Experimental Investigation of Magnesium-Base Nanocomposite Produced by Friction Stir Processing: Effects of Particle Types and Number of Friction Stir Processing Passes

In this research, nanosized SiC and Al₂O₃ particles were added to as-cast AZ91 magnesium alloy, and surface nanocomposite layers with ultrafine-grained structure were produced via friction stir processing (FSP). Effects of reinforcing particle types and FSP pass number on the powder distribution pattern, microstructure, microhardness, and on tensile and wear properties of the developed surfaces were investigated. Results show that the created nanocomposite layer by SiC particles exhibits a microstructure with smaller grains and higher hardness, strength, and elongation compared to the layer by Al₂O₃ particles. SiC particles do not stick together and are distributed separately in the AZ91 matrix; however, distribution of SiC particles is not uniform in all parts of the stirred zone (SZ), which causes heterogeneity in microstructure, hardness, and wear mechanism of the layer. Al₂O₃ particles are agglomerated in the different points of matrix and create alumina clusters. However, distribution of Al₂O₃ clusters in all parts of the SZ is uniform and results in a uniform microstructure. In the specimen produced by one-pass FSP and SiC particles, the wear mechanism changes in different zones of SZ due to the nonuniform distribution of particles. However, in the specimen produced by Al₂O₃ particles, the wear mechanism in all parts of the SZ is the same and, in addition to the abrasive wear, delamination also occurs. Increasing FSP pass number results in improved distribution of particles, finer grains, and higher hardness, strength, elongation, and wear resistance.     

Influence of Friction Stir Processing Parameters on the Microstructure of Aluminum Foams

In the present study, friction stir processing (FSP) has been used to fabricate aluminium foams. The effects of the number of FSP passes, FSP tool rotational speed, foaming time and temperature on the porosity have been investigated. Aluminium foam with porosity up to 40% was successfully fabricated. In the samples foamed at 923 K (650 °C), a few irregular pores were produced as a result of high aluminium matrix stiffness in this temperature. In general with increase in foaming temperature the porosity increased. However, in the samples foamed for 30 or 60 min, lower porosity was detected at higher foaming temperature. Also, in the samples which were produced with more FSP passes, the foaming time decreased and more uniform pore structure was obtained.     

Microstructure,Tensile and Impact Toughness Properties of Friction Stir Welded Mild Steel

 Microstructure, tensile and impact toughness properties and fracture location of friction stir welded AISI 1018 mild steel are revealed in this paper. The 5 mm thick AISI 1018 mild steel plates were friction stir welded with tool rotational speed of 1000 rpm and welding speed of 50 mm/min with tungsten base alloy tool. Tensile strength of stir zone is higher (8%) when compared to the base metal. This may be due to the formation of finer grains in the weld nugget region under the stirring action of the rotating tool. The ductility and impact toughness of the joints are decreased compared to the base metal and this is due to the inclusion of tungsten particles in the weld region.     

Effect of Friction Stir Processing on Microstructure and Tensile Properties of an Investment Cast Al-7Si-0.6Mg Alloy

Friction stir processing (FSP) is emerging as a promising tool for microstructural modification. The current study assesses the effects of FSP on the microstructure and mechanical properties of an investment cast Al-7Si-Mg alloy. FSP eliminates porosity and significantly refines eutectic Si particles. The extent of particle refinement varied with changes in processing conditions. A high tool rotation rate and a low-to-intermediate tool traverse speed generated a higher volume fraction of finer particles. Tensile ductility changed significantly as a result of FSP, whereas ultimate tensile strength improved only marginally. Yield strength was similar in both cast and FSP samples under various heat-treated conditions, with the highest value obtained after a T6 heat treatment. Furthermore, FSP caused significant grain refinement in the stir zone, subsequently transforming into very coarse grains as abnormal grain growth occurred during solution treatment at high temperature.     

Friction Stir Welding in Wrought and Cast Aluminum Alloys: Weld Quality Evaluation and Effects of Processing Parameters on Microstructure and Mechanical Properties

Friction stir welding (FSW) is a solid-state process widely used for joining similar and dissimilar materials for critical applications in the transportation sector. Understanding the effects of the process on microstructure and mechanical properties is critical in design for structural integrity. In this study, four aluminum alloy systems (wrought 6061-T651 and cast A356, 319, and A390) were processed in both as-fabricated and pre-weld heat-treated (T6) conditions using various processing parameters. The effects of processing and heat treatment on the resulting microstructures, macro-/micro-hardness, and tensile properties were systematically investigated and mechanistically correlated to changes in grain size, characteristic phases, and strengthening precipitates. Tensile tests were performed at room temperature both along and across the welding zones. A new method able to evaluate weld quality (using a weld quality index) was developed based on the stress concentration calculated under tensile loading. Optimum processing parameter domains that provide both defect-free welds and good mechanical properties were determined for each alloy and associated with the thermal history of the process. These results were further related to characteristic microstructural features, which can be used for component design and materials/process optimization.     

Friction Stir Welded AZ31 Magnesium Alloy: Microstructure, Texture, and Tensile Properties

This study was aimed at characterizing the microstructure, texture and tensile properties of a friction stir welded AZ31B-H24 Mg alloy with varying tool rotational rates and welding speeds. Friction stir welding (FSW) resulted in the presence of recrystallized grains and the relevant drop in hardness in the stir zone (SZ). The base alloy contained a strong crystallographic texture with basal planes (0002) largely parallel to the rolling sheet surface and $ \langle {11\bar{2}0} \rangle $ directions aligned in the rolling direction (RD). After FSW the basal planes in the SZ were slightly tilted toward the TD determined from the sheet normal direction (or top surface) and also slightly inclined toward the RD determined from the transverse direction (or cross section) due to the intense shear plastic flow near the pin surface. The prismatic planes $ (10\bar{1}0) $ and pyramidal planes $ (10\bar{1}1) $ formed fiber textures. After FSW both the strength and ductility of the AZ31B-H24 Mg alloy decreased with a joint efficiency in-between about 75 and 82 pct due to the changes in both grain structure and texture, which also weakened the strain rate dependence of tensile properties. The welding speed and rotational rate exhibited a stronger effect on the YS than the UTS. Despite the lower ductility, strain-hardening exponent and hardening capacity, a higher YS was obtained at a higher welding speed and lower rotational rate mainly due to the smaller recrystallized grains in the SZ arising from the lower heat input.     

Friction Stir Processing of Investment-Cast Ti-6Al-4V: Microstructure and Properties

Investment-cast titanium components are becoming increasingly common in the aerospace industry due to the ability to produce large, complex, one-piece components that were previously fabricated by mechanically fastening multiple pieces together. The fabricated components are labor-intensive and the fastener holes are stress concentrators and prime sites for fatigue crack initiation. The castings are typically hot-isostatically-pressed (HIP) to close internal porosity, but have a coarse, fully lamellar structure that has low resistance to fatigue crack initiation. The as-cast + HIP material exhibited 1- to 1.5-mm prior β grains containing a fully lamellar α + β microstructure consistent with slow cooling from above the β transus. Friction stir processing (FSP) was used to locally modify the microstructure on the surface of an investment-cast Ti-6Al-4V plate. Friction stir processing converted the as-cast microstructure to fine (1- to 2-μm) equiaxed α grains. Using micropillars created with a dual-beam focused ion beam device, it was found that the fine-grained equiaxed structure has about a 12 pct higher compressive yield stress. In wrought products, higher strength conditions are more resistant to fatigue crack initiation, while the coarse lamellar microstructure in the base material has better fatigue crack growth resistance. In combination, these two microstructures can increase the fatigue life of titanium alloy castings by increasing the number of cycles prior to crack initiation while retaining the same low-crack growth rates of the colony microstructure in the remainder of the component. In the current study, high-cycle fatigue testing of investment-cast Ti-6Al-4V was performed on four-point bend specimens. Early results show that FSP can increase fatigue strength dramatically. This article is based on a presentation given in the symposium entitled “Materials Behavior: Far from Equilibrium” as part of the Golden Jubilee Celebration of Bhabha Atomic Research Centre, which occurred December 15–16, 2006 in Mumbai, India.     

Effect of Process Parameters on Microstructure and Mechanical Properties in Friction Stir Welding of Aluminum Alloy

Friction stir welding process is a promising solid state joining process with the potential to join low melting point materials, particularly aluminum alloys. The most attractive reason for this is the avoidance of solidification defects formed during conventional fusion welding processes. Tool rotational speed and the welding speed play a major role in deciding the weld quality. In the present work an effort has been made to study the effect of the tool rotational speed and welding speed on mechanical and metallurgical properties of friction stir welded joints of aluminum alloy AA6082-T651. The micro hardness profiles obtained on welded zone indicate uniform distribution of grains in the stir zone. The maximum tensile strength obtained is 263 MPa which is about 85% of that of base metal. Scanning electron microscope was used to show the fractured surfaces of tensile tested specimens.     

Effect of Multiple-Pass Friction Stir Processing Overlapping on Microstructure and Mechanical Properties of As-Cast NiAl Bronze

As-cast Cu-9Al-4.5Ni-4Fe NiAl bronze alloy (NAB) was subjected to multiple-pass friction stir processing (FSP) with a 50 pct overlap. After FSP, the coarse microstructure of the base metal (BM) was transformed to defect-free material with fine microstructure. While the torchlike patterns in the stir zone (SZ) and the uplifted grains in the transitional zones (TZs) between two passes were observed in the multiple-pass FSP region, no grain coarsening was found in the remnant zone of the previous SZ after subsequent FSP pass. The hardness value of the FSP materials was higher than that of the BM and was homogeneously distributed throughout the entire multiple-pass FSP region. The FSP materials showed greatly improved tensile properties compared to the BM, and the TZs showed similar tensile strength and ductility to the single-pass FSP materials. The BM broke in a mixture mode of brittle cleavage and microvoid coalescence fracture, whereas the FSP and TZ samples failed in the latter fracture mode. The results showed that the multiple-pass overlapping (MPO) FSP was feasible to modify the microstructure of large-sized plate of the NAB.     

Effect of Friction Stir Processing on Microstructure and Mechanical Properties of a Cast-Magnesium–Rare Earth Alloy

Single-pass friction stir processing (FSP) was used to increase the mechanical properties of a cast Mg-Zn-Zr-rare earth (RE) alloy, Elektron 21. A fine grain size was achieved through intense plastic deformation and the control of heat input during processing. The effects of processing and heat treatment on the mechanical and microstructural properties were evaluated. An aging treatment of 16 hours at 200 °C resulted in a 0.2 pct proof stress of 275 MPa in the FSP material, a 61 pct improvement over the cast + T6 condition.     

Effect of Welding Parameters on Tensile Properties and Fatigue Behavior of Friction Stir Welded 2014-T6 Aluminum Alloy

The present work describes the effect of welding parameters on the tensile properties and fatigue behaviour of 2014-T6 aluminum alloy joints produced by friction stir welding (FSW). Characterization of the samples has been carried out by means of microstructure, microhardness, tensile properties and fatigue behaviors. The hardness in the softened weld region decreases with decreasing the welding speed. Irrespective of the tool rotation speeds, the best tensile and fatigue properties were obtained in the joints with the welding speed of 80 mm/min. The joint welded with a rotating speed of 1520 rpm at 80 mm/min has given a highest tensile and fatigue properties. The fatigue behaviors of the joints are almost consistent with the tensile properties, especially elongations. Higher ductility in FSW joints made the material less sensitive to fatigue. The location of tensile fractures of the joints is dependent on the welding parameters. On the other hand, the fatigue fracture locations change depending on the welding parameters and stress range. In addition, a considerable correlation could not be established in between heat indexes and mechanical properties of FSW 2014-T6 joints under the investigated welding parameters.     

Taguchi Optimisation of Friction Stir Processing Parameters to Achieve Maximum Tensile Strength of Mg AZ31B Alloy

In this investigation, the effect of friction stir processing process variables such as rotational speed, traverse speed and tool tilt angle on the tensile strength of magnesium alloy AZ31B was studied. The experiments were carried out according to the Taguchi parametric design L9 at various combinations of process parameters and statistical optimization technique ANOVA was used to determine the optimum levels and to find the percentage of contribution of the process parameters. The results indicate that the rotational speed is the most significant factor followed by the traverse speed and tool tilt angle for maximising the tensile strength of the friction stir processed magnesium alloy.     

A Review on Friction Stir Processing of Titanium Alloy: Characterization,Method, Microstructure,Properties

Metallurgical and Materials Transactions B - In the past two decades, friction stir processing (FSP) technology has received considerable attention. FSP can be used to adjust and control the...     

Processing, Microstructure, and Residual Stress Effects on Strength and Fatigue Crack Growth Properties in Friction Stir Welding: A Review

The purpose of this review is to provide a comprehensive overview of friction stir welding (FSW), as well as to introduce current research and applications involving this relatively new process. FSW is a new, efficient way of joining metal alloys that are considered unsuitable for welding via conventional fusion joining methods, and is capable of welding dissimilar metals with ease. This process also has the benefit of being solid-state, which mitigates the need for liquid filler metals that are common with conventional fusion welding techniques. This review will examine different facets of the FSW process, exploring the resulting static and dynamic properties and factors that influence these properties including weld zone boundaries, grain refinement, residual stress, and addition of reinforcing particles. Highlights of current research in this area and applications of this process in various industries will also be presented and discussed.     

6061铝合金搅拌摩擦焊接头组织与性能研究

采用搅拌摩擦焊方法(FSW)对6 mm厚的6061-T4铝合金板材进行对接,焊后利用光学显微镜(OM)和扫描电镜(SEM)分析、对比了焊接接头和母材的显微组织和断口形貌特征,并测试了其室温拉伸性能和显微硬度。实验结果表明:选择了适合于6061-T4铝合金板材搅拌摩擦焊的工艺参数:焊接时搅拌头旋转速度为1200 r.min-1,工件的进给速度为300 mm.min-1,在此参数下获得了与母材等强度、韧性接近于母材的焊接接头,为此种合金应用于汽车关键零部件提供了可靠的工艺方法。FSW板材接头焊核区的组织和性能明显优于其他区,热影响区是接头最薄弱的部分,焊核区的硬度最高,而热影响区的硬度最低,焊缝金属发生回复再结晶使晶粒细化。断口分析表明,断裂发生在热影响区,由于搅拌头的旋转运动和热量的累积,该区存在晶粒长大、组织粗化现象。对工艺参数的优化实验表明,搅拌头旋转速度与焊接速度对接头性能的影响存在一定的适配关系,通过工艺参数的调整可以有效地控制热影响区的焊缝组织和改善焊接接头的性能。细晶强化是搅拌摩擦焊接头强度与韧性提高的主要原因。     

Microstructure and Fatigue Properties of a Friction Stir Lap Welded Magnesium Alloy

Friction stir welding (FSW), being an enabling solid-state joining technology, can be suitably applied for the assembly of lightweight magnesium (Mg) alloys. In this investigation, friction stir lap welded (FSLWed) joints of AZ31B-H24 Mg alloy were characterized in terms of the welding defects, microstructure, hardness, and fatigue properties at various combinations of tool rotational rates and welding speeds. It was observed that the hardness decreased from the base metal (BM) to the stir zone (SZ) across the heat-affected zone (HAZ) and thermomechanically affected zone (TMAZ). The lowest value of hardness appeared in the SZ. With increasing tool rotational rate or decreasing welding speed, the average hardness in the SZ decreased owing to increasing grain size, and a Hall–Petch-type relationship was established. Fatigue fracture of the lap welds always occurred at the interface between the SZ and TMAZ on the advancing side where a larger hooking defect was present (in comparison with the retreating side). The welding parameters had a significant influence on the hook height and the subsequent fatigue life. A relatively “cold” weld, conducted at a rotational rate of 1000 rpm and welding speed of 20 mm/s, gave rise to almost complete elimination of the hooking defect, thus considerably (over two orders of magnitude) improving the fatigue life. Fatigue crack propagation was basically characterized by the formation of fatigue striations concomitantly with secondary cracks.