Impact of Selective Grain Refinement on Superplastic Deformation: Finite Element Analysis

Recently, Friction Stir Processing has been introduced as an effective tool to refine the grain structure of sheet metals and enhance their superplasticity. A possible application of friction stir processing is selective grain refinement to enhance deformation uniformity and minimize localized thinning. This selective treatment will result in a sheet with grain size gradient. Very limited studies have been directed toward investigating the effect of grain size gradient on the superplastic deformation during SPF. In this work, Finite Element simulations for the free bulging of a dome made of 7075Al alloy were conducted to examine the effects of initial grain size gradient within the sheet on SPF characteristics. The results clearly demonstrate that selective grain refinement can be utilized to eliminate severe thinning and improve the integrity of the superplastic formed part. This article was presented at the AeroMat Conference, International Symposium on Superplasticity and Superplastic Forming (SPF) held in Baltimore, MD, June 25-28, 2007.     

Analytical Modeling of Strain Rate Distribution During Friction Stir Processing

Friction Stir Processing (FSP) is becoming an acceptable technique for modifying the grain structure of sheet metals. One of the most important issues that hinder the widespread use of FSP is the lack of accurate models that can predict the resulting microstructure in terms of process parameters. Most of the work that has been done in the FSP field is experimental, and limited modeling activities have been conducted. In this work, an analytical model is presented that can predict the strain rate distribution and the deformation zone in the friction stir processed zone as a function of process parameters. In the model, the velocity fields within the processed zone are determined by incorporating the effects of both the shoulder and the pin of the tool on the material flow. This is achieved by introducing state variables and weight functions. The model also accounts for different interfacial conditions between the tool and the material. The effects of different process parameters and conditions on the velocity fields and strain rate distributions are discussed. The results clearly show that the model can successfully predict the shape of the deformation zone and that the predicted strain rate values are in good agreement with results reported in the literature. This article was presented at the AeroMat Conference, International Symposium on Superplasticity and Superplastic Forming (SPF) held in Baltimore, MD, June 25-28, 2007.     

A novel method to test the thinning limits of sheet metals in negative incremental forming

Single Point Incremental Forming (SPIF) is an emerging sheet-metal-forming technology, capable of manufacturing complex parts at low cost for small to medium-batch production. The present paper is focused on presenting an innovative and viable method to test the thinning limits of sheet metals in Negative Incremental Forming along with verification of the Cosine's law of thickness distribution. The Cosine's law was verified by comparing the experimentally measured thicknesses of incrementally formed parts with those predicted by the law. To test the thinning limit of a sheet metal, the idea is based on the forming of an axi-symmetric part varying its slope with depth corresponding to varying thinning. An arc of a circle was selected as generatrix to model such an axi-symmetric part. Based on the Cosine's law, mathematical expressions were derived to predict the thickness distribution along the depth of the part and the thinning limit of the sheet-metal. The Aluminum sheet metal was used as an experimental material. In order to test its thinning limit, the axi-symmetric part, modeled with a generatrix arc, was formed incrementally until it cracked. Thickness of the fractured part was measured at various points along its depth and compared with that predicted by the Cosine's law. The maximum thinning at a point, at which thickness followed the Cosine's law, was called the thinning limit of the sheet metal. In order to obtain accurate results, four such parts having the same generatrix design were formed. Based on these results, several axi-symmetric and asymmetric parts were formed at fixed slopes. It was found that the thinning limits obtained from the parts formed at fixed slopes were a little lower than those obtained from the parts modeled with the same generatrix design. As conclusion, a strategy to test the lowest possible thinning limits of sheet metals has been proposed. The proposed method is capable to test the thinning limits of sheet metals at reduced processing time and cost.     

7075-O铝合金板料的摩擦搅拌渐进成形：成形零件性能的实验研究

本文中,用不同的主轴转速来成形两种典型零件,研究了摩擦搅拌渐进成形零件的综合性能：成形性,表面质量,拉伸性能,显微硬度和壁厚分布。试验结果表明,板料的成形性随着转速增加而呈现增强趋势;接触面的表面粗糙度在水平和竖直方向表现出不同的变化趋势;非接触面的表面质量几乎不受转速影响;相比原板料,成形零件的拉伸性能和显微硬度明显增强,但转转速超过3000 rpm后,硬度值开始逐步下降;在高转速下成形零件的壁厚分布略微优于低转速下的成形零件。总体上,除了力学性能,高转速下成形零件的综合性能比低转速下的成形零件好。     

搅拌摩擦加工制备Al3Ni-Al原位反应复合体

通过在搅拌摩擦加工(Friction Stir Processing,FSP)过程中填加微米级Ni粉的方法,利用Al、Ni在FSP条件下的快速原位反应,在Al合金1100-H14表面层获得Al3Ni-Al复合体。采用SEM、EDS以及XRD对表面复合体微观结构及相组成进行分析,并对其显微硬度进行评测。结果表明,在FSP强烈的热、力耦合作用下,Ni粉产生了充分碎化,破碎后的Ni粒子与Al产生快速原位反应,生成亚微米甚至纳米级Al3Ni颗粒,而少量微米级残留Ni颗粒被Al3Ni包裹,并与细小的Al3Ni颗粒一同均匀分布于Al合金基体中,从而使得表面复合体的硬度显著提高,其平均值达到了818.3MPa,为基体硬度的2.4倍。     

Using torque sensitivity analysis in accessing Friction Stir Welding/Processing conditions

The use of Friction Stir Processing (FSP) techniques for the joining and/or transforming of metallic materials is being object of intensive research since the earliest development of the Friction Stir Welding (FSW) technology in 1991. Despite of this, an accurate understanding of the main welding/processing mechanisms and its relation with the process parameters is still missing. Current paper intends to provide some further insight on this subject by discussing the relations between processing parameters, classified as independent variables, and the corresponding welding results, classified as dependent variables, using torque sensitivity analysis. The relation between base materials properties, plate thickness, welding conditions and torque evolution were also explored, which constitutes a novelty relative to the previous studies on this subject.     

搅拌摩擦加工研究进展

搅拌摩擦加工(FSP),是一种新型的材料塑性变形加工方法,它是在搅拌摩擦焊(FSW)的基础上提出的。从发明至今,研究者已经成功将FSP用于铸造金属微观组织细化、超塑性材料的制备、材料表面改性以及各种复合材料的制备中。搅拌摩擦加工工艺与搅拌摩擦焊接工艺基本相同,工艺参数对搅拌摩擦加工材料质量有很大的影响。综述了搅拌摩擦加工近年来的研究进展,主要包括不添加增强相的FSP和添加增强相的FSP两大类。其中不添加增强相的FSP主要有铸造金属微观组织细化和超塑性材料制备,添加增强相的FSP主要有材料表面改性和复合材料制备。搅拌摩擦加工制备复合材料根据添加相是否与基体反应生成增强相,又分为非原位合成法制备复合材料与原位合成法制备复合材料。文中对以上内容分别进行了总结与评述,最后指出了FSP今后发展应用的方向。     

Friction Stir Knead Welding of steel aluminium butt joints

To develop steel aluminium-tailored hybrids in a butt joint for sheets in a thickness of about 1 mm conventional Friction Stir Welding is not feasible due to a high distortion of the welded specimen. Contrary to Friction Stir Welding the tool used for Friction Stir Knead Welding has no pin wherefore higher welding speeds can be realised. Due to the fact that this is a newer process, applied for patent in 2005, the cut contours of the edges and their variations have to be optimised by numerical analysis to transfer a maximum of load in order to improve the formability. The examined materials in this paper are steel DC04, as well as the aluminium alloys AA5182 and AA6016 in sheet thicknesses of 1 mm. Accompanying experimental investigations, as tensile tests, will evaluate the quality of the welding mechanism. As the mechanics of the new welding technology is not fundamentally investigated until now, metallographic investigations are performed, and additionally micro hardness measurements are carried out to verify the changes in the hardness distribution in the welding zone after stirring and welding.     

等离子旋转电极法制备钛铝粉末性能表征

采用等离子束旋转电极法制备Ti-47Al-2Cr-2Nb球形粉末,粉末的平均粒径d50为85 μm, 松装密度为2.65 g/cm3。截线法测得粉末的球形度为99.6%;粉末的流动性为10.40 s/50 g;粉末的平均氧、氮含量分别为0.05%和0.004%。XRD分析表明,粉末的相组成为α2相和γ相,主相为富钛α2相;粉末经热等静压后主相为γ相,伴随少量α2相。SEM背散射照片表明,粉末颗粒的晶粒为胞状组织,晶粒平均大小约为2 μm;对不同粒径的颗粒进行EPMA分析表明,颗粒内部化学成分与预合金棒接近,颗粒表面有部分Al元素挥发,约为2at%     

搅拌摩擦加工研究进展及前景展望

对搅拌摩擦加工工艺、加工对性能的影响及其在制备复合材料及合金等方面的研究做了比较详尽的综述,并指出搅拌摩擦加工研究进程中存在的问题、发展前景及今后的研究方向。认为对非正常晶粒长大的影响因素及其产生机制还有待深入研究。熔化焊结合搅拌摩擦加工能满足钛合金等高强材料焊接的需要。用搅拌摩擦加工制备纳米相增强金属基复合材料和金属间化合物将有良好的应用前景。为解决材料微观结构不均匀现象,基于搅拌摩擦加工基本思想,开发新的适合材料制备的新工艺将是该技术从实验室成功走向实际应用的瓶颈     

Surface modification of aluminium by friction stir processing

In this study, SiC particles were incorporated by using Friction Stir Processing (FSP), into the commercially pure aluminium to form particulate surface layers. Samples were subjected to the various tool rotating and traverse rates with and without SiC powders. Microstructural observations were carried out by employing optical microscopy of the modified surfaces. Mechanical properties like hardness and plate bending were also evaluated. The results showed that increasing rotating and traverse rate caused a more uniform distribution of SiC particles. The hardness of produced composite surfaces was improved by three times as compared to that of base aluminium. Bending strength of the produced metal matrix composite was significantly higher than processed plain specimen and untreated base metal.     

Experimental and numerical investigation of the process chain from composite extrusion to friction stir welding regarding the residual stresses in composite extruded profiles

The innovative production methods Composite Extrusion and Friction Stir Welding are used to manufacture lightweight frame structures made of steel reinforced aluminium. In order to optimise the processes themselves and the manufacturing of frame structures both processes are analysed in a structural simulation. One result of these simulations is the distribution of the residual stresses in the reinforced profiles. The simulation results are compared with measurements of the residual stresses by means of neutron scattering.     

Processing–Microstructure–Crystallographic Texture–Surface Property Relationships in Friction Stir Processing of Titanium

Friction stir processing (FSP) is a solid-state technique for microstructural modification of metallic materials. The aim of this work is to establish processing–microstructure–texture–surface properties relationship in commercially pure titanium (cp-Ti) processed by FSP under different processing conditions. The effect of processing conditions on the microstructural changes and the evolution of crystallographic texture is systematically studied. The changes in the chemical composition of the passive surface layer are characterized by x-ray photoelectron spectroscopy. The corrosion behavior of cp-Ti after FSP is evaluated in simulated body fluid and is related to the microstructure, texture and composition of passive layer. Substantial grain refinement was observed after FSP. Shear type deformation texture evolved during FSP with dynamic restoration processes weakening the overall shear texture. The corrosion resistance improved after processing at lower rotational speed due to the presence of basal planes at the surface and the incorporation of TiN in the passive layer. The results of this study suggest that surface properties of cp-Ti like passive layer and corrosion resistance are altered by FSP and can be controlled using appropriate processing parameters.     

一种替代传统电阻点焊的创新技术——搅拌摩擦点焊

搅拌摩擦点焊（Friction Stir Spot Welding,FSSW）是在“线性”搅拌摩擦焊接（linear friction stir welding,FSW）基础上,研究开发的一种创新的焊接技术。FSSW可以形成一种点焊的搭接接头,其焊缝外观与应用于汽车车体的电阻点髓类似,因而在汽车和其他工业领域引起很大的关注。综述分析了搅拌摩擦点焊的基本原理、工艺特点和工业应用。     

Study of top sheet thinning during friction stir lap welding of AZ31 magnesium alloy

Abstract

Friction stir lap welding was applied to AZ31 magnesium alloy sheets to investigate the effect of tool designs and welding variables on top sheet thinning. Three tools with different shoulder designs were used. Sheet thinning aroused by the hooking or cold lap feature was quantified, and the lap shear fracture load of the joints was evaluated and correlated with the effective top sheet thickness. The tool geometry has a significant effect on the morphology and extent of both hooking and cold lap features, as well as welding variables such as welding pitch and plunge depth. The morphology and extent of these features influenced the effective top sheet thickness, which exhibited a linear relationship with the unit width fracture load via the welded material strength. High fracture loads can be achieved by suppressing the hooking and cold lap features as well as by strengthening the material in the nugget.     

Laser-assisted asymmetric incremental sheet forming of titanium sheet metal parts

Asymmetric Incremental Sheet Forming (AISF) is a relatively new manufacturing process. In AISF, a CNC driven forming tool imposes a localized plastic deformation as it moves along the contour of the desired part. Thus, the final shape is obtained by a sequence of localized plastic deformations. AISF is suitable for small series production of sheet metal parts as needed in aeronautical and medical applications. Two main process limits restrict the range of application of AISF in these fields. These are the low geometrical accuracy of parts made from titanium alloys or high strength steels and, for titanium alloys, the limited formability at room temperature. In this paper a new concept for laser-assisted AISF is introduced including the required components. Furthermore, the CAX tools used for programming the NC path for the forming tool and the laser spot are illustrated. First experimental results show that the formability of the alloy Ti Grade 5 (TiAl6V4), which is usually used in aeronautic applications, can be increased.     

TA2工业纯钛表面搅拌摩擦加工组织及性能

对TA2工业纯钛成功实现了搅拌摩擦加工(Friction Stir Processing, FSP),研究FSP后搅拌区、热机影响区、热影响区组织特征,对比分析FSP加工区与母材的显微硬度及摩擦磨损性能。结果表明：TA2工业纯钛表面经FSP后,搅拌区晶粒发生了剧烈的塑性变形、混合和破碎,实现组织结构的致密化、均匀化和细化;加工区平均硬度相对母材提高37.5%,当摩擦磨损圈数分别为1000、1500、2000 r时,摩擦磨损质量损失分别比母材减少31.4%、36.6%和46.4%,经FSP后TA2工业纯钛表面硬度和抗摩擦磨损性能明显提高     

Improvement of the drawability based on the surface friction stir process of AA5052-H32 automotive sheets

Based on the imperative social demand for lighter vehicles, lightweight materials such as aluminum alloys are expected to replace conventional steels in many automotive applications. In automotive parts manufacturing, most of the components produced in conventional stamping operations are geometrically complex as the blanks are subjected to both stretching and drawing deformations. However, aluminum alloys have intrinsic drawbacks, such as the inferior formability of these materials, although the effects of the weight reduction in terms of performance are highly promising. In an effort to improve the formability of aluminum alloy sheets, the surface friction stir process is proposed in this study. This process locally modifies the surface of automotive aluminum alloy sheets via stirring and advancing on the surface of the sheet, similar to the Friction Stir Welding (FSW) process that utilizes a probe without a pin. When the surface of the sheet is modified locally by stirring, dynamic recrystallization due to the severe shear deformation along with heat resulting from the friction occur due to changes in the micro-structure and mechanical properties in the stirred zone, while the dislocation density and grain size refinement are curtailed. In this work, the drawability performance of AA5052-H32 sheets (thickness 1.5 mm) that were welded using the surface friction stir process was experimentally and numerically investigated in cylindrical cup drawing tests. When applied to AA5052-H32 automotive sheets, the surface friction stir process improved the drawability of the entire aluminum alloy sheet. For numerical simulations, the non-quadratic anisotropic yield function Yld2000-2d was employed along with isotropic hardening, while the formability was evaluated by utilizing theoretical forming limit diagrams (FLD) based on Hill's bifurcation and M-K theories.     

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 金属板材的成形极限是板材成形领域中的重要性能指标和工艺参数。目前,国内外均已开展以厚度减薄率作为成形极限判据的成形极限试验模拟研究,其厚度减薄率的测试过程较为复杂,不适合实验室大批量检验工作。提出了2种金属板材拉伸厚度减薄率的测试方法,一种为人工测试断后试样断口附近厚度分布,得到厚度减薄率变化曲线;另一种方法是通过公式推导,将厚度减薄率与试样纵向应变联系起来,通过测试纵向应变,得到集中性失稳前的厚度减薄率变化规律。     

Local enhancement of the material properties of aluminium sheets by a combination of additive manufacturing and friction stir processing

Friction stir processing (FSP) is combined with additive manufacturing (AM) with selective laser melting to locally enhance the material properties of a metallic part. A groove inside aluminium 1060 alloy sheet is filled with an aluminium 7075 alloy powder by AM. FSP is then applied to the AMed region along the groove. The suggested technique creates a heterogeneous microstructure with alternating reinforcement-enriched and matrix-enriched regions. While the overall hardness of the stir zone (SZ) increases significantly, the heterogeneous microstructure results in a unique uneven hardness distribution in the SZ. Tensile tests confirm the effectiveness of the suggested technique.