Friction Stir Welding of Low-Carbon AISI 1006 Steel: Room and High-Temperature Mechanical Properties

Friction stir welding (FSW) is an ecologically benign solid-state joining process. In this work, FSW of low-carbon AISI 1006 steel was carried out to study the microstructure and mechanical properties of the resulting joints at both room temperature (RT) and 200 °C. In the parameter space investigated here, a rotational tool speed and translation feed combination of 1200 rpm and 60 mm/min produced a defect-free weld with balanced mechanical properties and a superior Vickers microhardness profile compared to all other conditions and to base metal (BM). At faster translation feeds (100 and 150 mm/min), wormhole defects were observed in the weld microstructure and were attributed to higher strain rate experienced by the weld zone. Under tensile loading, welded material exhibited yield strength that was up to 86 and 91% of the BM at RT and 200 °C, respectively. On the other hand, tensile strength of welded material was nearly similar to that of the base metal at both RT and 200 °C. However, at both temperatures the tensile ductility of the welded joints was observed to be significantly lower than the BM. Annealing of the 1200 rpm and 60 mm/min FSW specimen resulted in tensile strength of 102% compared to base material and 47% increase in the strain at failure compared to the as-welded specimen. The Charpy impact values revealed up to 62 and 53% increase in the specific impact energy for the 1200 rpm and 60 mm/min welded joints as compared with the BM.     

Effect of the surface roughness of friction stir welded joints on the fatigue characteristics of welded joints in V-1461 and V-1469 aluminium–lithium alloys

Investigations were carried out into the effect of friction stir welding conditions of V-1461 and V-1469 aluminium–lithium alloys on surface roughness and mechanical properties of welded joints. The general parameter of the welding conditions was the heat input coefficient determined by the ratio of the speed of rotation of the tool to the welding speed. The optimum range of friction stir welding conditions of sheets of the investigated alloys was determined. In this stage, the surface finish of the welded joint after welding is much better (required for increasing the fatigue characteristics and high-quality deposition of protective coatings), there are no defects and the strength of the welded joint is equal to 0.75–0.85 of the strength of the parent material. The production of the welded joints with reduced surface roughness (R_z ≤ 40 μm) greatly increases the fatigue characteristics of the welded joints (low-cycle fatigue strength equals more than 300,000 cycles without fracture).     

Influence of tool material on mechanical and microstructural properties of friction stir welded 316L austenitic stainless steel butt joints

In the present research, the influence of friction stir welding (FSW) tool material on the mechanical and microstructural properties of friction stir (FS) welded 316L stainless steel butt joints is investigated. FS welds were produced using two different tungsten based FSW tools having identical tool shoulder and pin profiles. In both the cases, the FSW experimental runs were carried out using tool rotational speed of 600 rpm, welding speed of 45 mm/min, axial force of 11 kN and tool tilt angle of 1.5°. The results of the study show that the joints produced using the tungsten lanthanum oxide tool are having superior mechanical and microstructural properties when compared to the joints produced using tungsten heavy alloy tool. Furthermore, the tool degradation study by mass loss and photographic techniques suggests that the tungsten lanthanum oxide tool is more prone to degradation by plastic deformation, whereas the tungsten heavy alloy tool is more prone to degradation by wear.     

Friction stir welding of aluminium casting alloys

The results of testing friction stir welding quality in relation to EN AC-43200 (AK9) and EN AC-45000 (AK64) aluminium casting alloys are presented. The test joints were made with the use of a welding machine constructed on the basis of numerically controlled milling machines. The assessment of the joint quality was made based on visual inspection, mechanical testing, weldment structure analysis and hardness tests. The purpose of the investigation was to discover the possibility of friction stir welding of casting alloys and the influence of welding conditions on joint properties and structure. The test results show good weldability of aluminium casting alloys by the FSW method. Sound welds can be obtained in a relatively wide range of welding parameters while the weld strength is satisfactory. In order to obtain the highest quality joints, the workpieces must be pressed onto the other, while the welding process cannot run with excessively high speed. The best mechanical properties of the joints were achieved when the friction process was conducted at a rotational speed of the mixing tool of 900 rpm.     

Effect of Intermetallic Compound Phases on the Mechanical Properties of the Dissimilar Al/Cu Friction Stir Welded Joints

Types and distribution of intermetallic compound phases and their effects on the mechanical properties of dissimilar Al/Cu friction stir welded joints were investigated. Three different rotation speeds of 1000, 1200 and 1400 rpm were used with two welding speeds of 20 and 50 mm/min. The results show that the microstructures inside the stir zone were greatly affected by the rotation speed. Complex layered structures that containing intermetallic compound phases such as CuAl₂, Al₄Cu₉ were formed in the stir zone. Their amount found to be increased with increasing rotation speed. However, the increasing of the rotation speed slightly lowered the hardness of the stir zone. Many sharp hardness peaks in the stir zones were found as a result of the intermetallic compounds formed, and the highest peaks of 420 Hv were observed at a rotation speed of 1400 rpm. The joints ultimate tensile strength reached a maximum value of 105 MPa at the rotation speed of 1200 rpm and travel speed of 20 mm/min with the joint efficiency ranged between 88 and 96% of the aluminum base metal. At the travel speed of 50 mm/min, the maximum value of the ultimate tensile strength was 96 MPa at rotation speed of 1400 rpm with the joint efficiency ranged between 79 and 90%. The fracture surfaces of tensile test specimens showed no evidence for the effect of the brittle intermetallic compounds in the stir zones on the tensile strength of the joints.     

耐磨铝硅铜合金的双面搅拌摩擦焊

对10 mm厚的铝硅铜合金进行了双面搅拌摩擦焊接,研究了焊接接头的显微组织及力学性能.研究表明,铸态金属晶粒和硅粒子在双面搅拌摩擦焊接过程中得到细化,硅颗粒分布均匀化.反面焊道比正面焊道的晶粒细化和硅粒子均匀化作用更明显.异向焊接的接头强度较低,所有接头均断在焊缝的前进侧.同向焊接的接头强度较高,接头断裂位置取决于焊接参数.选用搅拌头旋转频率为950 r/min、焊接速度为10mm/min的参数焊接时,同向接头表面成形好、内部无缺陷,其强度可达母材的87.4%.     

Remote method of controlling crack propagation in materials and welded joints

A TRIP 800 steel was friction stir spot welded using three different tool rotational speeds, 1600, 2000 and 2400 rpm, and the dwell time was kept constant at 2 s. The resultant microstructures formed in each weld zone were analysed as well as their hardness. Higher hardness values were observed for the lowest rotational speed, 1600 rpm, where the heat input in theory was lower and, therefore, the cooling rate was faster. However, for this rotational speed, allotriomorphic ferrite was also observed in the stir zone. In the lap shear tests, samples welded at 1600 and 2400 rpm did not reach the minimum value recommended by the AWS D8.1M standard, which was attributed to the lower bonding ligament length and also lower distance between the keyhole left by the pin and the end of the zinc line, which is formed in the stir zone. The fracture of the samples occurred along this line. As a result, the influence of the microstructure on the failure process could only be inferred when the zinc line disappeared.     

Effect of Welding Parameters and Aging Process on the Mechanical Properties of Friction Stir-Welded 6063-T4 Al Alloy

6063-T4 Al alloy was friction stir welded at various tool rotations (800, 1120, and 1600?rpm) and welding speeds (200 and 315?mm/min) using a specially manufactured tool with a height-adjustable and right-hand-threaded pin. The postweld aging process (at 185?°C for 7?h) was applied to a group of the welded plates. In this study, the effects of the welding parameters and the postweld aging treatment on the microstructural and mechanical properties of 6063-T4 Al alloy were studied. The maximum weld temperatures during the welding process were recorded, and the fracture surfaces of tensile specimens were examined using a scanning electron microscope. The homogeneous hardness profiles were obtained for all the weldings with no trace of softening regions. It was observed that the ultimate tensile strengths (UTS) increased slightly (on average by approx. 8%) and the percent elongations decreased (on average by approx. 33%) by the postweld aging treatment. The maximum bending forces (F _max) of all the welds were less than that of the base metal. It was observed that the F _max values increased after the postweld aging process at the welding speed of 315?mm/min and decreased at the welding speed of 200?mm/min.     

Influence of Rotation Rate on Microstructure and Comprehensive Performance of FSWed Mg Alloy

Die-casting AZ31 Mg alloys were successfully friction stir welded at a constant welding speed and different rotation rates. More uniform and fine grains were obtained at the rotation rate of 1400 rpm due to more suitable temperature for dynamic recrystallization in this welding condition. In addition, the intensity of (0001) texture in stir zone increased with the increasing rotation rate. The results of mechanical property test indicated joint with rotation rate of 1400 rpm had better tensile property, which was associated with fine grains, uniform transition in the interface between thermo-mechanically affected zone and stir zone as well as more favorable Schmidt factor for basal slip and twinning. The corrosion resistance of joints increased with the increasing rotation rate, which is significantly related to the (0001) texture with basal plane parallel to the corroded surfaces.     

Friction stir welding of AZ31 magnesium alloys processed by equal channel angular pressing

Equal channel angular pressing (ECAP) is an effective thermo-mechanical process to make ultrafine grains.An investigation was carried out on the friction stir welding (FSW) of ECAPed AZ31 magnesium alloys with a thickness of 15 mm.For different process parameters,the optimum FSW conditions of ECAPed AZ31 magnesium alloys were examined.The basic characterization of weld formation and the mechanical properties of the joints were discussed.The results show that the effect of welding parameters on welding quality was evident and welding quality was sensitive to welding speed.Sound joints could be obtained when the welding speed was 37.5 mm/min and the rotation speed of the stir tool was 750 r/min.The maximum tensile strength (270 MPa) of FSW was 91% that of the base materials.The value of microhardness varied between advancing side and retreating side because of the speed field near the pin of the stir tool,which weakened the deformed stress field.The value of microhardness of the welding zone was lower than that of the base materials.The maximum value was located near the heat-affected zone (HAZ).Remarkable ductile character was observed from the fracture morphologies of welded joints.     

Friction stir welding of AZ-91 and AM-Lite magnesium alloys

The objective of this paper was to report the results of butt joint quality of friction stir welding (FSW) of samples of alloys of MgAlZn group. Welding has been carried out with the FSW tool of welding speed ranging from 140 to 355 mm/min. In the stir zone and the surrounding, neither porosity nor cracks have been obtained. Microhardness in the stir zone increased for all welds when compared to the base metals, as a consequence of the high grain refinement and increased precipitations at the grain boundaries. The decrease in plastic properties of metal in these regions is expected. The results show a strong relationship between properties of joints and welding parameters.     

Torque,temperature and hardening precipitation evolution in dissimilar friction stir welds between 6061-T6 and 2014-T6 aluminum alloys

Dissimilar friction stir butt welds between the 2014-T6 and the 6061-T6 Al alloys were performed with various sets of welding parameters including a lateral shift of the tool from the initial separation between the plates to be welded and by placing the alloys, either on the advancing, or on the retreating side of the weld. Torque and temperature measurements during welding as well as macrographies and hardness profiles measurements were performed after welding. It was found that the welding torque, the temperature, the metal flow and the welds’ hardness profile depend on the proportion of each alloy included in the stirred zone. Those results are attributed to the difference between the softening temperatures of both alloys. The 6061 alloy's HAZ is the weak link in all dissimilar welds. The evolution of the hardening precipitation, the hardness and the local yield strength in the 6061 alloy are therefore modeled. The model reasonably well predicts the experimentally measured hardness of both similar and dissimilar welds. It also explains the influence of the alloys placement or tool lateral shift on the welds hardness by their influence on the precipitate radius and volume fraction.     

6005A-T6铝合金型材双轴肩搅拌摩擦焊接头组织及力学性能

采用双轴肩搅拌摩擦焊接方法对4.5 mm厚6005A-T6铝合金型材在较高焊接速度下进行了对接试验。结果表明,较高的焊接速度下仍可获得外观及性能优良的接头,但易出现隧道型缺陷及裂纹缺陷。在试验参数下,接头性能与WP(焊接速度与搅拌头旋转速度的比值)有密切关系:接头抗拉强度随着WP值的增大基本呈现先增大后减小的趋势,在搅拌头转速为1 400 r/min以及焊接速度为1 400 mm/min时获得强度较高的接头,其抗拉强度为231 MPa,是母材强度的77%。断口扫描结果显示,在试验参数下,接头断裂方式随着WP值的增大由塑性断裂逐渐变为包含沿晶断裂、韧性断裂、解理断裂的混合型断裂。     

铁素体不锈钢搅拌摩擦焊工艺及缺陷形成机理

采用钨铼合金搅拌工具对T4003铁素体不锈钢进行搅拌摩擦焊接工艺试验,研究搅拌摩擦焊缝成形、接头组织特征及缺陷形成机理.结果表明,不同旋转速度下随焊接速度增加,轴向压力呈单调增加趋势;当转速为150,250 r/min时,可获得无缺陷致密焊缝;当转速为350 r/min时,靠近前进侧的焊缝区出现孔洞缺陷,随着焊接速度和轴向压力不断增加,焊接缺陷有减少趋势.焊接接头焊核区发生了相变和明显淬硬现象,组织为细小等轴铁素体和低碳马氏体,焊缝具有明显不均匀硬度分布.提出了一种焊缝热塑性金属平衡流动模型分析其缺陷形成机理.     

Microstructure and fatigue properties of friction stir welded high-strength steel plates

The weldability of high-strength steels (HSSs) is limited by their loss of strength, toughness and fatigue properties. In demanding applications, the fatigue properties of welds are among the strictest requirements. This paper presents a weldability analysis focusing on the microstructure and fatigue properties of 6?mm thick S690 and S355 HSS plates joined by friction stir welding. Their fatigue properties are compared to design recommendations and to fatigue properties obtained with other welding techniques. Results show that the high-quality friction stir welded steel joints outperform high-quality arc welds and FAT80 design recommendations. The fatigue strength of the friction stir welded joints is increased with material yield strength. The mechanisms governing crack initiation at different maximum stress levels are discussed.     

The influence of refill FSSW parameters on the microstructure and shear strength of 5042 aluminium welds

Refill friction stir spot welding (FSSW-Refill) was used to produce solid-state joints in an automotive 5042 aluminium alloy. The influence of plunge depth, rotational speed, plunge rate and time on the microstructure and shear strength was investigated. The Statistica software package was used to correlate process parameters with the mechanical properties of the joints. The most significant variables are plunge depth and tool rotational speed, while volumetric defects have a small influence on the mechanical performance of the welds. Reducing the rotational speed from 1900 rpm to 900 rpm increased the bonding ligament length. For joints produced at a higher tool rotational speed (1900 rpm) the material flow was more vertical, i.e., towards the surface of the joint, the bonding ligament length was reduced and the shear strength was impaired.     

Global and local mechanical properties and microstructure of Bobbin tool friction-stir-welded Al–Li alloy

AA2198–T851 sheets were welded by bobbin tool friction stir welding using a rotational speed of 800?rpm and welding speed of 42?mm?min^?1. The microstructure and precipitates within the joint were characterised by transmission electron microscopy. The global and local mechanical behaviour was determined using a digital image correlation system. Specific attention was given to the relationship between the local microstructure and properties across the joint, which govern the global strength and ductility of the welds. A lower global elongation of the joint is caused by the premature strain localisation in the softened zone.     

Static shear strength and fatigue life of refill friction stir spot welded AA 6061-T6 sheets

This study was aimed at evaluating the static shear strength and fatigue properties of the newly developed refilled friction stir spot welded AA 6061-T6 joints. The keyhole, the process disadvantage of conventional friction stir spot welding, was refilled successfully, using an additional filler plate, with specially designed tools. Two different tool profiles, namely, convex and concave, were used for the refilling process. Sound and defect free joints were obtained by the refilling process. Joints refilled with convex tools showed better static shear strength than those with the concave ones. The variation of microhardness in different regions of the weld was analysed. Fatigue tests were conducted on the lap shear specimens at a stress ratio of R?=?0·1. The optical micrographs of the welds after fatigue failure in both the conventional and refilled processes were examined to study the fatigue crack propagation and failure modes.     

Microstructure,mechanical properties and failure behaviour of protrusion friction stir spot welded 2024 aluminium alloy sheets

The current study investigates the mechanical and microstructure properties of 2024 aluminium alloy welded by protrusion friction stir spot welding as a novel method to produce keyhole-free welds. Tool rotation speed and anvil protrusion height are used as effective variables of the process to obtain optimum conditions. Results illustrate that the keyhole-free welds with the joint show superior mechanical properties in protrusion friction stir spot welding compared to conventional friction stir spot welding. Failure mode changes from interfacial mode to circumferential mode by increasing the nugget zone depth and joint length, while the effect of nugget zone is considerable. Finally, welding at a rotation speed of 1600?rev?min^?1 and a protrusion height of 0.4?mm presents significant mechanical properties with more joint length.     

Microstructural characteristics and mechanical properties of Al–Mg–Si alloy self-reacting friction stir welded joints

The 5?mm thick Al–Mg–Si alloy was self-reacting friction stir welded using the specially designed tool at a constant rotation speed of 400?rev?min^?1 with various welding speeds. Defect-free welds were successfully obtained with welding speeds ranging from 150 to 350?mm?min^?1, while pore defects were formed in the weld nugget zone (WNZ) at a welding speed of 450?mm?min^?1. Band patterns were observed at the advancing side of WNZ. Grain size and distribution of the precipitated phase in different regions of the joints varied depending on the welding speed. The hardness of the weld was obviously lower than that of the base metal, and the lowest hardness location was in the heat affected zone (HAZ). Results of transverse tensile tests indicated that the defective joint fractured in the WNZ with the lowest tensile strength, while the fracture location of the defect-free joints changed to the HAZ.