DETERMINATION OF MATERIAL PARAMETERS FOR COMPUTER SIMULATION OF SUPERPLASTIC FORMING PROCESSES

In order to analyze and simulate the complex super-plastic forming process by computer, a method of equal height bulging for determining material parameters m and K of the superplastic alloy is presented. The formulae related to the method are deduced in this paper. The accuracy of the method is available for evaluating the examples used in simulating the superplastic sheet-metal bulging processes.

Superplastic forming characteristics of AZ41 magnesium alloy

An AZ41 magnesium alloy in the hot-rolled condition without further thermomechanical processing to modify its microstructure was investigated to establish its suitability for use within a superplastic forming process and to establish optimum forming parameters.Formability was assessed using elevated temperature tensile testing and hot gas bulging,across a range of strain rates(1×10^(−1)−1×10^(−3)s^(−1))and temperatures(350−450℃).Circle grid analysis with GOM Aramis cameras was used to understand peak strains and material thinning in relation to industrial forming processes.Post forming EBSD and STEM analysis was conducted to understand the mechanisms responsible for the materials formability,with dynamic recrystallization being clearly evident.Peak elongation of 520%was achieved at 450℃ and 1×10^(−3)s^(−1);industrially relevant elongation was achieved at 1×10^(−2) s^(−1) at both 450℃(195%)and 400℃(170%).

Superplastic Microforming of Zn-A122 Alloy Ribs

Research on microforming of fine-grained superplastic Zn-AI22 alloy was carried out in this paper by means of superplastic forming. With the selected material, microformability tests have been carried out with the specially developed evaluation method by using two kinds of stainless steel dies. With these dies, micro-extrusion tests have been carried out and the characteristics of microformability of the material have been clarified. Effects of processing parameters and die structure on forming have been analyzed. SEM and metallographic microscope have been used to analyze the formed specimens. Through analysis of flow lines of the formed specimen, behavior of material could be understood well.

Superplastic forming of Ti6Al4V alloy using ZrO_(2)-TiO_(2) ceramic die with adjustable linear thermal expansion coefficient

Firstly,the relation between the coefficient of thermal expansion(CTE)and the volume fraction of TiO_(2) was investigated, and also the influence of relative density of ceramic on the CTE was studied.The results show that the volume fraction of TiO_(2) and the relative density both make influence on the CTE of ZrO_(2)-TiO_(2) ceramic.According to the results,the ZrO_(2)-TiO_(2)(volume fraction of TiO_(2) is 27%)ceramic die with the similar CTE(8.92×10^(-6) ℃^(-1))to Ti6Al4V was fabricated.Secondly,to evaluate the dimensional accuracy of the workpiece superplastically formed,the Ti6Al4V impression experiment was performed.The result shows that the dimensional inaccuracy of workpiece is 0.003.Thirdly,in order to evaluate the practicability,the experiment of superplastic forming Ti6Al4V using ZrO_(2)-TiO_(2) cylinder ceramic die was carried out.The Ti6Al4V cylinder shows good shape retention and surface quality,and high dimensional accuracy.The ceramic dies seem to be adequate for superplastic forming the high accuracy Ti6Al4V, and the trials have confirmed the potential of the ZrO_(2)-TiO_(2) ceramic die.

DETERMINATION OF PRESSURIZING CURVE AND COMPUTER CONTROL OF THICKNESS THINNING FOR SUPERPLASTIC BULGING

This paper proposes the assumption that the flow with viscous friction is the stretch of part of the sheet that lies along the walls of a die during the process of superplastic bulging according to superplastic flow equation and geometrical model of bulging of a sheet into a long trapezoid groove or truncated cone, by introducing the friction-factor P which describes the friction effect on the process. Also, the paper proposes the method of controlling thickness nonuniformity and develops the equipment which for uniform thickness of bulging, is automatically controlled with a computerl it also analyzes the important innuence of lubrication on thickness distribution of bulging materials. By the assumption, the relationship between bulging pressure and time is obtained in bulging of a sheet into the groove and cone, and p-t curve of multi-mould-cavity complicated bulging is discussed based on the analysis of single-mould-cavity bulging characteristics.

SUPERPLASTIC FORMING AND AP-PLICATION OF GH4169 SUPERALLOY

The superplastic forming and application of GH4169 superalloy are studied. It is shown that for the typical superplastic forming strain rates of 10- 3 s 1 to 10- 4 s -1, the available engineering strains of the fine-grain GH4169 superalloy are always higher than 250% within wide temperature scopes (920 -980℃), and the maximum elongation to failure obtained is 513%. Dynamic grain growth is presented during superplastic deforming for the alloy, and the grain becomes larger with the increase of the strain. A GH4169 fuel manifold of an aerocraft is superplastically formed, and the superplastically formed GH4169 mainfold is tested by 30 MPa hydraulic pressure for 10 min. So some GH4169 alloy complex components can be superplastically formed in the aerospace industry to decrease the costs of fabricating these components.

Effects of Heating Rate on the Process Parameters of Superplastic Forming for Zr_(55)Cu_(30)Al_(10)Ni_5

We investigated the effects of heating rate on the process parameters of superplastic forming for Zr55Cu30Al10Ni5 by differential scanning calorimetry. The continuous heating and isothermal annealing analyses suggested that the temperatures of glass transition and onset crystallization are heating rate-dependent in the supercooled liquid region. Then, the time-temperature-transformation diagram under different heating rates indicates that increasing the heating rate can lead to an increase of the incubation time at the same anneal temperature in the supercooled liquid region. Based on the Arrhenius relationship, we discovered that the incubation time increases by 1.08-1.11 times with double increase of the heating rate at the same anneal temperature, and then verified it by the data of literatures and the experimental results. The obtained curve of the max available incubation time reveals that the incubation time at a certain anneal temperature in the supercooled liquid region is not infinite, and will increase with increasing heating rate until this temperature shifts out of the supercooled liquid region because of exceeding critical heating rate. It is concluded that heating rate must be an important processing parameter of superplastic forming for Zr55Cu30Al10Ni5.

Deformation Behavior of Zr_(65)Cu_(17.5)Ni_(10)Al_(7.5)in Superplastic Forming with Silicon Mould: Simulation and Application

We investigated the deformation behaviors of Zr_65Cu_17.5Ni_10Al_7.5 in superplastic forming in silicon mould via numerical modeling and experiments. The data needed for the constitutive formulation were obtained from compressive tests to establish a material library for finite-element simulation using a DEFORM 3D software. A constant speed forming process of a micro gear was modeled where the loading force, feature size and amount of deformation in the micro gear in silicon mould were analyzed in detail for the optimal requirements of micro gear forming and the protection of silicon mould. Guided by the modeling parameters, an amorphous metal micro gear was successfully obtained by our home-made superplastic forming system with the optimized parameters （temperature of 683 K, top speed of 0.003 mm/s until the load force reaching limiting value at 1960 N, and a gradually decelerating process for holding the force to the end）. Our work gives a good example for optimization of superplastic forming and fabrication of BMGs in microparts.

Superplastic Forming and Diffusion Bonding for Sandwich Structure of Ti-6Al-4V Alloy

Superplastic forming and diffusion bonding (SPF/DB) is a well-established process for the manufacture of components almost exclusively from Ti-6AI-4V sheet material. The sandwich structure of Ti-6AI-4V alloy is investigated. The effects of the microstructure on the SPF/DB process were discussed. The microstructure at the interfaces and the distribution of thickness were researched.

Surface Modelling and Mesh Generation for Simulating Superplastic Forming

This paper presents an approach which enables surface modelling, mesh generation and the Finite Element (FE) analysis to be integrated together to simulate superplastic forming process for complex shaped components. Techniques have been developed to generate an FE mesh over non-four-sided surface areas, the boundaries of which are Bezier curves of arbitrary degree, using a consistent expression. Theoretical evidence is given to determine the number of Bezier triangular patches required for accurately re-constructing die surfaces within a commercial FE solver. The developed techniques have been successfully used in determining the process parameters for forming a 3D rectangular box.

Microstructure and texture evolution during high-strain-rate superplastic deformation of coarse-grained Mg-Gd-Y-Zr rolled sheet

Microstructure and texture evolution during high-strain-rate superplastic deformation of the rolled Mg-Gd-Y-Zr sheet were investigated.The tensile tests at the strain rate of 0.01 s-1 achieved the elongations of 180%-266% in the deformation temperature range of 400-500 ℃.Post-deforming microstructures were characterized by optical microscopy,scanning electron microscopy and transmission electron microscopy,while crystallographic orientation information was obtained from macro-texture analysis.The results show that the high strain-rate superplasticity was attributed to class-I dislocation creep accommodated by dynamic recrystallization （DRX）.During preheating at 435 ℃ for 600 s,twinning-induced recrystallization occurred.The initial strain of 80% made original grains fragmented and produced homogenous DRX grains.The interaction between dynamic recrystallization and dynamic precipitation yielded out such a phenomenon that finer DRX grains were often accompanied by denser particles.The macro-texture evolution exhibited some characteristics of the crystal rotation arising from basal slip and prismatic slip despite the occurrence of DRX.

DETERMINATION OF THE PARAMETERS OF SUPERPLASTIC FORMING FOR LONG RECTANGULAR THIN SHEET TITANIUM ALLOY Ti-6A1-4V

A method for determining the value of the strain rate sensitivity parameter m, of a thin sheet superplastic material, which is based on the results of constant gas pressure tests, has been developed in this paper. Unlike the conventional procedures the method involved provides the test conditions similar to those occurring during an industrial technological process. Such an approach enables one to estimate reliable the superplas- tic properties of the material under study. Theoretical analysis is based upon use of the standard equations of the membrane theory. The experimental investigations have been carried out for Ti-based alloy VT6 (Ti--6Al-4V). Theoretical predictions show satisfactory agreement with experimental data. The results obtained are compared with those measured by means of standard procedures (constant cross-head velocity experiments and load relaxation tests). It is shown that the use of specimens having reduced gage length (which is less than specimen's width) could lead to considerable errors in experimental estimation of the m value which, in its turn, leads to inaccurate calculating the technological parameters.

Numerical Simulation of Superplastic Forming and Diffusion Bonding of Ti Alloys

The research on numerical simulation for combinative process of SPF/DB is carried out in this paper. The contacting problem of sheets is analyzed by using the penalty method. In order to solve the contact problem of different parts of the sheet, a new algorithm for contacting judgment is proposed. According to the relation of the distance vector and the vector of contacting element area, and the condition of contact, it can be judged whether or not a node on the slave surface and the corresponding master surface are in the state of SPF/DB. The Mindlin shell element is employed to simulate SPF/DB process of an asymmetry double-cell cup of Ti-6Al-4V to examine the efficiency of the new algorithm using ARVIP-3D. The results of the numerical simulation are in good agreement with experimental results.

Superplastic forming gas pressure of titanium alloy bellows

The complex superplastic forming (SPF) technology applying gas pressure and compressive axial load is an advanced forming method for titanium alloy bellows,whose forming process consists of the three main forming phases namely bulging, clamping and calibrating phase. The influence of forming gas pressure in various phases on the forming process was analyzed and the models of forming gas pressure for bellows were derived according to the thin shell theory and the plasticity deformation theory. Using the model values, taking a two-convolution DN250 Ti-6Al-4V titanium alloy bellows as an example, a series of superplastic forming tests were performed to evaluate the influence of the variation of forming gas pressure on the forming process. According to the experimental results these models were corrected to make the forming gas pressures prediction more accurate.

Superplastic formability of Ti-6Al-4V butt-welded plate by laser beam welding

The superplasticity of Ti-6Al-4V butt-welded plates by laser beam welding(LBW)was studied in virtue of hot tensile tests and superplastic bulging tests.Furthermore,microstructural evolution of weld metal upon superplastic forming was systematically analyzed via metallographical tests and scanning electron microscope(SEM).The relation between the microstructure of weld metal and its superplastic ability was discussed.The experimental results show that Ti-6Al-4V butt-welded plates by LBW possess superplasticity.The maximum elongation is up to 154%and the maximum bulge height can be up to 1.81 times the internal radius of the female die.There is an optimum value of the bulge height for bulging gas pressure.

GRAIN SHAPE EFFECT IN SUPERPLASTIC DEFORMATION

The method controlling grain shape in TMT processing and the effect of grain shape on char- acteristic parameters in superplastic deformation were discussed.The accommodation velocity of grahl boundary sliding,which is the dominant mechanism in superplastic deformation,and the contribution of each mechanism to the total strain,as influenced by grain shape,were ana- lyzed.Grain shape has been shown to be an essential structural factor for superplasticity.Then an analysis was made about the effect of grain shape on the region transition strain rate so that a new concept,critical aspect for superplasticity,was worked out.These predictions were compared with the measured results in an Al-Zn-Mg alloy.

Superplastic behavior of in situ synthesized (TiB+TiC)/Ti matrix composite

The superplastic deformation behavior of titanium alloy matrix composites reinforced with TiB and TiC was studied. Superplastic tension tests under conditions from 750℃ to 1100℃ and initial strain rates ranging from 2×10^-2 s^-1 to 10^-4s^-1 were carried out. A maximum elongation of 659% was obtained. The micro-structural evolution was studied by means of optical microscopy （OM） and transmission electron microcopy （TEM）. The deformation mechanism and the effect of reinforcement on superplasticity are also discussed.

Surface hardening of die steel by superplastic deformation of thermo-sprayed powder alloy

The superplastic forming is an advanced processing technique and has been used in manufacturing some dies, especially extrusion die and forging die. In heavy load condition, the superplastic formed dies often exhibit low service life owing to the low surface strength and poor wear resistance. A commercial cold working die steel GCr15 pre treated into ultra fine grain sizes was chosen as the substrate material, on which the surface was thermal sprayed with the powders of a wear resistant alloy. At some appropriate temperatures and strain rates, the accommodated superplastic deformation of the substrate and coating occurred and the good cohesion was well performed. The coated layer shows high microhardness values and fairly smooth transition in hardness from substrate to the surface. The wear resistance of superplastically formed specimen has also been improved, compared with the GCr15 steel coated with the same alloy powder and treated by flame remelting. [

Effect of Post-Rolling after ECAP on Superplastic Behavior of Commercial Al-Mg Alloy

A commercial Al-Mg alloy was subjected to equal channel angular pressing of 4 passes with and without postrolling, and the effects of post-rolling on the deformation characteristics of the alloy at 723 K were examined. Post-rolling was found to influence the deformation behavior significantly. The deformation behavior of the alloy processed only by equal channel angular pressing was characterized by (a) localized deformation indicated by severe surface prominence and depression, (b) the strain rate sensitivity of 0.33, and (c) moderate high strain rate superplastic elongations. By contrast, that of the alloy processed by equal channel angular pressing and post-rolling (70 % thickness reduction) was manifested by (a) uniform deformation associated with grain boundary sliding throughout the sample, (b) a sigmoidal behavior showing the strain rate sensitivity of 0.45 at the intermediate strain rates in the logarithmic stress-strain rate curve, and (c) very large high strain rate superplastic elongations.

VISCOUS AND RHEOLOGIC SUPERPLASTIC TECHNIQUE

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