Fracture mechanisms of aluminium alloy AA7075-T651 under various loading conditions

The fracture behaviour of the aluminium alloy AA7075-T651 is investigated for quasi-static and dynamic loading conditions and different stress states. The fracture surfaces obtained in tensile tests on smooth and notched axisymmetric specimens and compression tests on cylindrical specimens are compared to the fracture surfaces that occur when a projectile, having either a blunt or an ogival nose shape, strikes a 20 mm thick plate of the aluminium alloy. The stress state in the impact tests is much more complex and the strain rate significantly higher than in the tensile and compression tests. Optical and scanning electron microscopes are used in the investigation. The fracture surface obtained in tests with smooth axisymmetric specimens indicates that the crack growth is partly intergranular along the grain boundaries or precipitation free zones and partly transgranular by void formation around fine and coarse intermetallic particles. When the stress triaxiality is increased through the introduction of a notch in the tensile specimen, delamination along the grain boundaries in the rolling plane is observed perpendicular to the primary crack. In through-thickness compression tests, the crack propagates within an intense shear band that has orientation about 45° with respect to the load axis. The primary failure modes of the target plate during impact were adiabatic shear banding when struck by a blunt projectile and ductile hole-enlargement when struck by an ogival projectile. Delamination and fragmentation of the plates occurred for both loading cases, but was stronger for the ogival projectile. The delamination in the rolling plane was attributed to intergranular fracture caused by tensile stresses occurring during the penetration event.     

A Comparative Study of Friction Self-Piercing Riveting and Self-Piercing Riveting of Aluminum Alloy AA5182-O

In this paper, self-piercing riveting (SPR) and friction self-piercing riveting (F-SPR) processes were employed to join aluminum alloy AA5182-O sheets. Parallel studies were carried out to compare the two processes in terms of joint macrogeometry, tooling force, microhardness, quasi-static mechanical performance, and fatigue behavior. The results indicate that the F-SPR process formed both rivet–sheet interlocking and sheet–sheet solid-state bonding, whereas the SPR process only contained rivet–sheet interlocking. For the same rivet flaring, the F-SPR process required 63% less tooling force than the SPR process because of the softening effect of frictional heat and the lower rivet hardness of F-SPR. The decrease in the switch depth of the F-SPR resulted in more hardening of the aluminum alloy surrounding the rivet. The higher hardness of aluminum and formation of solid-state bonding enhanced the F-SPR joint stiffness under lap-shear loading, which contributed to the higher quasi-static lap-shear strength and longer fatigue life compared to those of the SPR joints.     

金属夹层对自冲铆接头强度和失效形式的影响

基于自冲铆技术,分别制备了铝-铝(A0)、铝-泡沫铁镍夹层-铝(A1)和铝-涂胶泡沫铁镍夹层-铝(A2)的3种自冲铆接头。为研究泡沫金属夹层以及涂胶泡沫金属夹层对自冲铆接头强度及失效形式的影响,通过力学试验测试接头静力学性能,从宏观角度分析接头失效形式,并采用扫描电子显微镜(SEM)对接头失效断口进行微观分析。研究表明:泡沫铁镍金属夹层和涂胶泡沫铁镍金属夹层可有效提高接头静失效载荷,但接头能量吸收值呈下降趋势,A0组接头最大平均失效载荷为3586 N,能量吸收值为11.779 J,A1组和A2组接头静失效载荷比A0组分别提高了9.9%和30%,能量吸收值比A0组分别降低了0.9%和7.2%;3组接头均为拉脱失效,且上板搭接区均出现不同程度的曲翘,A0组接头铆钉从下板拉脱,而A1和A2组接头下板底部与铆钉一并从下板拉脱;3组接头失效模式微观特征为:铆钉腿与铆接孔一侧发生剧烈摩擦和挤压,最终铆钉在拉剪载荷下完全从下板拉脱。     

Microstructural evolution in friction self-piercing riveted aluminum alloy AA7075-T6 joints

Friction self-piercing riveting(F-SPR)is an emerging technique for low ductility materials joining,which creates a mechanical and solid-state hybrid joint with a semi-hollow rivet.The severe plastic deforma-tion of work materials and localized elevated temperatures during the F-SPR process yield complex and heterogeneous microstructures.The cut-off action of the work materials by the rivet further compli-cates the material flow during joint formation.This study employed the F-SPR process to join AA7075-T6 aluminum alloy sheets and systematically investigated the microstructural evolutions using electron backscatter diffraction(EBSD)techniques.The results suggested that as the base material approached the rivet,grains were deformed and recrystallized,forming two distinct fine grain zones(FGZs)surround-ing the rivet and in the rivet cavity,respectively.Solid-state bonding of aluminum sheets occurred in the FGZs.The formation of FGZ outside the rivet is due to dynamic recrystallization(DRX)triggered by the sliding-to-sticking transition at the rivet/sheet interface.The FGZ in the rivet cavity was caused by the rotation of the trapped aluminum,which created a sticking affected zone at the trapped aluminum/lower sheet interface and led to DRX.Strain rate gradient in the trapped aluminum drove the further expansion of the sticking affected zone and resulted in grain refinement in a larger span.     

碳纤维增强树脂复合材料和铝合金温热自冲铆接工艺及接头力学性能

为研究传统自冲铆(SPR)工艺连接碳纤维增强树脂复合材料(CFRP)和铝合金的损伤问题,制备三种典型铺层结构的自冲铆接头,研究铺层结构对接头表面宏观损伤形貌的影响。在不同测试温度下对CFRP进行力学试验,研究温度对CFRP力学性能及失效的影响。基于CFRP的温热力学性能,以减小接头损伤为目的,创新性提出了CFRP和铝合金的温热自冲铆接(WSPR)工艺,对比了两种铆接工艺获得接头中CFRP的损伤差异。制备CFRP和铝合金的WSPR接头,研究铺层角度对接头力学性能和失效过程的影响。研究表明:常温下铆接时,钉头附近区域易出现宏观裂纹缺陷,主要以平行于纤维方向的基体裂纹和垂直于纤维方向的纤维裂纹形式存在。在树脂基体的玻璃化转变温度下,CFRP在横向和剪切方向的延展性大幅度提高,导致WSPR接头的CFRP表面无宏观裂纹,同时减小了分层损伤面积。铺层角度影响接头的拉剪力学性能及失效过程,[0/90/0]_s铺层接头的力学性能最优。      

冷弯薄壁型钢锁铆连接力学性能及其本构模型研究

为将锁铆连接引入冷弯薄壁型钢结构中构件的连接,对锁铆连接及自攻螺钉连接的试件进行了抗拉、抗剪性能试验,探讨了铆钉端距、基板厚度差、铆钉长度等参数对锁铆连接抗剪性能的影响;基于传染病传播动力学SIR模型建立了铆接本构模型,提出了锁铆连接抗剪承载力设计计算方法。研究结果表明:锁铆连接的主要破坏模式为延性破坏模式,表现为铆钉腿部剥离下层板材并伴随铆钉头部局部脱离上层板材,且刚度、强度和耗能性能均明显优于自攻螺钉连接;所建立的本构模型能够较精确反映锁铆连接荷载-变形曲线的变化趋势,且抗剪承载力的理论值和试验值误差较小;锁铆连接用于冷弯薄壁型钢板间连接时,其组合厚度不宜大于4 mm,厚板与薄板的厚度比不宜大于1.5,锁铆接头的端距应大于2倍铆钉直径。     

Effects of post-weld heat treatment on microstructure and mechanical properties of friction stir welded joints of 2219-O aluminium alloy

Abstract

Post-weld heat treatment (PWHT) of 2219-O aluminium alloy friction stir welding joints was carried out at solution temperatures of 480, 500 and 540°C for 32 min followed by aging at 130°C for 9 h. The effects of PWHT on the microstructure and mechanical properties of the joints were investigated. Experimental results show that PWHT causes coarsening of the grains in the weld, and the coarsening degree increases with increasing solution temperature. The tensile strength of the heat treated joints increases with increasing solution temperature. The maximum tensile strength can reach 260% that of the base material at the solution temperature of 540°C. PWHT has a significant effect on the fracture locations of the joints. When the solution temperature is lower than 500°C, the joints fracture in the base material; when the temperature is higher than 500°C, the joints fracture in the weld. The change of the fracture locations of joints is attributed to the presence of precipitate free zones beside the grain boundaries and coarsening equiaxed grain structures in the weld.     

Rotation friction pressing riveting of AZ31 magnesium alloy sheet

A kind of joining method for magnesium alloys, rotation friction pressing riveting (RFPR), is proposed in this paper. In RFPR operation, a rivet with a plug rotating at high speed is brought to contact with the riveted sheets, generating frictional heat between the rivet and riveted sheets, which softens the sheet materials and enables the rivet to be drilled into the sheets under reduced force. When fully inserted, the rivet is stopped rotating, and the plug is immediately pressed into the shank of the rivet by a punch. The expansive deformation of the rivet shank occurs under the action of the plug, thereby forming a mechanical interlock between the rivet and the sheets to fasten the sheets together. The studies show that RFPR of AZ31 magnesium alloy sheet can be carried out at ambient temperature, and provides the joints with superior shear strength and fatigue property when compared with self-piercing riveting (SPR). The effects of the operating parameters of RFPR process on the quality of the joints were investigated in the study. The results shows that while the rivet rotation speed little affects the shear strength of RFPR joints, the punch pressure has a significant influence on the mechanical properties of the RFPR joints. A numerical analysis was also performed to understand the effect of the punch pressure on the interlock between the rivet and the sheets, and the stress and strain distribution inside the sheet materials around the rivet. The results show that the interlock increased with the punch pressure and there is residual compressive stress inside the sheet materials, which seems to explain the good fatigue property of RFPR joints observed.     

Effect of aging treatment on fracture toughness in ZM61 magnesium alloy

The effect of aging treatment on fracture toughness in Mg–6Zn–1Mn (wt-%) was investigated by optical microscopy, scanning electron microscopy, transmission electron microscopy, scanning transmission electron microscopy, uniaxial tensile and fracture toughness tests, respectively. The results showed that the fracture toughness of Mg–6Zn–1Mn alloy can be enhanced by aging treatment. The fracture toughness and strength showed a reverse trend in single aged and double aged alloy. Synergetic effect of fine grains and precipitates improved the fracture toughness more sharply than aging treatment. The precipitate free zones and grain boundary precipitates made the largest contribution to the reduction of toughness. Under as extruded and aged conditions, the main origins of cracks were elastic incompatibility and plastic deformation.     

轻合金自冲铆微动磨损及疲劳性能研究

选择4组轻合金自冲铆进行疲劳实验,用扫描电子显微镜和能谱仪对其断口进行微动磨损机理分析,并系统地研究了接头疲劳寿命和失效形式的影响因素。结果表明,下板与钉腿区的微动磨损是导致下板沿纽扣断裂和铆钉断裂的主要原因,两板间的微动磨损是导致上板靠钉头断裂的主要原因;微动磨屑主要成分为金属板材氧化物,并对微动磨损起缓冲作用。增加板厚可提高接头疲劳寿命,且疲劳载荷较大时寿命提高更为显著;增加板强可提高接头疲劳寿命,且寿命提高程度受疲劳载荷影响较小。增加板厚使失效形式从上板断裂变为下板断裂,增加板强使失效形式从板材断裂变为铆钉断裂。     

Dynamic Tensile Fracture Behaviours of Selected Aluminum Alloys under Various Loading Conditions

Experiments investigating dynamic tensile fracture were performed on the extruded rods of 2024‐T4 and 7075‐T6 aluminum alloys under varying loading conditions. The initial yield stress and fracture strain of 7075‐T6 alloy obtained in spilt Hopkinson tension bar tests are higher than that of 2024‐T4 alloy. But the initiation fracture toughness and spall strength of 2024‐T4 alloy are higher than those of 7075‐T6 alloy in three‐point bending and plate impact experiments, which indicates that 2024‐T4 alloy has better crack initiation tolerance and stronger spall failure resistance. Based on metallurgical investigations by using optical and scanning electron microscopes, it is revealed that the microstructure has a profound effect on the dynamic tensile fracture mechanism of each aluminum alloy. The 2024‐T4 alloy is relatively brittle due to voids or cracks nucleated at many coherent CuMgAl₂ precipitate phases in the grain interiors, and the fracture mode is predominantly transgranular. The 7075‐T6 alloy exhibits relatively ductile fracture because voids or cracks growth is partly intergranular along the grain boundaries and partly transgranular by void formation around coarse intermetallic particles. The obvious differences of damage distribution and void coalescence mechanisms for 2024‐T4 and 7075‐T6 alloys under plate impact are also discussed.     

TA1异质自冲铆接头力学性能及失效机理

通过自冲铆接对比试验获得接头最优铆接参数,并以此制备TA1钛合金板分别与Al5052铝合金板和H62铜合金板的异质自冲铆接头。通过静力学实验和疲劳实验研究异质接头的力学性能,并运用疲劳三参数经验公式拟合S-N曲线,最后利用扫描电镜和能谱仪进行断口分析和能谱分析进而研究接头的疲劳失效机理。结果表明,TA1-H62(STH)接头静失效载荷优于TA1-Al5052(STA)接头;且前者在低载荷下疲劳寿命优于后者,STA接头则在高载荷下优势明显。STA接头疲劳失效模式为下板断裂,STH接头则出现了两种失效模式;两板间及铆钉与上下板之间接触区域发生的剧烈微震磨损是导致疲劳裂纹萌生的主要原因。     

淬火速率对7055铝合金组织和力学性能的影响

通过常温力学性能测试和透射电镜(TEM)研究了淬火速率对7055铝合金组织和力学性能的影响.结果表明,一定时效条件下,合金的力学性能随淬火速率降低而下降.组织观察发现,淬火速率小时,合金在冷却过程中于A l3Zr粒子和晶界非均匀形核析出粗大η平衡相,降低固溶体过饱和程度,削弱时效强化效果.时效时这些粗大η平衡相继续长大并在周围形成无沉淀析出带.晶界无沉淀析出带宽度随淬火速率降低而增大.对合金力学性能下降的原因进行了分析和探讨.     

SiCp/Al-Li复合材料的微观结构性能、及断裂特征

本文对粉末冶金法制备的SiCp/Al-Li复合材料进行了不同温度的拉伸试验和透射电镜分析,结果表明,该复合材料具有高的室温强度和低的延伸率。在复合材料基体晶界和SiC颗粒界面处均存在一定宽度的无沉淀带。微裂纹常在基体晶界和SiC颗粒界面处形成。复合材料的断裂形貌为韧窝加沿晶断裂。      

Effect of Solution Annealing on Microstructure and Mechanical Properties of a Ni-Cr-W-Fe Alloy

The effect of solution annealing on the microstructure and mechanical properties of a Ni-Cr-W-Fe alloy developed for advanced 700?C ultra-supercritical power plants was investigated. Test samples in this study were subjected to different solution treatments and the same aging treatment(at 760?C for 1 h).When solution annealing temperature was elevated from 1020?C to 1150?C, the stress-rupture life at750?C/320 MPa was increased from 60 h to 300 h, the stress-rupture elongation was enhanced from12% to 17%, and the elongation of the tensile at 750?C was improved from 11% to 24%. All tensile and stress-rupture samples displayed an intergranular dimple mixed fracture. Intergranular micro-cracks had a great relationship with the morphology of grain boundary carbides. Most carbides retained the morphology of globular shape and continuous thin plate. After tensile and stress-rupture tests, a few carbides were converted into lamellar. The results showed that intergranular micro-cracks were easier to form at continuous thin plate carbides than at globular shape carbides. Lamellar carbides hardly caused the nucleation of micro-cracks. Besides, grain boundaries sliding and elements diffusion during stressrupture tests led to the formation of precipitate free zones, which accelerated the extension of microcracks and influenced the stress-rupture life.     

On the self-piercing riveting of aluminium blanks and carbon fibre composite panels

In the present paper the possibility to join aluminium alloys blanks and carbon fibre composites panels by self-piercing riveting operation is considered. In particular a few case studies were carried out at the varying of the process parameters. The effectiveness of the obtained joints was tested through tensile tests and through fatigue ones; what is more the process mechanics was highlighted through proper macro and micro observations of the transverse sections of the joints. The failure mechanics of the obtained joints were also considered in order to highlight the mechanisms which occur and determine the lost of the load carrying capability of the joints. Finally a numerical model of the process was carried out and the residual stress state after piercing was highlighted. The developed experiments and simulations demonstrated that self-piercing riveting can be effectively used to join carbon fiber composite panels and aluminum blanks.     

Microstructure, tensile deformation and fracture behaviour of aluminium alloy 7055

The microstructure, tensile deformation and fracture behaviour of aluminium alloy 7055 were studied. Detailed optical and electron microscopy observations were made to analyse the as-received microstructure of the alloy. Detailed transmission electron microscopy observations revealed the principal strengthening precipitates to be the hexagonal disc-shaped η′ phase of size 2 mm×20 mm and fully coherent with the aluminium alloy matrix, the presence of spheroidal dispersoids, equilibrium grain-boundary η precipitates and narrow precipitate-free zones adjacent to grain-boundary regions. It is shown that microstructural characteristics have a profound influence on tensile deformation and fracture behaviour. Tensile test results reveal the alloy to have uniform strength and ductility in the longitudinal and transverse orientations. Strength marginally decreased with an increase in test temperature but with a concomitant improvement in elongation and reduction in area. No change in macroscopic fracture mode was observed with sample orientation. Fracture, on a microscopic scale, was predominantly ductile comprising microvoid nucleation, growth and coalescence. The tensile deformation and fracture process are discussed in the light of the competing influences of intrinsic microstructural effects, matrix deformation characteristics, test temperature and grain-boundary failure. This revised version was published online in November 2006 with corrections to the Cover Date.     

铝合金自冲铆接微动行为研究

选择两组不同厚度铝合金板制成SA15和SA25自冲铆接头并进行疲劳实验,用威布尔分布验证实验数据有效性,用疲劳三参数经验公式拟合接头S-N曲线并对比分析接头疲劳性能,用扫描电子显微镜研究接头微动行为以探索疲劳失效与微动磨损的关系。实验数据服从威布尔分布,表明数据可靠。三参数经验公式较好地拟合接头S-N曲线,SA25接头疲劳性能优于SA15接头,且在中低疲劳寿命区域SA25接头疲劳性能的优越性更为突出。SA15接头失效于上板靠铆钉头,而SA25接头失效于下板沿纽扣。SA15接头在铆接区两板间微动磨损较严重,SA25接头在下板与铆钉腿部微动磨损较严重。疲劳失效部位与微动磨损剧烈部位重合,表明微动磨损是导致接头疲劳失效的重要因素之一。     

Influence of rolling direction on strength and ductility of aluminium and aluminium alloys produced by accumulative roll bonding

Sheets from commercial purity aluminium AA1050 and aluminium alloy AA6016 were processed by accumulative roll bonding to obtain an ultrafine-grained microstructure. The accumulative roll bonded samples showed a significant increase in specific strength paired with high ductility. Despite a strongly elongated grain structure, tensile testing of samples oriented 45° to the rolling direction revealed considerable improvement in elongation to failure compared to the samples oriented parallel to the rolling direction. From hydraulic bulge tests, it was observed that the accumulative roll bonded samples reached higher burst pressures and slightly lower equivalent strains in comparison to the as-received conventionally grain-sized samples. This behaviour reflects the extraordinary mechanical properties of the ultrafine-grained materials and indicates promising metal sheet formability.     

Influence of pre-compression on the ductility of AA6xxx aluminium alloys

Reversed loading experiments were conducted to study the influence of pre-compression on the ductility of three aluminium alloys. Diabolo-shaped specimens were machined from extruded profiles along the transverse direction, and heat treated to peak strength (T6 temper). The specimens were subjected to five different levels of pre-compression (0, 10, 20, 30, 40%), i.e., the specimens were first compressed to a prescribed strain and then pulled to fracture in tension. Using a laser-based measuring system, the minimum diameter in the extrusion direction and thickness direction were continuously measured during the tests until fracture. The three aluminium alloys AA6060, AA6082.25 and AA6082.50 had different grain structure and texture. The AA6060 and AA6082.50 alloys had recrystallized grain structure with equi-axed grains and large elongated grains, respectively. The AA6082.25 alloy had a non-recrystallized, fibrous grain structure. It was found that pre-compression has a marked influence on the ductility of the aluminium alloys, which depends on the microstructure and strength of the alloy. Using the compressed configuration as the reference configuration, the relative failure strain could be calculated. For the AA6060 alloy, the relative failure strain increased for increasing pre-compression, and was approximately doubled for 40% pre-compression compared to pure tension. For the AA6082.25 alloy, a slight increase in the relative failure strain was observed for increasing pre-compression, while for the AA6082.50 alloy the relative failure strain was low and approximately constant for different levels of pre-compression.