂ and Al₂Ca compounds distributed along the grain boundaries. Friction stir welding produced much refined α-Mg grains accompanied by the dissolution of the eutectic β-Mg₁₇Al₁₂ phase, while Al₂Ca phase was dispersed homogeneously into the Mg matrix. An increase in rotation speed increased the α-Mg grain size but not significantly, while microstructure in the heat affected zone was almost not changed compared with the base material. The hardness tests showed uniform distributed and slightly increased harness in the stir zone. Results of transverse tensile tests indicated that the defect-free joints fractured at the base material, while longitudinal tensile tests showed that the strength of the defect-free welds was improved due to microstructural refinement and uniform distribution of intermetallic compounds.     

Microstructure and mechanical properties of mild steel joints prepared by a flat friction stir spot welding technique

With the successful application of the flat spot friction stir welding technology to aluminum alloys, this technique was expanded to the spot lap welding of 1 mm thick mild steel in this study. It reveals that sound joints can be successfully obtained with smooth surfaces and without any internal welding defects. Two welding strategies based on the welding parameter can be used to obtain the welds that fracture through plug failure mode at high shear tensile strength. One way is to weld the sheet at low heat input in the first step and the second step is used to generate large stir zone and flatten the sample surface. However, the microstructure in the stir zone is not homogeneous and a coarse columnar grain structure forms at the bottom of the stir zone. Another way is to make the stir zone penetrate into the lower sheet during the first step and the second step is only aimed to flatten the sample surface. In this case, the total heat input can be reduced and the microstructure of the stir zone can be remarkably refined. The sound joints fractured along the circumstance of the stir zone and reached about 6600 N during the shear tensile tests.     

Characterization of centrifugal cast functionally graded aluminum-silicon carbide metal matrix composites

The present investigation is on characterization of functionally graded composites based on 356 cast and 2124 wrought aluminum alloys reinforced with SiC particles of 23 μm average particle size processed by liquid metal stir casting followed by horizontal centrifugal casting. A maximum of 45 and 40% SiC particles are obtained at the outer periphery of the Al(356)-SiC and Al(2124)-SiC FGMMC casting respectively. The maximum hardness obtained at the outer periphery after heat treatment for Al(356)-SiC and Al(2124)-SiC FGMMC are 155 BHN and 145 BHN respectively. The freezing range of the matrix alloy has been found to dictate the nature of transition from particle enriched to depleted zone. These composites are suitable for making engineering components, which require very high surface hardness and wear resistances with high specific strength.     

Effect of tool pin eccentricity on microstructure and mechanical properties in friction stir welded 7075 aluminum alloy thick plate

Four different tools with the pin eccentricity of 0.1 mm, 0.2 mm, 0.3 mm and 0.4 mm were designed to friction stir weld 10 mm thick AA7075-O plate. The effect of pin eccentricity on microstructure, secondary phase particles transformation and mechanical properties of the joints was investigated. The results show that the nugget area (A_NZ) increases firstly and then decreases with increasing the pin eccentricity. When the pin with 0.2 mm eccentricity is applied, the A_NZ is the largest; meanwhile the grains size is the smallest which is about 3 μm and secondary phase particles are the most dispersive in nugget zone compared with other tools. While the grains are coarsened to 7–11 μm as the eccentricity is more than 0.4 mm, some coarse hardening particles get to cluster in the thermo-mechanically affected zone. The joints produced by the pin with 0.2 mm eccentricity perform the highest tensile strength and elongation, which is attributed to better interfaces, finer grains and more dispersive secondary phase particles.     

Microstructure and tensile strength of friction stir welded joints between interstitial free steel and commercially pure aluminium

In the present study, the joining of interstitial free steel and commercial pure aluminium was carried out by friction stir welding (FSW) technique using tool rotational speeds of 600, 900, 1200 rpm and traverse speed of 100 mm/min. The microstructure and micro-hardness of the weld interface have been investigated. Optical microscopy was used to characterize the microstructures of different regions of friction stir welding joints. The scanning electron microscopy-back scattered electron (SEM-BSE) images show the existence of the different reaction layers in the welded zone. The Al₃Fe intermetallic compound has been observed in the weld interface and their thickness increase with the increase in tool rotational speed. Tensile strength was also evaluated and maximum tensile strength of ∼123.2 MPa along with ∼4.5% elongation at fracture of the joint have been obtained when processed at 600 rpm tool rotational speed.     

Quantitative measurements of small scaled grain sliding in ultra-fine grained Al–Zn alloys produced by friction stir processing

An Al-15 wt.% Zn alloy was processed by friction stir processing to produce grain sizes of ~ 0.5 μm, ~ 1 μm, and ~ 2 μm. A simple and effective method was developed to determine the true strain by scribing marker lines with scaled division using focused ion beam micromachining prior to deformation. The “microscopic” grain boundary sliding, with displacements of adjacent grains of the order of a nanometer, can easily be detected by the proposed technique, providing a surface analysis with high accuracy that could be used to observe the changes in relief with increasing strains. Moreover, the occurrence of grain boundary sliding at room temperature was considered a major cause for higher strain rate sensitivity in fine-grained Al–Zn alloys.     

Effect of welding parameters on microstructure and mechanical properties of AA6061-T6 butt welded joints by stationary shoulder friction stir welding

Stationary shoulder friction stir welding (SSFSW) butt welded joints were fabricated successfully for AA6061-T6 sheets with 5.0 mm thickness. The welding experiments were performed using 750–1500 rpm tool rotation speeds and 100–300 mm/min welding speeds. The effects of welding parameters on microstructure and mechanical properties for the obtained welds were discussed and analyzed in detail. It is verified that the defect-free SSFSW welds with fine and smooth surface were obtained for all the selected welding parameters, and the weld transverse sections are obviously different from that of conventional FSW joint. The SSFSW nugget zone (NZ) has “bowl-like” shapes with fairly narrow thermal mechanically affected zone (TMAZ) and heat affected zone (HAZ) and the microstructures of weld region are rather symmetrical and homogeneous. The 750–1500 rpm rotation speeds apparently increase the widths of NZ, TMAZ and HAZ, while the influences of 100–300 mm/min welding speeds on their widths are weak. The softening regions with the average hardness equivalent 60% of the base metal are produced on both advancing side and retreating side. The tensile properties of AA6061-T6 SSFSW joints are almost unaffected by the 750–1500 rpm rotation speeds for given 100 mm/min, while the changing of welding speed from 100–300 mm/min for given 1500 rpm obviously increased the tensile strength of the joint and the maximum value for welding parameter 1500 rpm and 300 mm/min reached 77.3% of the base metal strength. The tensile fracture sites always locate in HAZ either on the advancing side or retreating side of the joints.     

Effects of tool rotation speed on microstructures and mechanical properties of AA2219-T6 welded by the external non-rotational shoulder assisted friction stir welding

The external non-rotational shoulder assisted friction stir welding (NRSA-FSW) was applied to weld high strength aluminum alloy 2219-T6 successfully, and effects of the tool rotation speed on microstructures and mechanical properties were investigated in detail. Defect-free joints were obtained in a wide range of tool rotation speeds from 600 rpm to 900 rpm, but cavity defects appeared on the advancing side when the tool rotation speed increased to 1000 rpm. The microstructural deformation and heat generation were dominated by the rotating tool pin and sub-size concave shoulder, while the non-rotational shoulder helped to improve the weld formation. Microstructures and Vickers hardness distributions showed that the NRSA-FSW is beneficial to improving the asymmetry and inhomogeneity, especially in the weld nugget zone (WNZ). At the tool rotation speed of 800 rpm, both the tensile strength and the elongation reached the maximum, and the maximum tensile strength was up to 69.0% of the base material. All defect-free joints were fractured at the weakest region with minimum Vickers hardness in the WNZ, while for the joint with cavity defects the fracture occurred at the defect location.     

Friction stir processing of a D2 tool steel layer fabricated by laser cladding

Friction stir processing of a D2 tool steel layer fabricated by laser cladding was carried out to modify its as-cast microstructure. The microstructural evolution in the stir zone was observed and micro-Vickers hardness was measured. A maximum hardness of 857 HV was obtained by friction stir processing at 200 rpm. This value is higher than that of the clad layer when it is subjected to austenitizing heat treatment without friction stir processing. This superior property is attributed to the decrease of volume fraction of retained austenite after friction stir processing. The friction stir processing condition with low heat input led to high volume fractions of fine M₇C₃ carbide particles and martensite.     

Formation and mechanical properties of stationary shoulder friction stir welded 6005A-T6 aluminum alloy

6005A-T6 aluminum alloy is welded by stationary shoulder friction stir welding (SSFSW). At a constant rotational velocity of 2000 rpm, the effect of welding speed on mechanical properties of SSFSW joint are investigated in detail. Defect-free joint with gloss surface and small flash is attained and no cracks appear at the bending angle of 180°. Compared with traditional friction stir welding (FSW), width of rotational shoulder affected zone is relatively small because of the smaller diameter of rotational shoulder. Increasing welding speed is benefit for reducing the width of softening region and the softening degree. The fracture position of welding joint locates in thermo-mechanically affected zone and the fracture surface morphology presents the typical ductile fracture. The maximum tensile strength of joint at the welding speed of 400 mm/min reaches 82% of base metal (BM).