Influence of grain orientations on the initiation of fatigue damage in an Al–Li alloy

The variation in microstructure and texture in a rectangular bar extruded from a billet of spray-cast 8090 Al–Li alloy has been examined. The fine grain size of the as sprayed billet and the moderate extrusion ratio (≈ 25 : 1) were seen to cause geometric dynamic recrystallization (GDR) in regions of higher strain towards the edge of the bar. The grain morphology varied from the expected elongated grains at the centre of the bar to equiaxed grains where GDR occurred at the bar edges. A <111> + <100> double fibre texture, significantly distorted towards rolling components and varying through the bar thickness, was found using electron backscatter diffraction. Fatigue resulted in a high density of short secondary cracks, many of which had arrested at grain boundaries. The cracks preferentially nucleated in grains from the <100> fibre texture corresponding to high Schmid factors.     

Rate-dependent transient extrusion

Plane strain slip line field solutions are developed for deformation at the edge of a cylindrical billet when this is backward extruded to form a thin-walled cup. The extrusion pressure for a perfectly plastic material is then derived assuming homogeneous compression in the centre of the billet. A geometric factor, derived from the slip line field, is identified which permits the extrusion pressure to be determined for a nonlinear viscous material.Theoretical results agree well with recent experimental data on back extrusion of highly rate-sensitive superplastic alloys. It is shown that an apparent steady state exists in the process and that extrusion pressure increases with ram speed with a rate index approximately equal to that of the material. The punch profile shape has a greater effect on extrusion pressure for viscous materials than for perfectly plastic ones. The method of analysis appears to have a generality beyond the particular process considered and may be used to optimize extrusion equipment designs.     

The effect of billets extruded by a curved and flat-face die on the semisolid characteristics and tensile properties of thixoformed products

In this study, A356 aluminum billets in different extruded states are used as feedstock for the thixoforming. The extrusion billets were fabricated by a hot extrusion process through a flat-face and a curved die. After the induction reheating of the extrusion billets into a semisolid state, the microstructural evolution was thoroughly investigated. For the extrusion alloy by the flat-face die, there was a large variation in the average grain size (20 %) and the mean roundness (17 %) of equiaxed α-Al grains. This, together with evidence of elongated grains in the interior regions of the billet, indicated that a noticeably nonuniform globular microstructure had been obtained. In contrast, for the extrusion alloy through the curved die, the obtained globular microstructure was more uniform. There were slight variations of 5 % and 7 % in the average grain size and the mean roundness, respectively. By using the extrusion billets, some parts fabricated via the thixoforming process those underwent T6 heat treatment. The tensile test results for the fabricated parts showed that when the extrusion billet through the conventional flat-face die was used as the feedstock, there was a large scattering in the tensile properties throughout the part. In contrast, when the extruded billet through the curved die was used as the feedstock, limited variation was observed in the tensile properties.     

Evaluating the effects of process parameters on maximum extrusion pressure using a new artificial neural network-based (ANN-based) partial-modeling technique

Metal extrusion process accounts for the production of the majority of industrial and domestic aluminum sections. A major limitation to the success of any extrusion operation is the capability of the particular extrusion press to meet the maximum pressure requirements for that operation. In the present work, the effects of industrial extrusion process parameters and their interactions on the resulting maximum extrusion pressure, of an industrially extruded aluminum alloy, have been studied using a newly devised ANN-based partial modeling technique. Two operating parameters (initial billet temperature and ram speed) and three geometrical parameters (extrusion ratio, profile average thickness, and number of die cavities) were investigated. The main objective for developing this modeling technique is to overcome the limitations of presently available statistical modeling tools, as foreseen by the modeling needs for a complex thermo-mechanical process such as extrusion. The main present limitations are accounting for non-linearity in the process behavior, incorporating interaction effects and a meaningful determination of the highly significant process parameters and/or interactions. These three features have been, collectively, incorporated into the present model by means of combining statistical analysis of variance into ANN and by using a partial sum of squares analysis, which we propose to call the “present factor analysis.” Normal linear regression has been also employed for comparison purposes. According to the present model, maximum extrusion pressure has shown various degree of non-linearity in behavior with respect to the different process parameters and their significant interactions. It has been found that variations in the maximum extrusion pressure are mainly a function of initial billet temperature and its interactions with other process parameters, especially the ram speed. The present ANN-based model has shown superior prediction capabilities compared to the linear model with a marginal overall prediction error value of ±2.5 %.     

Microstructure analysis of aluminum extrusion: grain size distribution in AA6060, AA6082 and AA7075 alloys

Microstructure and material flow of aluminum alloys have a significant influence on the mechanical properties and surface quality. In extrusion of aluminum billets at high temperatures the microstructure is dependent on the alloy and the forming and temperature history. A prediction of grain size and precipitation is of increasing importance in order to design the process by adjustment of parameters such as punch speed, temperatures, and quenching. To give references for microstructure prediction based on material flow, and with it strain and strain rate history, this paper deals with the microstructure during the extrusion process of AA6060, AA6082, and AA7075 alloys. Billets have been partly extruded to axisymmetric round profiles and the microstructure of the press rests consisting of the billet rests in container and die has been considered. Furthermore, these rests have been analyzed to show the material flow, dynamic and static recrystallization based on macro etchings and visible microstructure under different conditions, e.g. as in the area of high strain rate near the container wall, or in dead zones [1]. To allow an accurate simulation of the extrusion process, punch force and temperature conditions during the tests have been measured and are presented in this paper, too.     

Deformation behavior of commercial Mg-Al-Zn-Mn type alloys under a hydrostatic extrusion process at elevated temperatures

This paper presents the deformation behavior of commercial Mg-Al-Zn-Mn type alloys during hydrostatic extrusion process at elevated temperatures. In the current study commercial Mg-Al-Zn-Mn type alloys with different Al contents were subjected to hydrostatic extrusion process at a range of temperatures and at ram speeds of 4.5, 10 and 17 mm/sec. Under the hydrostatic condition at 518K, the alloy with Al contents of 2.9 wt% was successfully extruded at all applied speeds. The alloys with Al content of 5.89 and 7.86 wt% were successful up to 10mm/sec, and finally extrusion of alloy with Al content 8.46wt% was successful only at 4.5 mm/sec. These results show that the deformation limit in the Mg alloys in terms of extrusion speed greatly extended to higher value in the proximity of lower Al content. It is presumed that deformation becomes harder as Al content increases because of strengthening mechanism by solute drag to increase of supersaturated Mg₁₇ Al₁₂ precipitates. Also, microstructures of cast and extruded Mg alloys were compared. Defect-wide microstructure of cast alloy completely evolved into dense and homogeneous microstructure with equiaxed grains.     

Portholes-equal channel angular pressing: novel technique for extrusion of 6061 aluminum alloy tube by subsize billet

Wrought 6061 Al alloy exhibits the prospective applications in the form of tube extrusions. In this study, billets of 6061 Al alloy were extruded under optimized conditions by a novel technique namely portholes-equal channel angular pressing (P-ECAP) extrusion. This technique is different from the conventional extrusion as the dimension of the product is greater than that of the billet. The extruded tube produced by the method was characterized for their microstructure as well as for their physical and mechanical properties. The tube that was fabricated using P-ECAP die showed significant refinement in microstructure with improved mechanical properties outside the seam joint portion. However, the extrusion loads using porthole die were less compared to that in the conventional method by using columniform billet because of the decrease in billet dimension or extrusion ratio. Furthermore, microstructure at seam joints of 6061 Al alloy extrusion was discussed in detail in this study. Thus, the P-ECAP technique has significant potential for substantial energy saving.     

Optimization of an aluminum profile extrusion process based on Taguchi’s method with S/N analysis

Taguchi’s design of experiment and numerical simulation were applied in the optimization of an aluminum profile extrusion process. By means of HyperXtrude, the extrusion process was simulated and the effects of process parameters on the uniformity of metal flow and on the extrusion force were investigated with the signal to noise ratio and the analysis of variance. Through analysis, the optimum combination of process parameters for uniform flow velocity distribution was obtained, with the billet diameter of 170?mm, ram speed of 2.2?mm/s, die temperature of 465°C, billet preheated temperature of 480°C, and container temperature of 425°C. Compared with the initial process parameters, the velocity relative difference in the cross-section of extrudate was decreased from 2.81% to 1.39%. In the same way, the optimum process parameters for minimum required extrusion force were gained, with the billet diameter of 165?mm, ram speed of 0.4?mm/s, die temperature of 475°C, billet preheated temperature of 495°C, and container temperature of 445°C. A 24.7% decrease of required extrusion force with optimum process parameters was realized. Through the optimization analysis in this study, the extrusion performance has been greatly improved. Finally, the numerical results were validated by practical experiments, and the comparison showed that the optimization strategy developed in this work could provide the effective guidance for practical production.     

An investigation of the tribological interaction between die damage and billet deformation during MMC extrusion

This study deals with material flow behaviour during the extrusion process of a metal-matrix composite (MMC), and the effects of this behaviour on the damage to die flat surfaces. AA 6063 aluminium matrix composite billets reinforced with SiC particles (167 μm) were prepared using the stir-casting method for extrusion. Extrusion of the MMC billets were conducted at 500 °C with a ram speed of 2 mm s⁻¹ and an extrusion ratio of 25:1 under laboratory conditions. The extrusion die with two different channel profiles was manufactured from AISI H13 steel that was hardened, tempered and grounded. The flow patterns of the deformed billet during the MMC extrusion determine the positions of the SiC particles in the deformation zone. While some of the SiC particles flow within the deformed material, some flow at the deformed billet surface; these SiC particles play the most important role in the damage mechanism of the die-bearing surface and the geometry of the dead metal zone (DMZ). The possible damage to the die-bearing surfaces is severe at the entrance of the die bearing. On the other hand, some SiC particles are broken in this zone due to the severe deformation stress of the MMC billet.     

挤压态镁合金AZ80动态再结晶机理及性能研究

探讨了挤压态镁合金AZ80的显微组织及动态再结晶机理。利用扫描电镜分析了材料的拉伸性能,并采用T6工艺研究了其热处理性能。结果表明：镁合金AZ80挤压后,出现细小的动态再结晶晶粒,其动态再结晶的机制属于连续动态再结晶;挤压后,材料的强化机制主要是晶粒细化作用。镁合金经过固溶处理后,β-Mg17Al12相已全部溶解到了α-Mg基体中,形成了过饱和的α-Mg固溶体,随着时效温度的升高,β-Mg17Al12相析出机理为从不连续析出到连续析出。     

A computational study of die geometry and processing conditions effects on equal channel angular extrusion of a polymer

Equal channel angular extrusion (ECAE) is an efficient process to obtain enhanced microstructures via super-plastic deformation. In view of its optimisation, it is of prime importance to assess the relationships between processing conditions and material flow. More precisely, detailed knowledge of the plastic strain distribution in the extruded material in relation to the ECAE processing variables is required. The key parameters of the ECAE process are primarily die geometry, ram speed, extrusion temperature, use of back-pressure, number of extrusion sequences and processing route (e.g. rotation of the sample between successive passes). A numerical investigation was achieved to check out the influence of these parameters on the homogeneity of plastic strain distribution in the case of a conventional thermoplastic polymer. Material parameters of a phenomenological elastic viscoplastic model were deduced from compressive deformation tests at different temperatures and strain rates on high-density polyethylene (HDPE). Recommendations on tool geometry and processing conditions can then be provided, according to the numerical results.It was found that optimum ECAE die geometry is strongly material dependent. The application of a back-pressure significantly contributes to reduce the corner gap and consequently promotes the homogeneity of the plastic strain field. A slight sensitivity of plastic strain to ram speed and friction conditions was pointed out. The extrusion temperature strongly influences the magnitude of the plastic strain and has a slight effect on its homogeneity. The number of passes has a significant effect on the magnitude of the plastic strain but has a negligible influence beyond a certain temperature. The extruded material reaches a stationary strain state after few passes. The homogeneity of the plastic strain field is strongly affected by the processing route.     

A numerical model for simulating temperature and speed effects in hot extrusion of rod

In this paper, a two-dimensional numerical model with upper-bound coupled thermal analysis has been developed. The model is capable of simulating the hot rod extrusion process with variable ram speeds. The temperature distributions and the speed effects in hot extrusion are predicted in detail by the proposed numerical model. A generalized kinematically-admissible velocity field without velocity discontinuity is adopted. The temperatures are calculated by considering simultaneously the heat generation due to deformation and friction and heat transfer. A finite-difference method with an implicit time integration scheme is utilized to solve the two-dimensional heat conduction problem. Two mathematical models for variable ram speed profiles are proposd. Ram speed profiles satisfying the exit temperature and the load requirements are obtained. The proposed numerical simulation has been demonstrated to be a powerful tool for the design of hot extrusion processes.     

Forward extrusion through steadily rotating conical dies. Part I: experiments

Forward extrusion of a cylindrical rod from a round billet was carried out through steadily rotating conical dies. Die rotation was shown to decrease the extrusion load and impose a twist shear strain into the partially extruded billet. The material twisting occurred both inside the container and inside the convergent die. However, not all the rotary work was transferred into shearing the bulk of the material, and this led to circumferential slippage at the rotating tool/material interface. The twisting degree on the outer surface of the material is quantified by a simple measurement technique. The influences of various process parameters on the extrusion load are also studied, including die semi-cone angle, die rotating speed and lubrication condition.     

Forward extrusion through steadily rotating conical dies. Part II: theoretical analysis

Forward extrusion of a cylindrical rod from a round billet was carried out through steadily rotating conical dies. Die rotation was shown to decrease the extrusion load and impose a twist shear strain into the partially extruded billet. The material twisting occurred both inside the container and inside the convergent die. However, not all the rotary work was transferred into shearing the bulk of the material, and this led to circumferential slippage at the rotating tool/material interface. The twisting degree on the outer surface of the material is quantified by a simple measurement technique. The influences of various process parameters on the extrusion load are also studied, including die semi-cone angle, die rotating speed and lubrication condition.     

马氏体组织Ti-6Al-4V钛合金多向等温锻造组织演变及力学性能强化研究

将铸态Ti-6Al-4V钛合金经过β相区热处理水淬之后获得马氏体组织,经过两步多向等温锻造之后获得了平均晶粒尺寸为1.5 μm的均匀等轴细晶组织,其室温拉伸屈服强度为906 MPa,抗拉强度为954 MPa,伸长率为16.7%,相比铸态Ti-6Al-4V钛合金,其室温力学性能得到了极大提升。研究表明,获得马氏体组织对钛合金晶粒细化有着巨大促进作用。第一步等温锻造之后的钛合金坯料组织并不均匀,存在变形区和“变形死区”,在变形区域内,心部位置应变量最大,组织细化最为明显,从心部到两端应变量逐渐减小,片层组织变形量相应减小;经过90°换向后的第二步等温锻造之后,钛合金坯料组织内的片层组织基本全部细化,形成了均匀的等轴晶组织,从心部到两端,随着应变量的减小,晶粒取向变化相应减小。     

Surface permeation and die design during rod extrusion processes

During a rod extrusion process, oxidation and segregation layers on the billet surface can be drawn into the billet and become part of the product. The surface-permeated portion has to be severed, which affects the productivity of the extrusion process. This study uses finite element analysis to investigate the permeation mechanism of the oxidation and segregation layers on the billet surface. The study presents a discussion of the effects of various extrusion conditions, such as the extrusion ratio, inclination angle, billet length, and thickness of the oxidation layer, on the critical ram stroke length and on the length of the product with surface permeation defects. Optimal inclination angles for optimal product ratios under various extrusion ratios are determined. Finally, extrusion experiments with aluminum rods are conducted, and comparisons of the lengths of the oxidation layers in the products are made to validate the proposed analytical model.     

The influence of temperature and preheating time in extrudate quality of solid-state recycled aluminum

Recovering waste metal without the need for remelting in solid-state recycling of metal chips can create green production. The overall process of solid-state recycling should be run at the lowest possible cost to remain competitive. High temperature and prolonged preheating time for billet’s homogenization in hot extrusion to consolidate the chips conflicts with the aim of minimizing energy usage. Therefore, optimizing the effect of preheating temperature and time prior to hot extrusion is important. This study investigates the effects of preheating temperature and preheating time on the extrudates’ quality. Milling chips of AA6061 were cold compacted and hot extruded through a flat-face die using preheating temperatures of 400, 500, and 550 °C for 1–6 h of preheating time. The mechanical and physical properties and microstructure of the extruded profiles were compared. The results revealed that higher acceptable strength and ductility were obtained at 500 °C with 2 h of preheating time. On top of that, temperature increase was the main criterion that results in the highest tensile strength; nevertheless, it was subjected to trade-off in ductility. The profile extruded at 500 and 550 °C had gained a close tensile strength. The study includes the prediction of the chip’s welding quality through the damage evolution on the extrudate’s structure. It was implemented with the help of DEFORMTM 3D finite element method (FEM) software, and the normalized Cockcroft and Latham (C & L) fracture criterion was chosen. The results of the simulations were compared and validated by the experimental results.     

铝型材挤压成型数值模拟及优化设计

应用有限元法,通过对AA6063铝合金方管挤出过程进行数值模拟,优化了分流组合模的工作带,以模孔出口处型材挤出速度的流速均方差为目标,使模孔出口处型材挤出速度均匀;同时通过对影响型材焊合品质的3个重要工艺参数:挤压速度、模具预热温度和坯料预热温度进行正交试验,以焊合面上的压力大小作为评定型材焊合品质好坏的标准,获得了AA6063铝型材挤压的最佳工艺参数组合。     

The effect of microstructure on creep behavior of a powder metallurgy (PM) beta gamma alloy

Pre-alloyed beta gamma titanium aluminide powder with a nominal composition of TiAl-2Nb-2Mo (G2) is consolidated by hot isostatic pressing. After consolidation, a step cooled heat treatment is performed to homogenize the material and produce a fully lamellar microstructure. Various aging heat treatments are then performed to form interfacial beta phase precipitates along lamellar interfaces. The step cooled heat treatment produces a relatively fine microstructure with an average lamellar grain size of 40 ??m. The aging heat treatments generate beta phase precipitates along lamellar grain boundaries as well as along lamellar interfaces, and result in limited lamellar degradation and grain growth. However, coarse intergranular grains consisting of beta and gamma grains form during aging. Constant load tensile creep tests are performed on step cooled heat treated and aged specimens. Primary creep resistance, generally, improves with aging time, even with interfacial precipitation, and the limited lamellar degradation occurs with aging. However, total creep life of aged samples decreases with aging time. The microstructures of the tested specimens are characterized and related to the creep behaviour of the TiAl-2Nb-2Mo alloy in the un-aged and aged conditions.     

铁、镍对2618合金挤压前后组织及性能的影响

研究了铁、镍含量对2618合金挤压前后及挤压加淬火时效后组织与性能的影响。结果表明,当铁、镍含量分别增加至1．4％和1．45％时,铸态组织中出现了粗大的针片状Al9FeNi相,挤压后破碎为粗大颗粒相;增加铁、镍含量对合金时效后的晶粒度没有影响,且降低2618合金的室温力学性能,但能提高合金300℃强度。