Numerical Simulation for the Optimization of Polygonal Pin Profiles in Friction Stir Welding of Aluminum

The tool with polygonal pin profile has been widely employed in friction stir welding(FSW) of aluminum, but there is hardly an effective optimization methodology existed as the thermomechanical characteristics affected by pins with various flats number have not been understood comprehensively. Therefore, the present work employs a 3-dimensional computational fluid dynamics(CFD) model to have an integrated observation of the FSW process with the effect of polygonal pin profiles. Both the heat generation modes due to contact friction at the tool–workpiece interface and volumetric viscous dissipation in the vicinity of the tool are considered. The model is utilized to give a quantitative analysis of the heat generation, temperature distribution, plastic material flow and welding loads during the FSW process for various tools with polygonal pin profiles, as well as a variety of shoulder diameters, welding speeds and tool rotation speeds. The calculated results of thermal cycles, tool torques and joint cross sections for some typical polygonal pins and welding parameters are all found to be compared well with the experimental ones, which demonstrates the feasibility and applicability of the present numerical model. Particularly, a methodology is developed for the optimization of the flats number by identifying the torque components in both parallel and vertical direction of the pin-side flat region. The results show that the optimized pin flats number increases with increasing tool rotation speed, while the influence of both welding speed and shoulder diameter can be supposed to be insignificant. Moreover, the dependability of the optimized results is also discussed by considering wear tendency and service life of the pin for multiple welding conditions.     

Some effects of tool geometric features on friction stir weld response parameters

Pin geometries (shape, dimensions, and surface features) have significant effects on friction stir welding, dictating, to some extent, the material flow which in turn affects process variables and joint quality. In this paper, volumetric defect content and process response variables (in-plane forces, torque, power, and temperature) as a function of pin thread forms and features (thread/flat/flute) on mildly tapered conical pins were examined in AA6061-T6 for a range of welding parameters. The coarser thread pitch with different pin features performed better for producing good quality welds compared to finer threaded pins. Forge force requirements can be minimised using counter-flow flutes in threaded pin. Variations of nugget geometry, torque, power, and temperature were reported to correlate their dependence on pin features.     

基于涡状搅拌针的2024-T4铝合金搅拌摩擦扩散焊接头成形与断裂载荷

以2024-T4铝合金为对象进行了搅拌摩擦扩散焊试验,首次提出并使用涡状搅拌针,且对比分析了涡状搅拌针数量对横截面形貌、材料流动行为与力学性能的影响. 结果表明,涡状搅拌针作用下的搭接界面未出现明显的弯曲现象,增加涡状针的数量有利于增加焊核区底部的宽度. 当采用4个涡状结构的搅拌针时,发生拉伸断裂模式的搭接接头有效连接宽度与断裂载荷均大于具有6个涡状结构的搅拌针,其值分别为11.74 mm及17 531.83 N.     

Material Flow Tracking for Various Tool Geometries During the Friction Stir Spot Welding Process

This study applied powder-tracing techniques to mount Cu and W powders on A6061-T6 aluminum sheets to investigate the material flow mechanism of friction stir spot welding (FSSW) using various geometric tools. The experimental results showed that the geometry of the tools plays a crucial role and determines the entrances of material flow during FSSW. It was believed that instantaneous voids were filled up with material flow in all directions for triangular pins, and the voids were located at the pin bottom for cylindrical pins. In accordance with the plastic rule of material flow, the pressure gradient is the necessary condition to cause material flow during FSSW; therefore, the transient constraint space (TCS) is required to generate pressure in this space. Enlargement of the TCS accompanies the evolution of the stir zone (SZ). A generated void causes a steep pressure gradient, which is regarded as the entrance of material flow. A tool with screw threads causes downward driving force, which determines the intermixing behavior between the upper and lower sheets, and also affects the size of the SZs.     

阶梯形正反螺纹搅拌针对2A12-T4铝合金FSLW接头力学性能的影响

搅拌针上螺纹分布情况会对搅拌摩擦焊接头内部材料流动行为产生重要影响,进而影响接头成形及力学性能. 采用锥形螺纹搅拌针和阶梯形正反螺纹搅拌针进行2A12-T4铝合金搅拌摩擦搭接焊试验,对比分析了两种搅拌针下搅拌摩擦搭接焊接头横截面形貌、显微组织、拉剪性能及接头断裂位置. 结果表明,两种搅拌针下接头横截面形貌均呈现“碗状”. 然而,在阶梯形正反螺纹搅拌针焊接下搭接界面后退侧出现特有的“括号”形貌. 相对于锥形螺纹搅拌针,阶梯形正反螺纹搅拌针下的接头热力影响区与热影响区晶粒分布相差不大,但焊核区晶粒细化程度更加明显;接头在焊接速度80 mm/min下可获得最大拉剪性能,其值为10.39 kN. 阶梯形正反螺纹搅拌针下接头界面后退侧出现的“括号”形貌阻碍了裂纹向焊核区进一步扩展,断裂模式表现为拉伸断裂.     

Understanding the material flow path of friction stir welding process using unthreaded tools

Material flow during friction stir welding is very complex and not fully understood. Most of studies in literature used threaded pins since most industrial applications currently use threaded pins. However, initially threaded tools may become unthreaded because of the tool wear when used for high melting point alloys or reinforced aluminium alloys. In this study, FSW experiments were performed using two different pin profiles. Both pins are unthreaded but have or do not have flat faces. The primary goal is to analyse the flow when unthreaded pins are used to weld thin plates. Cross-sections and longitudinal sections of welds were observed with and without the use of material marker (MM) to investigate the material flow. Material flow with unthreaded pin was found to have the same features as material flow using classical threaded pins: material is deposited in the advancing side (AS) in the upper part of the weld and in the retreating side (RS) in the lower part of the weld; a rotating layer appears around the tool. However, the analysis revealed a too low vertical motion towards the bottom of the weld, attributed to the lack of threads. The product of the plunge force and the rotational speed was found to affect the size of the shoulder dominated zone. This effect is reduced using the cylindrical tapered pin with flats.     

基于MORFEO的采用不同搅拌针形状时沿板厚方向流动性的数值分析

基于专业搅拌摩擦焊有限元分析软件MORFEO的局部分析模块,对5 mm厚2024-T3铝合金薄板对接焊缝不同厚度处的材料流动性进行分析. 焊接速度为80 mm/min,转速为600 r/min时,对圆柱形和圆锥形搅拌针分别建模,后处理提取不同厚度水平面准稳态流线,得到材料流动特点:靠近轴肩质点运动最快;搅拌针前方质点绕过后退侧,最终沉积在搅拌针后方的后退侧,类似于回填作用;搅拌针为圆锥形时,质点可在搅拌针后方两侧均匀沉积. 在该焊接参数下,使用圆锥形搅拌针,铜箔作为标记材料进行试验,模拟结果得到了验证.     

Effects of pin geometry on the material flow behavior of friction stir spot welded 2A12 aluminum alloy

A three dimensional finite volume model was established by the ANSYS FLUENT software to simulate the material flow behavior during the friction stir spot welding (FSSW) process. Effects of the full-threaded pin and the reverse-threaded pin on the material flow behavior were mainly discussed. Results showed that the biggest material flow velocity appeared at the outer edge of the tool shoulder. The velocity value became smaller with the increase of the distance away from the tool surface. In general, material flows downwards along the pin thread when the full-threaded pin is used. Meanwhile, both the materials of the upper and the lower plates flow towards the lap interface along the pin thread when the reverse-threaded pin is used. The numerical simulation results were investigated by experiment, in which 2A12 aluminum alloy was used as the research object. The effective sheet thickness (EST) and stir zone (SZ) width of the joint by the reverse-threaded pin were much bigger than those by the full-threaded pin. Accordingly, cross tension failure load of the joint by the reverse-threaded pin is 23% bigger than the joint by the full-threaded pin.     

Analysis and Modeling of Friction Stir Processing-Based Crack Repairing in 2024 Aluminum Alloy

A friction stir processing-based method was used to repair cracks in the 2024 aluminum alloy plates.The temperature field and plastic material flow pattern were analyzed on the basis of experimental and finite element simulation results.Microstructure and tensile properties of the repaired specimens were studied.The results showed that the entire crack repairing was a solid-phase process and plastic materials tended to flow toward the shoulder center and then resulted in the repairing of cracks.Meanwhile,the coarse grain structures were refined in repaired zone(RZ),while the grains in thermal-mechanically affected zone and heat-affected zone were elongated and driven to grow up.Meanwhile,large phases are crushed into small particles and dispersed inside the RZ.Finally,the strength of the repaired specimens can be restored dramatically and their ductility can be partially restored.After heat treatment,the tensile properties of the repaired specimens can be further enhanced.     

搅拌针形状影响搅拌摩擦焊过程金属塑性流动规律的数值模拟

考虑材料参数随温度的变化关系以及搅拌工具的实际结构,利用Fluent流体力学软件建立了搅拌摩擦焊的有限体积模型,对搅拌针的形状影响材料塑性流动行为的规律进行了研究.结果表明,材料的流动速度随着到焊件表面以及到搅拌针旋转轴的距离增加而减小;当减小搅拌针的锥角以及减小搅拌针的螺纹槽宽度时,焊件内部材料的流动速度得到提高.当搅拌工具在焊接过程中顺时针旋转时,对于左螺旋搅拌针,搅拌针附近的材料向下流动,而热力影响区材料的流动方向向上,此规律与右螺旋搅拌针时相反.     

A new pin design for pressure measurement in metal forming processes

In the past pin pressure transducers for measuring workpiece-die contact stresses have been cylindrical. At high forming pressures the workpiece material is extruded into the pin-bore clearance, hindering pin movement. In this paper a new pin design is described. It is a self-release conic pin lapped to fit into a conic bore. A mandrel containing two of the new pins situated obliquely to the surface, was designed and tested. Friction and normal stresses were measured simultaneously in a forging die. The output from the pins always returned to zero after the forging operation, showing that no metal had extruded between a pin and its bore, even when pressures as high as 30 times the yield stress of the material, were imposed on the die cavity.     

A study of erosion in die casting dies by a multiple pin accelerated erosion test

An accelerated erosion test was developed to evaluate the erosion resistance of die materials and coatings for die casting application. An acceleration in wear was achieved by selecting pyramid-shaped core pins, hypereutectic aluminum silicon casting alloy, high melt temperatures and high gate velocities. Multiple pin design was selected to enable multiple test sites for comparative evaluation. Apilot run was conducted on a 300 ton commercial die casting machine at various sites (pins) to verify the thermal and flow similarities. Subsequently, campaigns were run on two different 300 ton commercial die casting machines to evaluate H13 die material and different coatings for erosive resistance. Coatings and surface treatments evaluated included surface micropeening, titanium nitride, boron carbide, vanadium carbide, and metallic coatings—tungsten, molybdenum, and platinum. Recent campaigns with different melt temperatures have indicated a possible link between soldering phenomena and erosive wear. This paper presents the details of the test set up and the results of the pilot and evaluation tests.     

Filling exit holes of friction stir welding lap joints using consumable pin tools

Abstract

In this study, filling friction stir welding was used to remove the exit hole of friction stir welding lap joints made from AA5456 sheets. For this purpose, the exit holes were filled by consumable pins with various geometries and different pin applying methods. Then, the structures and mechanical properties of the resulting joints were investigated. Results showed that the strength of 7% higher than the strength of the joint with the non-filled exit hole, ～91% of the corresponding defect free joints, is obtainable with this technique. The best results were found by a pin with 11° cone angle, 8?mm diameter and 7?mm length, and with a 6?mm plunge without rotation.     

基于神经元网络的薄壁筒滚珠旋压成形缺陷诊断

作为一种连续局部塑性成形工艺,滚珠旋压被应用于制造高强度、高精度的纵向内筋薄壁筒形件.通过使用铝合金作为旋压材料,在实验的基础上分析了滚珠旋压过程中金属材料非稳定流动的基本原理及旋压件产生表面质量缺陷的原因.以人工神经元网络为基础,对旋压件的表面质量缺陷进行了预测.实验证明,神经元网络能够精确地诊断旋压件的表面质量缺陷.     

Investigation of the material flow and texture evolution in friction-stir welded aluminum alloy

The material flow and crystallographic orientation in aluminum alloy sheets joined by friction stir welding (FSW) were investigated by electron back scattered diffraction (EBSD). The microstructure and microtexture of the material near the stir zone was found to be influenced by the rotational behavior of the tool pin. It was found that, during FSW, the forward movement of the tool pin resulted in loose contact between the tool pin and the receding material at the advancing side. This material behavior inside the joined aluminum plates was also observed by an X-ray micrograph by inlaying a gold marker into the plates. As the advancing speed of the tool increases at a given rotation speed, the loose contact region widens. As the microtexture of the material near the stir zone is very close to the simple shear texture on the basis of the frame of the tool pin in the normal and tangent directions, the amount of incompletely rotated material due to the loose contact could be estimated from the tilt angle of the shear texture in the pole figure around the key hole.     

Material adhesion and stresses on friction stir welding tool pins

During friction stir welding, polygonal tool pins experience severe stresses and, under certain conditions, loss of functionality due to adhesion of plasticised material on their surfaces. The extent of adhesion is analysed for various pin geometry and welding conditions based on the theory of machining. The effective stresses on the polygonal pins are evaluated following the principles of mechanics. The results show that the polygonal pins with fewer sides can avoid permanent adhesion of plasticised material at higher weld pitch, which is defined as a ratio of welding speed and tool rotational speed. The computed pin geometries for minimum adhesion are compared with the pin profiles recommended by various investigators based on independent experiments. The computed stresses show that pins with larger number of sides will experience lower stresses for any given set of welding variables.     

Numerical simulations on material flow behaviors in whole process of friction stir welding

A finite element model based on solid mechanics was developed with ABAQUS to study the material flow in whole process of friction stir welding (FSW), with the technique of tracer particles. Simulation results indicate that the flow pattern of the tracer particles around the pin is spiral movement. There are very different flow patterns at the upper and lower parts of the weld. The material on the upper surface has the spiral downward movement that is affected by the shoulder and the lower material has the spiral upward movement that is affected by the pin. The velocity of the material flow on the periphery of the stirring pin is higher than that at the bottom of the stirring pin. The material can be rotated with a stirring pin a few times, agreeing well with the previous experimental observation by tungsten tracer particles.     

Tribological studies of a Zr-based bulk metallic glass

In the present study, the tribological behavior of a Zr_52.5Cu_17.9Ni_14.6Ti₅Al₁₀ bulk metallic glass (BMG) was investigated using pin-on-disk sliding measurements under an argon atmosphere, rubbing against a type 303 stainless steel counterface. The tested pins and disk were examined using X-ray diffractometry, optical microscopy, profilometry, scanning electron microscopy and transmission electron microscopy. The results showed that the wear of the BMG pins was substantially larger compared with previous tests performed against a zirconia counterface. Strain softening was found in the near-surface region of the glassy pin due to the highly localized shearing. Frictional heating contributed to the occurrence of viscous flow and material transfer on the worn surface of the wear pin and the disk, respectively. Thus, the pin exhibited a severe adhesive-dominated sliding wear.     

半螺纹搅拌针对2024铝合金搅拌摩擦搭接焊力学性能影响

作为影响搅拌摩擦焊（FSW）过程中塑性材料上下流动的重要因素,搅拌针螺纹形貌同样对搅拌摩擦搭接焊（FSLW）的材料流动和力学性能有着重要影响.为研究半螺纹搅拌针对FSLW接头显微组织和力学性能的影响,文中将全螺纹搅拌针和半螺纹搅拌针用于包铝2024铝合金FSLW试验,并对不同搅拌针作用下的FSLW接头的横截面形貌、剪切拉伸载荷、断裂位置等方面进行分析对比.结果表明,半螺纹搅拌针会使钩状缺陷向下弯曲,从而使FSLW接头具有较大的有效板材厚度以及搭接宽度.断裂模式同为剪切断裂,但半螺纹搅拌针作用下的FSLW接头拥有更大的拉断载荷.     

焊接速度对BT-FSW 2219铝合金接头流动行为及组织与力学性能影响分析

以4 mm厚2219-T87铝合金为研究对象,采用数值模拟和试验研究相结合的方法,研究了焊接速度对双轴肩搅拌摩擦焊接(BT-FSW)过程中金属流动行为、接头组织特征和力学性能的影响. 模拟结果表明,随着焊接速度增加,焊缝高温区逐渐变窄,轴肩附近的材料流动减弱,而搅拌针附近的材料流动增强. 试验结果表明,随着焊接速度增加,材料汇流区及S线缺陷均向前进侧(AS)靠近,焊接速度过大时AS附近的搅拌区(SZ)形成不连续组织. 接头SZ的硬度值随着焊接速度增加而增大,而拉伸强度先增大后稍降低. 当转速为300 r/min,焊接速度为150 mm/min时,接头获得最大的拉伸强度,为414 MPa ± 4MPa.