Superplastic forming characteristics of fine-grained 5083 aluminum

Superplastic forming characteristics of a fine-grained 5083 aluminum sheet have been investigated by means of gas-pressure forming of a rectangular pan. This part geometry lends itself to a simple representation in terms of nearly one-dimensional sheet stretching and permits reasonably rigorous control of strain rate throughout the forming cycle. This study followed a study of the uniaxial tensile properties carried out on this alloy. A two-stage forming cycle, which comprised a short, rapid prestraining stage followed by a stage of slower rate of superplastic straining, was used because the uniaxial tensile work showed enhancement of superplastic response of this alloy under this condition. The study examined the effect of process parameters such as initial gas pressurization rate, level of hydrostatic pressure, and lubricants on the thinning characteristics of the sheet, especially along the die entry radii. The gas pressure/time cycle was suitably modified to avoid premature sheet failure due to excessive sheet thinning or cavitation. Cavitation under the biaxial forming condition and the effect of hydrostatic pressure on cavitation suppression were evaluated. A defect-free pan with sharp corners was formed.     

空洞敏感材料超塑性胀形过程的有限元模拟

基于刚粘塑性有限元技术采用最大应变速率恒定的压力控制策略和超塑性成形空洞损伤演变模型对空洞敏感材料的超塑性胀形过程进行了数值模拟。分别以半球壳和圆筒形零件为例 ,给出了优化后自由胀形和充模胀形的加压曲线 ,预测了成形零件的壁厚分布及空洞体积损伤情况。本文分析结果对指导超塑性胀形工艺设计具有实际意义。     

Deformation characteristics and cavitation during multiaxial blow forming in superplastic 8090 alloy

This work examined the effect of multiaxial stress on deformation characteristics of a superplastic aluminum alloy 8090 by deforming the sheet into a die with a cylindrical cavity. Several interrupted tests were performed to bulge the sheets to various depths for different strain rates, the formed parts were utilized to evaluate the deformation status, thickness distribution, local strain states, and cavitation. It was found that evolution of cavity volume fraction with forming time could be related to the thinning behavior of the deformed sheet during forming. Decrease in cavity volume fraction at the central region was observed in the later stage of forming as the thickness of the deformed sheet remained constant for all test forming rates.     

Designing superplastic forming process of a developmental AA5456 using pneumatic bulge test experiments and FE-simulation

Relatively low tooling costs, high design complexity coupled with low forming speeds make the superplastic sheet metal forming process attractive, especially for smaller lot sizes. Due to the relatively small lot size, the effort and budget for designing superplastic forming processes is usually limited (Kappes and Liewald in J Mater Sci Eng B1:472?C478, 2011). For this reason the tool design and corresponding pressure profiles in superplastic forming processes are often based on trial and error (Franchitti et al. in 11th international Esaform conference on material forming, 2008; Barnes in J Mater Eng Perform 4:440?C454, 2007). Consequently a process chain should be established to design superplastic forming processes accurately and efficiently. This paper deals with the process chain to form an aluminium part superplastically. At the beginning of the process chain, there is a new, developmental aluminium alloy sheet (AA5456, s₀?=?1.6?mm) designed for superplastic forming supplied by Hydro Aluminium Rolled Products GmbH. The relevant material parameters of this sheet are then determined via pneumatic bulge testing with and without in situ measurement of strains. Using these experimentally determined parameters superplastic forming process can be simulated by FE modelling (PAM-STAMP 2G). Due to in situ measurement of strains during pneumatic bulging, the comparison of experiment and FE-simulation results over the whole pneumatic bulging process could be done. This comparison shows good correlation for the observed conditions. Furthermore a cylindrical cup was simulated, evaluated via determined isobar Superplastic Forming Limit Curve (at fracture) and finally formed by pneumatic bulging. Material characterisation of the bottom of this cup showed that excessive cavitation was observed as a result of the iron-silicon particles. Superplastic forming of a bracket usually formed out of AA5083 was also simulated using material parameters of AA5456. The simulation was able to show that this part is not able to be manufactured out of AA5456 under these forming conditions, which was confirmed by forming trials performed at ALU-SPF AG.     

MODELING OF SUPERPLASTIC FORMING PROCESS FOR ALUMINUM ALLOYS WITH STRAIN HARDENING EFFECT

ＭＯＤＥＬＩＮＧＯＦＳＵＰＥＲＰＬＡＳＴＩＣＦＯＲＭＩＮＧＰＲＯＣＥＳＳＦＯＲＡＬＵＭＩＮＵＭＡＬＬＯＹＳＷＩＴＨＳＴＲＡＩＮＨＡＲＤＥＮＩＮＧＥＦＦＥＣＴＹ．Ｎ．ＫｗｏｎａｎｄＹ．－Ｗ．Ｃｈａｎｇ（ＣｅｎｔｅｒｆｏｒＡｄｖａｎｃｅｄＡｅｒｏｓｐａｃ...     

钣金成形中润滑剂润滑效果试验研究

钣金成形中润滑状况是影响成形的一个重要因素,成形过程中局部润滑状况的改善可以提高成形性。通过杯形件拉深试验,在需要润滑的局部涂抹润滑剂,采用3种不同的成形性评价方法研究润滑剂的润滑效果。对试验中使用的4种润滑剂进行了润滑效果研究,结果表明,4种润滑剂的润滑效果(由好到差的顺序)为,聚四氟乙烯薄膜,肥皂,猪油,鸡油。     

Superplastic Forming of Triangular Channels with Sharp Radii

The superplastic forming process is used to form light weight components with complex features in one manufacturing step. However, the non-uniformity of the produced part thickness and the possibility of severe thinning are among its major disadvantages. The goal of this parametric study was to investigate feasible geometries for triangular channels to be manufactured by superplastic forming. The channels considered had sharp radii and served as secondary features extending along a circular path at the base of a shallow cup. An axisymmetric finite element model in ABAQUS? was used to simulate the forming process. Effects of the aspect ratios of both the cup and the triangular channel on the thickness distribution and the pressure profiles were investigated. An experimental setup was used for validating the simulation results for AA5083 at 500 °C.     

The Effects of Stress State and Cavitation on Deformation Stability During Superplastic Forming

The current available models describing superplastic deformation do not account for a number of important characteristics, leading to the current limited predictive capabilities of deformation and failure. In this work, the effects of cavitation and stress state on deformation stability during superplastic forming are investigated using Finite Element simulations. The simulations are performed using constant strain rate forming and using a proposed optimization approach based on a multiscale failure criterion that accounts for stress state, geometrical necking, and microstructural evolution including grain growth and cavitation. The simulations are conducted for the superplastic copper-based alloy Coronze-638 and the superplastic aluminum alloy Al-5083 which are known to develop significant cavitation during deformation. The results clearly show the importance of accounting for microstructural evolution during superplastic forming, especially when the state of stress is biaxial. Furthermore, the results highlight the effectiveness of the proposed optimization technique in reducing the forming time and maintaining the integrity of the formed parts. This article was presented at the AeroMat Conference, International Symposium on Superplasticity and Superplastic Forming (SPF) held in Seattle, WA, June 6-9, 2005.     

一种耦合晶粒长大的超塑性本构及数值模拟

本文基于弹粘塑性有限元技术 ,将经过修正的晶粒长大模型耦合到材料粘塑模型中 ,得到一个考虑晶粒行为的超塑材料本构模型。利用所建立的考虑晶粒效应的本构模型和经典的Backofen本构模型 ,分别对Ti 6Al 4V圆杯进行超塑性成形模拟 ,其厚度分布结果表明 ,新本构能很好地描述晶粒敏感型材料的超塑性能。     

Finite element modeling and optimization of superplastic forming using variable strain rate approach

Detailed finite element simulations were carried out to model and optimize the superplastic blow forming process using a microstructure-based constitutive model and a multiscale deformation stability criterion that accounts for both geometrical instabilities and microstructural features. Optimum strain rate forming paths were derived from the multiscale stability analysis and used to develop a variable strain rate forming control scheme. It is shown that the proposed optimization approach captures the characteristics of deformation and failure during superplastic forming and is capable of significantly reducing the forming time without compromising the uniformity of deformation. In addition, the effects of grain evolution and cavitation on the superplastic forming process were investigated, and the results clearly highlight the importance of accounting for these features to prevent premature failure. This paper was presented at the International Symposium on Superplasticity and Superplastic Forming sponsored by the Manufacturing Critical Sector at the ASM International AeroMat 2004 Conference and Exposition, June 8–9, 2004, in Seattle, WA. The symposium was organized by Daniel G. Sanders, The Boeing Company.     

超塑成形薄壁构件壁厚分布的预测

壁厚分布对薄壁构件的结构性能有重要影响。本文研究超塑成形件壁厚分布的预测技术,实现了超塑成形过程的有限元数值模拟的成形件厚度分布曲线的自动预测,以半球壳和矩形盒成形为例,为自由胀形和约束胀表两种情形形件厚度变化进行了分析,预测结果与实验数据吻合。     

On Practical Forming Limits in Superplastic Forming of Aluminum Sheet

In this paper, the superplastic forming (SPF) potential of two fine-grained 5083 aluminum alloys were studied under various stress states with the use of both high temperature tensile testing and pneumatic bulge testing. Experiments with the pneumatic bulge test were performed at temperatures ranging from 475 to 525 °C under three different strain paths ranging from equi-biaxial to approaching plane strain. The effects of temperature on total elongation, m-value, final thickness distribution, dome height, and cavitation were investigated for the case of uniaxial and equi-biaxial stretching. Increased temperature in bulge forming was found to improve the thickness distribution in the formed parts, but did not have a significant effect on dome height. The shape of the forming limit diagram (FLD) was found to be significantly different than that of FLDs commonly used in room temperature stamping. Results indicate that determination of forming limits in SPF cannot be represented with a simple FLD and additional metrics such as external thinning and internal cavitation need to be considered to determine a material’s SPF potential. This article was presented at Materials Science & Technology 2006, Innovations in Metal Forming symposium held in Cincinnati, OH, October 15-19, 2006.     

Superplastic sheet metal forming of a generalized cup part i: uniform thinning

Superplasticity is the phenomenon observed in certain materials that deform on the order of 300 to 500% under very low flow stress, high temperature, and fine grain structure. Superplastically formed parts find application as both structural and nonstructural components in simple and complex shapes. Mathematical models that describe the forming process with optimum strain rate and tool geometry as input and pressure-time and thickness as output are essential for successful forming. This article describes the deformation of a generalized cup assuming uniform thinning in the unsupported region. Closed form equations are developed relating process parameters like pressure-time loading and thickness distribution to the shape of the cup and material properties. The generalized cup formulation is applicable to the superplastic forming of domes, right circular cylinders, deep slanted cups, and cones.     

Investigations of ductile damage during the process chains of toothed functional components manufactured by sheet-bulk metal forming

Sheet-bulk metal forming processes combine conventional sheet forming processes with bulk forming of sheet semi-finished parts. In these processes the sheets undergo complex forming histories. Due to in- and out-of-plane material flow and large accumulated plastic strains, the conventional failure prediction methods for sheet metal forming such as forming limit curve fall short. As a remedy, damage models can be applied to model damage evolution during those processes. In this study, damage evolution during the production of two different toothed components from DC04 steel is investigated. In both setups, a deep drawn cup is upset to form a circumferential gearing. However, the two final products have different dimensions and forming histories. Due to combined deep drawing and upsetting processes, the material flow on the cup walls is three-dimensional and non-proportional. In this study, the numerical and experimental investigations for those parts are presented and compared. Damage evolution in the process chains is simulated with a Lemaitre damage criterion. Microstructural analysis by scanning electron microscopy is performed in the regions with high mechanical loading. It is observed that the evolution of voids in terms of void volume fraction is strongly dependent on the deformation path. The comparison of simulation results with microstructural data shows that the void volume fraction decreases in the upsetting stage after an initial increase in the drawing stage. Moreover, the concurrent numerical and microstructural analysis provides evidence that the void volume fraction decreases during compression in sheet-bulk metal forming.     

SPF model applications for aluminum

A closed-form, plane strain model (PS2) is compared with a finite element model (SUPFORM3) in analyzing the forming of rectangular pans from 2090-OE16 aluminum sheet. The errors introduced by improperly using a plane strain model are quantified. Some preliminary verification of SUPFORM3 is shown, and the effect of friction on the distribution of thickness in the superplastic forming (SPF) of a rectangular pan is presented. SUPFORM3 is used to predict thickness distribution of male formed access doors, selecting proper starting gauges, assessing producibility, estimating forming time, and developing pressure time cycles. SUPFORM3 will be used to form complex parts and flat-bottomed pans such that post-SPF properties samples can be cut from the flat areas of the flat-bottomed pans, yet have the same strain and strain rate history as the critical areas in the complex part. It is concluded that SUPFORM3 is a useful tool for predicting thickness distributions resulting from and pressurization schedules for superplastic forming.     

Grain structure and microtexture evolution during superplastic forming of a high strength AlZnMgCu alloy

Grain structure and microstructure evolution during superplastic forming were studied on an unrecrystallized sheet of a modified 7050 superplastic alloy. A SEM-based local orientation technique was used to cover a large number of (sub)grain boundaries in combination with other metallographic techniques. The gradual boundary misorientation and microtexture evolution during superplastic forming (SPF) confirmed that a continuous evolutionary process was occurring. There was no evidence of dynamic recrystallization at the stress maximum. The fraction of high angle boundaries increased rapidly once the mean misorientation reached a critical value. These and other results suggest that both grain boundary sliding (GBS) and dislocation slip were operative initially until the stress maximum was approached, beyond which GBS was predominant. The results of quantitative orientation distribution function (ODF) analyses suggest that grain rotation, which resulted in texture randomization, became important from slightly beyond the stress maximum through most of the stress-strain curve.     

快速超塑性的研究与应用

综述了高应变速率超塑材料种类、变形机理和应用技术的最新进展。高应变速率超塑材料主要是铝基复合材料及铝合金,最近,对镁合金、纳米材料、钛合金高应变速率超塑性能的研究也已开始。高应变速率超塑性在工业中的应用已经起步,例如快速超塑成形技术、一模多件技术等,可以实现中等批量、甚至大批量生产,但是主要集中在铝合金上。未来激光辅助超塑成形技术、电塑性辅助超塑成形技术值得期待。     

Pressure Profile Designing in Superplastic Forming Based on the Strain Rate and on Post-forming Properties

In this work, the feasibility of reducing the cycle time in superplastic forming through a selective approach in the algorithm that calculates the forming pressure profile was investigated. First, a 3D numerical model of the blow forming process is created. Then, the blank was partitioned in different characteristic areas according to their strain and strain rate histories. Thus, different pressure profiles were numerically calculated choosing different combinations of those partitions of the blank. Experimental trials were finally carried out in order to explore the potential reduction of the forming time that can be achieved through the described approach without affecting the post-forming properties of the formed specimens. Post-forming properties were measured in terms of thickness distribution, mean grain size, and cavitation effects along the formed sheet. In particular, experiments were performed both with the conventional approach (with the whole sheet being monitored) and considering only the area of the sheet that experiences the highest strain values at the end of the forming process. Results highlighted that this latter approach can efficiently reduce the cycle time.     

Superplastic Response of Continuously Cast AZ31B Magnesium Sheet Alloys

Magnesium sheet is typically produced for commercial applications with the traditional DC-ingot casting method. As a result of the hexagonal close-packed crystallographic structure in magnesium, multiple rolling passes and annealing steps are required to reduce the thickness of the ingots. Thus, high fabrication costs characterize the creation of magnesium sheet suitable for common forming operations. Recently, continuous casting (CC) technology, where molten metal is solidified directly into sheet form, has been applied to magnesium alloys; this method has shown the potential to significantly reduce the cost of fabricating magnesium sheet alloys. In order to understand the viability of the CC process, a study was conducted to investigate the superplastic potential of alloys produced by this method. This study focused on AZ31B Mg that was continuously-cast on twin-roll casters from three different suppliers. These three materials were compared with a production DC-cast AZ31B alloy in terms of microstructure, elevated-temperature tensile properties, and superplastic forming response. The data from this study found that microstructural features such as grain size and segregation can significantly affect the forming response. Additionally, the CC alloys can have equivalent or superior SPF response compared to DC-cast alloys, as demonstrated in both elevated temperature tensile tests and superplastic forming trials using a rectangular pan die.     

大型双曲率非等厚TC4钛合金壁板整体SPF/DB成形工艺及优化

针对大型双曲率非等厚TC4钛合金壁板整体SPF/DB成形工艺进行了研究.由于零件尺寸超过1800 mm,型面复杂(双曲率,弦高为330 mm),壁厚分布不均匀,成形后出现了严重开裂(多于6处)、明显缩沟(深度大于1.1 mm)和不贴模等缺陷,且在成形后难于通过化铣精确控制壁厚分布,提出了预变形、化铣、扩散连接和超塑成形...