Dynamic recrystallization in adiabatic shear band in α-titanium

The microstructure and microtexture in adiabatic shear bands (ASBs) on the titanium side of the titanium/mild steel explosive cladding interface are investigated by means of OM, SEM/EBSD and TEM. The highly elongated subgrains and fine equiaxed grains with low dislocation density are observed in ASBs. Recrystallization microtextures (28°, 54°, 0°), (60°, 90°, 0°) and (28°, 34°, 30°) are formed within ASBs. According to the misorientation distribution, the grain boundaries in ASBs are geometrical necessary boundaries (GNBs) with high-angles. The temperature in the ASBs is estimated to be about 776∼1142 K (0.4∼0.6 T_m). Based on the rotational dynamic recrystallization (RDR) mechanism, the fine equiaxed grains in the core of ASBs are formed during the deformation processing. These results indicate that the dynamic recrystallization take place in ASBs.     

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.     

Microstructure evolution in adiabatic shear band in α-titanium

The microstructure and microtexture of adiabatic shear bands (ASBs) on the titanium side of a titanium/mild steel explosive cladding interface are investigated by means of optical microscopy (OM), scanning electron microscopy/electron back-scattered diffraction (SEM/EBSD) and transmission electron microscopy (TEM). Highly elongated subgrains and fine equiaxed grains with low dislocation density are observed in the ASBs. Recrystallization microtextures (28°, 54°, 0°), (60°, 90°, 0°) and (28°, 34°, 30°) are formed within ASBs. The grain boundaries within ASBs are geometrical necessary boundaries (GNBs) with high-angles. Based on the relations between temperature and the engineering shear strain, the temperature in the ASBs is estimated to be about 776–1142 K (0.4–0.6 T _m). The rotation dynamic recrystallization (RDR) mechanism is employed to describe the kinetics of the nano-grains’ formation and the recrystallized process within ASBs. The small grains within ASBs are formed during the deformation and do not undergo significant growth by grain boundary migration after deformation.     

Adiabatic Shear Bands in 30CrNi3MoV Structural Steel Induced during High Speed Cutting

The width and spacing of adiabatic shear bands (ASBs) in the serrated chips generated during high speed orthogonal cutting of 30CrNi3MoV structurai steel were measured by opticai microscopy (OM), the temperature rise in the shear band was estimated. The microstructures of the ASBs were also characterized by SEM and TEM. The results show that the width and spacing of ASBs decrease with the increase of the cutting speed. The further observations show that the microstructure between the matrix and the center of the ASB gradually changes, and that the martensitic phase transformation, carbide precipitation and recrystallization may occur in the ASB.     

Adiabatic Shear Bands in 30CrNi¬3MoV Structural Steel Induced during High Speed Cutting

The width and spacing of adiabatic shear bands (ASBs) in the serrated chips generated during high speed orthogonal cutting of 30CrNi¬3MoV structural steel were measured by optical microscopy (OM), the temperature rise in the shear band was estimated. The microstructures of the ASBs were also characterized by SEM and TEM. The results show that the width and spacing of ASBs decrease with the increase of the cutting speed. The further observations show that the microstructure between the matrix and the center of the ASB gradually changes, and that the martensitic phase transformation, carbide precipitation and recrystallization may occur in the ASB.     

Forming tubular hexahedral screws—Process development by means of a combined finite element-boundary element approach

The aim of the present paper is twofold: first, to propose a new forming process that is capable of producing hexahedral heads with washers in tubular screws and, second, to present a numerical approach to solve the plastic deformation of the tubular preforms and the elastic deformation of the dies.The methodology draws from independent determination of mechanical properties and fabrication of prototypes with a forming tool designed and fabricated by the authors to the development of a direct boundary integral element formulation for solving the elastic deformation of the dies inside an existing in-house finite element computer program.The performance of the proposed forming process is assessed by experimentation and results and observations are explained in the light of the proposed numerical approach based on the combination of the finite element (FE) with the boundary element (BE) methods.Because project and design of tooling is often only dependent on boundary solutions for the elastic deformation of dies and for the contact between the workpiece and the dies, the proposed FE-BE approach seems as a cost-effective methodology that avoids discretization of dies by finite elements, reduces the overall size and improves the performance of the resulting computer models.     

Microstructure and mechanical performance of metal-composite hybrid joints produced by FricRiveting

The mechanical performance and microstructure of friction riveted metallic-insert joints made of polyether ether ketone composite reinforced with 30% short carbon fibers and titanium grade 3 was studied. The metallic-insert joints reached a maximal pull-out tensile force of 10.6 kN, which corresponds to 100% of the titanium base material strength. It was shown the pull-out force increased as the rivet tip widened. Frictional heat during the process was mainly generated by the friction between the tip of the rivet and the composite substrate in the friction zone. Microstructural analyses of the metallic part of the joint revealed the presence of different microstructural zones: a friction zone, and two thermomechanically affected zones 1 and 2. Based on the composite morphology, the composite part of the joint was categorized into three different zones: the stir zone, a thermomechanically affected zone and a heat-affected zone. A study of the material flow showed that the flow of the composite was strongly affected by the rotation and axial movement of the rivet.     

The driven rivet head dimensions as an indication of the fatigue performance of aircraft lap joints

Solid aluminium alloy rivets are used in joints of aircraft structures. Riveting implies a squeezing process of the rivet stem with large plastic deformations to form the driven rivet head. The dimensions of the driven rivet head (diameter D and height H) depend on the applied squeeze load F_sq. High squeeze loads are beneficial for the fatigue properties of riveted joints. The present investigation is focused on the relation between F_sq and the rivet head dimensions during controlled squeezing. Measurements of D or H will then allow estimation of the applied squeeze load for quality control of the riveting process with respect to the fatigue properties of the joint. Extensive test series were carried out with five rivet materials, three rivet diameters and two sheet materials (2024-T3 and Glare). An equation was derived for F_sq(D) and F_sq(H) based on assuming a zero volume change of the rivet during plastic deformation. The two material constants in the equations were empirically determined for the rivet materials. A satisfactory correlation was obtained.     

连续变截面直接挤压成形技术研究

实现晶粒细化的传统塑性加工工艺所需工序多且要求较高,为了解决这些问题,本文提出了一种能够实现金属材料制备及成形一体化的挤压成形新工艺——连续变截面直接挤压成形技术.对不同工艺条件的连续变截面挤压成形过程的数值模拟研究表明:当模角由90°增大至140°时,塑性区范围明显扩大,金属流动均匀性随之提高;模口处轴向拉应力数值减小了20%,降低了表面产生开裂的可能性;模角为90°时变截面型腔处存在死区缺陷,且流线折叠倾向较严重,模角为140°时成形过程中坯料与型腔间易出现空隙.分析认为,本文条件下模角为120°的情况比较理想.     

Numerical investigation of an arc inlet structure extrusion die for large hollow sections

This paper aims to improve the working life of extrusion dies by optimal structure design, which plays an important role in mass production. First, an arc-shaped inlet die structure for an aluminum large-hollow-section profile was developed. Second, a three-dimensional finite-element model of the porthole extrusion process was established using an arbitrary Lagrangian-Eulerian method. Third, the comparison of the formability was analyzed and discussed, including the diversity of extrusion forces and uniformity properties between the proposed design and two traditional design schemes using the same extrusion process. A group of square-profile extrusion dies was used to set up a L16_4_3 orthogonal experimental scheme, considering the side length of profiles, L, with four levels, 110, 100, 90, and 80 mm; inlet angles, α, of the porthole bridge of 0°, 10°, 20°, and 30°; and profile wall thicknesses, t, of 1, 1.5, 2, and 2.5 mm. The results of the orthogonal tests were similar to those of the actual production die model. Two different analysis models reached the same conclusion: the inlet angle or the arc inlet structure has a small effect on the metal flow and the forming distribution, but the arc inlet structure can alleviate the stress load of the dies. The die testing and production validation results indicate that the novel structural design of the arc inlet die will have a long working life.