异形管材分流模挤压焊合过程金属流变及模具受力的模拟分析

针对采用有限元模拟时焊合面不能设置为刚性对称面的空心型材,其焊合过程不能模拟计算的问题,提出了基于有限元法结合逆向工程技术的焊合面网格重构技术的解决方法,并采用该方法对异形管材分流模挤压的焊合过程进行了模拟分析.研究结果表明:当挤压行程为33.1 mm时完成了焊合,焊合温度范围477~496℃,焊合室内静水压力为169~3 463 MPa,满足焊合要求;分流桥的焊合角部位所受应力最大,约为205 MPa.模具结构设计合理.     

Microstructural and mechanical characterisation of 7075 aluminium alloy consolidated from a premixed powder by cold compaction and hot extrusion

The present work concerns the processing of 7075 Al alloy by cold compaction and hot extrusion of a premixed powder. To this end, a premixed Al–Zn–Mg–Cu powder, Alumix 431D, was uniaxially cold pressed at 600 MPa into cylindrical compacts 25 mm in diameter and 15 mm thick. Subsequently, selected green compacts were subjected to either a delubrication or presintering heat treatment. Extrusion of the powder compacts was performed at 425 °C using an extrusion ratio of 25:1. No porosity was present in the microstructures of the extruded alloys. Heat treatment prior to extrusion had a great effect on the degree of alloy development in powder compacts and, as a direct consequence, remarkably affected the extrusion process and the as-extruded microstructures and mechanical properties of the processed materials. Hot extrusion caused banded structures for the alloys consolidated from the green and delubricated powder compacts. The alloy extruded from the presintered powder compact showed a fine, recrystallized microstructure which resulted in a superior combination of mechanical properties for the consolidated material.     

花键冷挤压用组合凹模内径收缩量的计算

为研究花键组合凹模压入预应力圈时内径的变化规律,实验研究了轴向压合量对凹模内径变化的影响规律.在理论分析的基础上,推导了组合凹模压合后凹模内径的计算公式,并通过实验进行了验证.研究表明:花键挤压用组合凹模压合时,大径比等直径的圆凹模内径收缩得慢,而小径则比等直径的圆凹模内径收缩得快.     

Fracture life prediction and sensitivity analysis for hollow extrusion dies

It has been reported in literature that extrusion dies most often fail by fatigue fracture. Experimental studies have shown that cracks pre‐exist in dies because of various factors including machining, heat treatment and surface hardening. High levels of repeated mechanical and thermal loads result in crack propagation leading to ultimate fracture failure. In an earlier work by the authors, a simplified approach of plate‐with‐edge‐crack was used to develop a fracture mechanics based fatigue life prediction model for tube dies. In the current work, extrusion die is modeled as a pressurized‐cylinder‐with‐internal‐crack, a more realistic approach for a hollow (tube) die. Stochastic nature of various fatigue‐related die parameters has been used to reflect their variability. Monte Carlo simulation has been performed to forecast fracture failure of extrusion dies under a given set of operating conditions and mechanical properties. Predicted mean‐time‐to‐failure is quite close to actual average extrusion die life data from the industry. Using tube die as a basis, fracture life of other hollow profiles can be estimated through their shape complexity values. Analysis has also been carried out to evaluate how sensitive fracture life of hollow extrusion dies is to material and process parameters. Major findings are that die life is highly sensitive to initial crack size, wall thickness, profile outer diameter and billet length; moderately sensitive to Paris constant and extrusion ratio; and only slightly sensitive to fracture toughness and ram speed. These results can contribute to a deeper understanding of the factors responsible for fracture failure of an extrusion die exposed to thermo‐mechanical fatigue environment.     

衔铁连续冷挤压模具失效分析及工艺改进

通过对衔铁连续冷挤压工艺进行试验,对挤压过程中模具失效进行了分析,并提出了合理的工艺改进,对薄壁、非对称零件连续冷挤压工艺设计及试验有参考作用。     

热处理对粉末热挤压法制备6061铝合金强度和塑性的影响

研究了单级固溶及峰值时效处理对粉末热挤压法制备6061铝合金显微组织及室温力学性能的影响,观察了合金挤压态、固溶态及时效处理后的显微组织,并对其力学性能进行了测试.结果表明:挤压态材料的晶粒均匀细小,基体合金中存在大量的第二相颗粒,主要为Mg2Si相;热挤压后的6061铝合金经固溶时效处理(530℃×1 h水冷+170℃×6 h)后,晶粒内部析出大量的β″(Mg5Si6)相,并伴有少量棒状的β’(Mg2Si)相析出,拉伸强度和延伸率分别为311 MPa和10%,相比挤压态铝合金,其拉伸强度提高了近160%.     

Effect of Dy and Nd on ZK10 alloy processed by hot extrusion

About 0.6%Dy and 0.2%Nd are simultaneously added to ZK10 alloy processed by hot extrusion, and their effects are investigated. Obvious refined grains and weakened texture are observed after addition of Dy and Nd. Significantly improved mechanical performance with increase of 16% in yield strength and of 114% in ductility is obtained compared with ZK10 alloy. The strengthening effect is attributed to the grain refinement and solid solution, and the enhanced ductility results from the refined structure and weakened texture. Better optimization in mechanical performance with enhanced strengthening is generated, compared with the alloy with individual RE element addition. Combined addition of trace Dy and Nd throws light on the wide-range production and application of extruded ZK10 alloy.     

Indigenization of high-strength,light-weight Al-Li alloys for aerospace

A research programme was initiated at the Defence Metallurgical Research Laboratory, Hyderabad, a decade ago for the indigenous development of Al-Li alloys in order to finally meet the requirements of the space and aircrft industries in the country. This paper describes the systematic studies carried out in the laboratory to overcome the initial difficulties in producing sound ingots, optimize the subsequent heat treatments and processing schedules, and to finally obtain reproducible microstructure and mechanical properties in the semi products (i.e. sheets and extrusions) developed. Laboratory-scale sheet and extrusion products meet tensile property specification of 8090 alloy. One of these semiproducts, i.e. round bar extrusion, is currently being supplied for the stallite programmes. Commercial-scale sheet products made in Russia under an Indo-Russian joint programme have been made available for the aircraft programme. Recent alloy development studies in the laboratory are discussed within the context of the present paper.     

In-process rheometry as a PAT tool for hot melt extrusion

Real time measurement of melt rheology has been investigated as a Process Analytical Technology (PAT) to monitor hot melt extrusion of an Active Pharmaceutical Ingredient (API) in a polymer matrix. A developmental API was melt mixed with a commercial copolymer using a heated twin screw extruder at different API loadings and set temperatures. The extruder was equipped with an instrumented rheological slit die which incorporated three pressure transducers flush mounted to the die surface. Pressure drop measurements within the die at a range of extrusion throughputs were used to calculate rheological parameters, such as shear viscosity and exit pressure, related to shear and elastic melt flow properties, respectively. Results showed that the melt exhibited shear thinning behavior whereby viscosity decreased with increasing flow rate. Increase in drug loading and set extrusion temperature resulted in a reduction in melt viscosity. Shear viscosity and exit pressure measurements were found to be sensitive to API loading. These findings suggest that this technique could be used as a simple tool to measure material attributes in-line, to build better overall process understanding for hot melt extrusion.     

Diffusion samples as a high-throughput screening method for alloy development

ABSTRACT

Diffusion couples of copper alloys are presented as screening tool for high-throughput alloy development. It allows estimating the precipitation hardening potential in multicomponent alloys of a vast number of compositions within one sample. The components were diffusion welded and annealed to form compositional gradients which were characterised through a grid of energy dispersive X-ray spectroscopy measurements. After splitting the different sample parts underwent different tempering treatments. Thereafter micro hardness grids matching the spectroscopy data were recorded and displayed in contour plots as a function of alloy composition that instantly show composition areas with hardening potential. A critical assessment of the reliability of the method is provided and checked for cu-Al/mn-Sn-ti systems against literature data.

This paper is part of a Thematic Issue on Copper and its Alloys.     

An analytical approach for simple shear extrusion process with a linear die profile

In the present work, for the first time, an analytical approach based on upper-bound theorem is proposed to analyze the simple shear extrusion process. In this regard new die parameter named maximum inclination angle is introduced. By this model, the power dissipated on all frictional and velocity discontinuity surfaces is determined and the total power is optimized for two types of die, fixed and movable inlet channel die. To check the validation of the upper-bound model, the process is simulated by the commercial finite element code, Deform-3D. To compare the theoretical results with experiments, a fixed inlet channel die and two dies with movable inlet channel are used to determine the processing force for different cross sections. The developed model predicts that the relative extrusion pressure increases with increasing the constant friction factor; also, for a given value of the constant friction factor and the maximum distortion angle, there is an optimized maximum inclination angle which minimizes the power. Comparing the fixed inlet channel die with the movable inlet channel one, it is seen that the optimized maximum inclination angle is higher in the FIC die.     

Constitutive base analysis of a 7075 aluminum alloy during hot compression testing

In the field of deformation process modeling, the constitutive equations may properly represent the flow behavior of the materials. In fact, these valuable relationships are used as a calculation basis to simulate the materials flow responses. Accordingly, in the present study a hot working constitutive base analysis has been conducted on a 7075 aluminum alloy. This has been performed using the stress–strain data obtained from isothermal hot compression tests at constant strain rates of 0.004, 0.04 and 0.4 s⁻¹ and deformation temperatures of 450, 500, 520, 550 and 580 °C up to a 40% height reduction of the specimen. A set of constitutive equations for 7075 Al alloy have been proposed employing an exponent-type equation. The related material constants (i.e., A, n and α) as well as the activation energy Q for each temperature regime have been determined. The correlation of flow stress to strain rate and temperature can be deduced from the proposed equations. Furthermore, a change in deformation mechanism has been realized in the semi-solid temperature range. This has been related to the onset of lubricated flow mechanism during processing.     

Failure analysis of NiTi wires used in medical applications

Superelastic nitinol (NiTi) wires have been fractured under various loading, temperature, and environmental conditions. The fracture surfaces of these NiTi specimens were then characterized using scanning electron microscopy (SEM) to provide a reference for future NiTi failure analysis investigations and to provide comparison with the existing literature. The results of this study indicate that NiTi fracture modes and morphologies are generally consistent with those for ductile metals, such as austenitic stainless steel, with one important exception: NiTi exhibits a unique compressive damage mechanism that occurs at high strains in bending. Compression-induced surface damage is observed at the compression-side of tight bends or kinks in NiTi wire. In these same samples, no cracking or surface damage is observed on the tensile side of the bend. Compressive damage begins as slip line formation due to plastic deformation, and at high strain levels, cracking is observed. These compression-damage induced cracks appear to initiate from slip lines and extend in a mode II/shear mechanism. Reprinted from Proc. of the Materials & Processes for Medical Devices Conference, St. Paul, MN, Aug 25–27, 2004, M. Helmus and D. Medlin, ed., ASM International, Materials Park, OH, May 2005, pp. 44–49.     

Dependencies of grain refinement on processing route and die angle in equal channel angular extrusion of bcc materials

The efficiency of grain refinement in equal channel angular extrusion of body-centered cubic (bcc) materials is investigated based on slip activities from crystal plasticity simulations, which account for both the macroscopic and crystallographic features of deformation. It is shown that the characteristics of slip activities, especially the relative contributions of slip systems newly activated or reversed at the transitions between successive passes, vary significantly with the processing routes (A, B and C) and die angles ( = 90° and 120°). The simulations assuming {1 1 0}111 slip suggest that routes B and A lead to the most significant contributions of newly activated slip systems and hence are most efficient for grain refinement with  = 90° and 120°, respectively. Further incorporation of {1 1 2}111 slip systems leads to the highest efficiency by route B for both die angles. These predictions are in partial agreement with experimental observations in the literature. Comparison of these results with those of face-centered cubic materials reveals the relevance of crystal structure and deformation mechanism during grain refinement.     

Microstructure of a recycled AA7050 alloy processed by spray forming followed by hot extrusion and rotary swaging

The aim of this study is to evaluate the microstructure of a recycled AA7050 alloy processed by spray forming followed by hot extrusion and swaging. Machining chips from an aircraft manufacturer were used as raw material. The microstructure was investigated by optical microscopy (MO) and scanning electronic microscopy (SEM). The spray formed deposit was homogenized at 743 K, 8 h, extruded at 693 K with a deformation rate of 0.1 s^–1 and an extrusion rate of 156:1, producing a bar with 8.0 mm in diameter. Due to a specific combination of high extrusion temperature and deformation rate, partial recrystallization occurred and different grain sizes were obtained through the extruded cross section. After extrusion, the 8.0 mm bar was processed by rotary swaging in several steps up to a 2.0 mm final diameter. The resulting microstructure revealed a cold worked structure, with no recrystallization. Tensile tests were performed in both cases and the slant type of fracture reveals that Portevin Le‐Chatelier effect took place in this alloy. Moreover, the efficacy of extrusion and rotary swaging parameters to reduce the porosity, intrinsic to the spray form process, was analyzed, as well the distribution of intermetallic particles.     

12 vol%SiCP/2024Al基复合材料热挤压过程有限元模拟与分析

基于MSC.Marc软件平台,建立了含SiC_P体积分数为12%的SiC_P/2024Al复合材料热挤压轴对称刚-塑性热力耦合有限元分析模型。利用该模型对复合材料的热挤压过程进行模拟,分析了热挤压过程中的载荷-行程曲线和材料流动状态,讨论了模具温度及挤压速度对挤压载荷的影响。模拟结果表明,该坯料在挤压比为30∶1、挤压温度为400～450℃、挤压速度为0.1～1.0 mm/s、挤压载荷为4.0 ×106～5.0 ×106N之间能够顺利挤出表面无缺陷的复合材料棒材。最后通过在 700 t水压机上采用相同工艺挤出高质量的复合材料棒材验证该工艺的可行性。     

Orientation dependencies of mechanical response, microstructure and texture evolution in hot compression of AZ31 magnesium alloy processed by equal channel angular extrusion

The anisotropic mechanical behavior during hot compression of an AZ31 Mg alloy processed by equal channel angular extrusion (ECAE) was evaluated and then discussed in correlation with the concurrent microstructure and texture evolution. The results revealed apparent orientation-dependencies in the mechanical responses, microstructure, and texture development in uniaxial compression along two perpendicular directions. Compression along the transverse direction (TD) led to a higher hardening rate, higher peak stress, and earlier softening than those obtained in compression along the extrusion direction (ED). This can be attributed to the differences in the initial textures prior to compression along the two directions, which led to a more significant contribution of tensile twinning at the early stage of straining and consequently more extensive dynamic recrystallization in loading along TD than along ED. These results suggest that the deformation behavior in compressive loading of the ECAE-processed Mg alloy is highly anisotropic, which needs to be taken into account in their applications.     

Application of X-ray microtomography to analysis of cavitation in AZ61 magnesium alloy during hot deformation

The efficiency of particles in acting as cavity formation sites during hot deformation was investigated for a fine-grained wrought magnesium-aluminium-zinc (AZ series) alloy using X-ray micro tomography. Two methodologies were developed to determine the particle/cavity association from 3-dimensional data, each clearly demonstrating that particles act as a major formation site for cavitation. The particles forming cavities were identified and characterised. It is shown that progressively smaller particles nucleate cavities as strain increases. This is due to concurrent grain growth which reduces the critical particle diameter for cavity nucleation during testing, leading to a continuous cavity nucleation. Particle agglomerates are shown to be particularly potent sites for cavity formation, leading to large and complex shaped cavities even if the individual particles within the agglomerate are below the critical particle diameter for cavity nucleation.     

Bio-corrosion of a magnesium alloy with different processing histories

High rates of degradation in corrosive media represent the Achilles heel of Mg alloys, which hinders their applications in various areas, particularly in prosthetics. We present an investigation of the degradation behaviour of magnesium alloy AZ31 in Hank's solution that simulates bodily fluids. The degradation rate is shown to be significantly reduced by grain refinement produced by mechanical processing. In particular, hot rolling does lead to a desirable retardation of degradation, while subsequent equal channel angular pressing does not result in any further reduction of degradation rate.