Analytical modeling and numerical simulation for three-roll bending forming of sheet metal

The three-roll bending forming of sheet metal is an important and flexible manufacturing process due to simple configuration. It is suitable for forming large sheet parts with complex, curved faces. Most researches on roll bending forming of large workpiece are mainly based on experiments and explain the process through macroscopic metal deformation. An analytical model and ABAQUS finite element model (FEM) are proposed in this paper for investigating the three-roll bending forming process. A reasonably accurate relationship between the downward inner roller displacement and the desired springback radius (unloaded curvature radius) of the bent plate is yielded by both analytical and finite element approaches, which all agree well with experiments. Then, the three-roll bending forming process of a semi-circle-shaped workpiece with 3,105 mm (length)?×?714 mm (width)?×?545 mm (height) is simulated with FEM established by the optimum tool and process parameters. Manifested by the experiment for three-roll bending forming of this workpiece, the numerical simulation method proposed yields satisfactory performance in tool and process parameters optimization and workpiece forming. It can be taken as a valuable mathematical tool used for three-roll bending forming of large area sheet metal.     

Fatigue life prediction model of Ti-6Al-4V alloy forgings based on forging process parameters

Forging processing parameters have an important impact on the fatigue resistance of metal in the process of plastic forming, which directly threatens the safety and reliability of the metal’s service. Taking Ti-6Al-4V alloy as the research object, this study investigates the effect of processing parameters, including forging temperature and deformation degree on the Young’s modulus, ultimate tensile strength, and reduction of fracture area on the basis of the mechanical property testing of Ti-6Al-4V alloy forgings and obtains the regression equations at the same time. Combined with the existing stress fatigue life prediction formula, the fatigue life prediction based on forging process parameters is realized. Results show that forging temperature and deformation degree have a significant effect on fatigue life. The forging processing parameters of Ti-6Al-4V alloy with optimum fatigue life are as follows: 985 °C–990 °C forging temperature and 46%–50% deformation degree.

     

Superplastic-like forming of Ti-6Al-4V alloy

Superplastic forming of titanium alloys is used for producing structural components, since it is an effective way to manufacture complex-shaped parts in a one-step operation. An optimized sheet-forming process has been designed incorporating a non-isothermal heating system to establish a fast forming process. This work sought to expand the advantages of the technology to the forming of Ti-6Al-4V alloy at 800 °C and shorter cycle time. The minimum thicknesses area was found at the outward corners, showing a maximum percent thinning of 54 %. In addition to stress variations, the cracks resulting from hot drawing and the oxidation on the sheet surface are the other reasons leading to thickness reduction. From the oxidization behavior of Ti-6Al-4V alloy, it was revealed that the decrease in forming temperature from 900 to 800 °C significantly reduced the formation rate of oxide film on the sheet surface. The study also showed that the main microstructure evolution of Ti-6Al-4V alloy under these conditions was recrystallization.     

Electric hot incremental forming of Ti-6Al-4V titanium sheet

Electric hot incremental forming of metal sheet is a new technique that is feasible and easy to control to form hard-to-form sheet metals. In the present study, Ti-6Al-4V titanium sheet was studied because it was wildly used in the aeronautics and astronautics industries. Although Ti-6Al-4V titanium can be well-formed in high temperature, the surface quality is a problem. In order to enhance the surface quality, it is very important to select the proper lubricant. At the same time, because Ti-6Al-4V titanium has a lively chemical property, it is very important to choose a processing temperature range in order to acquire excellent plastic property and to prevent oxidation. Various lubricants were selected in processing to compare the effect, and some workpieces were formed at different temperatures to find the best forming temperature. The results show that using the lubricant film of nickel matrix with MoS₂ self-lubricating material, Ti-6Al-4V titanium workpiece was formed with high surface quality, and the optimum thickness of composite coating is 20 μm for Ti-6Al-4V titanium sheet of 1.0-mm thickness. In fact, the lubricant film also does help to prevent oxidation of Ti-6Al-4V titanium sheet. The appropriate temperature range of Ti-6Al-4V forming with slightly oxidized is 500–600°C in processing, and the maximum draw angle formed in this range was 72°.     

Experimental investigations on effects of tool flank wear on surface integrity during orthogonal dry cutting of Ti-6Al-4V

Machining titanium alloy Ti-6Al-4V is a challenging task since tool flank wear adversely affects surface integrity. Quantitative effects of predetermined tool flank wear values (VB) on the surface integrity were investigated through the orthogonal dry cutting of Ti-6Al-4V. Experimental results indicated that three-dimensional (3D) average surface roughness increased with the VB ranging from 0 to 0.2 mm but decreased at VB = 0.3 mm. Given the effects of rubbing and ironing enhanced, surface material burning and plastic flows emerged on the machined surface at VB = 0.3 mm. Not only the plastic deformation layer became deeper but also the grains were greatly distorted with the increase of tool flank wear. When machined by using the tool at VB = 0.3 mm, the β phase of Ti-6Al-4V decreased near the machined surface layer than that of using the fresh tool. Besides, the depth of work-harden layer increased from 20 to 60 μm with the VB increasing from 0 to 0.3 mm. The softened layer was generated near the machined surface by using the tool at VB = 0.3 mm. In addition, the residual compressive stresses of the machined surface had the trend of decreasing. Experimental results indicated that the VB less than 0.2 mm was the most suitable condition for better surface integrity during orthogonal dry cutting of Ti-6Al-4V. This study aims at providing experimental data for optimizing the processing parameters and improving the surface integrity of Ti-6Al-4V.     

Analytical and numerical analyses of local loading forming process of T-shape component by using Coulomb,shear and hybrid friction models

The Coulomb friction model, shear friction model and hybrid friction model were employed to analyze the local loading forming process by using slab method (SM) and finite element method (FEM). The results indicated that predicted results by shear friction model and hybrid friction model are almost the same, but there exists a notable difference between Coulomb friction and the other two friction models. The rib cavity will be filled better under Coulomb friction model, and difference of rib height after forming process between Coulomb friction model and shear friction model increases at first and then decreases with the increase in width of local loading.     

Robust optimization of the billet for isothermal local loading transitional region of a Ti-alloy rib-web component based on dual-response surface method

Billet optimization can greatly improve the forming quality of the transitional region in the isothermal local loading forming (ILLF) of large-scale Ti-alloy rib-web components. However, the final quality of the transitional region may be deteriorated by uncontrollable factors, such as the manufacturing tolerance of the preforming billet, fluctuation of the stroke length, and friction factor. Thus, a dual-response surface method (RSM)-based robust optimization of the billet was proposed to address the uncontrollable factors in transitional region of the ILLF. Given that the die underfilling and folding defect are two key factors that influence the forming quality of the transitional region, minimizing the mean and standard deviation of the die underfilling rate and avoiding folding defect were defined as the objective function and constraint condition in robust optimization. Then, the cross array design was constructed, a dual-RSM model was established for the mean and standard deviation of the die underfilling rate by considering the size parameters of the billet and uncontrollable factors. Subsequently, an optimum solution was derived to achieve the robust optimization of the billet. A case study on robust optimization was conducted. Good results were attained for improving the die filling and avoiding folding defect, suggesting that the robust optimization of the billet in the transitional region of the ILLF was efficient and reliable.     

Finite element simulation and process optimization for hot stretch bending of Ti-6Al-4V thin-walled extrusion

Ti-6Al-4V is widely utilized to manufacture airframe component structures with curvature, because of its excellent strength to weight ratio, outstanding resistance to corrosion, and inherent thermal and electrical compatibility with carbon fiber composite. Hot stretch bending (HSB) is an effective technology to manufacture these kinds of structures. When comparing the thin-walled extrusion with the thick-walled one, however, it is more difficult to form. The reason is that the local temperature of extrusion decreases more because of the heat transfer between extrusion and die. In this study, the material properties of Ti-6Al-4V were measured experimentally, such as the tensile property within the temperatures from 873 to 1023 K and the strain rates from 0.0005 to 0.005 s^?1, the stress relaxation behavior in a wide range of temperatures (773–973 K) and prestrains (0.7–10%), as well as the heat transfer rule between Ti-6Al-4V (extrusion material) and asbestos cement (die material) under different pressures (8–25 MPa). The heat transfer coefficients (HTCs) were determined by an inverse analysis procedure, which was based on the comparison between measured and calculated temperature. Then, the coupled thermomechanical finite element (FE) model considering the effect of heat transfer was established. The influence of preheating temperature of die, initial temperature of extrusion, and dwell time on spring-back was researched based on orthogonal array testing strategy (OATS). The optimized parameters were verified by process test. It was showed that the established FE model could be used to predict spring-back within a relative deviation of 8.05%.     

Preform design for large-scale bulkhead of TA15 titanium alloy based on local loading features

Large-scale TA15 (Ti–6Al–2Zr–1Mo–1V) titanium alloy bulkhead is a key lightweight load-bearing structure part in an aircraft, which has a large plane view and has a complex shape with high ribs and thin webs. In its forging process, the forming defects, such as folding and under-filling are prone to occurrence. The near-net shape forming with saving force of this large-scale complex component can be realized with proper preform design combining local loading condition. By analyzing isothermal local loading process characteristic of large-scale bulkhead, it indicates that the simple unequal-thickness billet is suitable for small lot manufacture of large-scale TA15 titanium alloy bulkhead. Considering the geometry and forming characteristics, such as large dimension, complex shape, mass data, etc., a design method of unequal-thickness billet using analytical analysis and numerical simulation is proposed. The preform for a large-scale TA15 titanium alloy bulkhead is designed by the proposed method. The basic three-dimensional shape of billet is determined by the analytical models based on local loading features, and the basic billet is modified according to numerical simulation result and considering the local loading forming characteristic, and then the preform without resulting in folding and under-filling can be obtained after two modifications.     

Forming process optimization for non-axisymmetrical complex component based on FEM simulation and experiment

Numerical and experimental investigations were carried out in order to reduce the forming load and defects for non-axisymmetrical complex components. The component with intricate shape and V-type high ribs was widely used as key load-bearing structures. Single-step integral loading with different billet shapes and two-step local loading schemes were used to simulate the forming process and study the metal flow laws. Forming processes of preforming and finisher were analyzed to predict the detail characteristics of material flow using the 3D finite element method models. The simulated results showed that the two-step local loading schemes significantly reduced the forming load and improved the metal filling formability without defects, the required forming load obviously lower than single-step integral. The occurrences of defects, forming loads, and velocity vector distributions were studied and a suitable preform and corresponding die designs were obtained. The experimental results also showed that the component could meet the requirements of dimensional accuracy and mechanical properties.     

Study on multiple-step incremental air-bending forming of sheet metal with springback model and FEM simulation

A mathematical model of springback radius was developed with dimensional analysis and orthogonal test. With this model, the punch radius could be solved for forming high-precision semiellipse-shaped workpieces. With the punch radius and other geometrical parameters of a tool, a 2D ABAQUS finite-element model (FEM) was established. Then, the forming process of sheet metal multiple-step incremental air bending was simulated with the FEM. The result showed that average errors of the simulated workpiece were +0.68/?0.65 mm, and provided the process data consisting of sheet feed rate, punch displacement and springback angle in each step. A semiellipse-shaped workpiece, whose average errors are +0.68/?0.69 mm, was made with the simulation data. These results indicate that the punch design method is feasible with the mathematical model, and the means of FEM simulation is effective. It can be taken as a new approach for sheet metal multiple-step incremental air-bending forming and tool design.     

Metal flow controlled by back pressure in the forming process of rib-web parts

In the forming process of rib-web parts, which are widely used in automobiles and aircrafts, severe streamline disorder often occurs because of unreasonable metal flow. In this study, a back-pressure-controlled forming method that can control the direction of metal flow was introduced. The effects of key parameters on metal flow were investigated, including the back-pressure value, loading mode, and back-pressure distance. The results showed that streamline defects can be avoided and the final forming load can be reduced by reasonably controlling the back-pressure parameters. The constant back-pressure mode was more conducive to achieving good forming quality compared with the linear-increased back-pressure mode. From the stress analysis of metal flow, it was demonstrated that the stress state in the rib root is changed by the back pressure, which makes the metal flow more easily along either the tangential or the radial direction. The advantages of the back-pressure-controlled forming method were confirmed by applying it to a compressor scroll rotor.     

Electrochemical corrosion property of nanostructure layer of Ti-5Al-2Sn-2Zr-4Mo-4Cr titanium alloy

Ti-5Al-2Sn-2Zr-4Mo-4Cr titanium alloy was nanocrystallized with supersonic fine particles bombarding (SFPB). The microstructure features of nanocrystalline layer were determined by XRD, TEM, and microhardness tester. The electrochemical corrosion properties of the surface of original sample and the nanocrystallized sample surface were tested by CHI660 tester. That random crystallographic oriented particles (average grain size of 16 nm) were observed in the top surface layer of Ti-5Al-2Sn-2Zr-4Mo-4Cr titanium alloy, which could be attributed to the surface nanocrystallization. The electrochemical corrosion results show that the impedance of the sample nanolayer is reduced after SFPB with 30 min, and the corrosion resistance is lower than the original sample. The residual internal stress from the process of SFPB is one of the main factors to decrease the nanolayer corrosion resistance of Ti-5Al-2Sn-2Zr-4Mo-4Cr titanium alloy. However, the corrosion resistance is significantly recovered after stress relief annealing with 250–350 °C.     

单榫头叶片锻造过程成形缺陷分析

折叠和模腔充不满是锻造过程中常见的成形缺陷 ,利用自行开发的叶片锻造过程三维刚粘塑性有限元模拟分析软件 ,通过对叶片锻造过程不同截面金属流动规律的三维有限元模拟分析 ,可以预测叶片锻造过程成形缺陷折叠和充不满的形成。通过改变毛坯尺寸和成形工艺参数 ,可以消除这些成形缺陷的产生 ,并获得合格叶片锻件。该研究对其它种类叶片锻造过程中成形缺陷的分析具有一定的指导意义     

Quantitative analysis of the material flow in transitional region during isothermal local loading forming of Ti-alloy rib-web component

The material flow in transitional region plays an important role in the forming quality of transitional region in the isothermal local loading forming of titanium alloy large-scale rib-web component. To study the material flow in transitional region, the finite element (FE) model of transitional region was established based on DEFORM-2D software and validated by physical experiment. Then, a quick and easy method, which can measure the area of different local regions of forged part in DEFORM-2D via user subroutine, was proposed to achieve the quantitative analysis of material flow mechanism. This technique can also be used in the analysis of other forming process, such as the calculation of fill ratio in forging process. The material flow pattern of transitional region during local loading forming was analyzed step by step and compared with integral forming. The results show that the material flow of transitional region during local loading process can be divided into six stages according to the material flow pattern and load-time curve. Twice transverse material flow with opposite directions occurred in the first and second loading steps sequentially, which does not exist in the integral forming. Four characteristic values evaluating the transverse flow of material, which are associated with the formation of defects and their severities, are defined and quantitatively measured at various processing conditions. It is found that decreasing the spacer block thickness and increasing friction both can decrease the four characteristic values, thus weaken the transverse material flow, which are helpful to improve the forming quality in transitional region. However, the transverse flow of material is little affected by the loading speed.     

A study of the cutting-induced heating effect on the machined surface in ultra-precision raster milling of 6061 Al alloy

Ti-6Al-4V alloy is an attractive material in many industries due to its unique and excellent combination of strength to weight ratio and their resistance to corrosion. However, because of its low thermal conductivity and high chemical reactivity, Ti-6Al-4V alloy is generally classified as a difficult-to-cut material that can be characterized by low productivity and rapid tool wear rate even at conventional cutting speeds. It is well known that tool wear has a strong relationship with the cutting forces and a sound knowledge about correlation between cutting forces variation and tool wear propagation is vital to monitor and optimize the automatic manufacturing process. In the present study, high-speed end-milling of Ti-6Al-4V alloy with uncoated cemented tungsten carbide tools under dry cutting conditions is experimentally investigated. The main objective of this work is to analyze the tool wear and the cutting forces variation during high-speed end-milling Ti-6Al-4V alloy. The experimental results show that the major tool wear mechanisms in high-speed end-milling Ti-6Al-4V alloy with uncoated cemented tungsten carbide tools are adhesion and diffusion at the crater wear along with adhesion and abrasion at the flank wear. The cutting force component in the negative y-direction is more dominant of the three components and displays significantly higher magnitudes than that of the other two components in x- and z-directions. The variation of cutting force component F _y has a positive correlation with the tool wear propagation, which can be used as a tool wear indicator during automatic manufacturing process.     

Deformation behavior of variable-thickness region of billet in rib-web component isothermal local loading process

Using the unequal-thickness billet, a good cavity fill can be obtained in the local loading process of the large-scale complex component with high ribs and thin webs. However, the folding/lap defect is prone to occur in variable-thickness region of billet (VTRB) in the forming process. In this paper, the finite element method is adopted to study the deformation behavior of VTRB in local loading process. The evolution and reasons for the folding under the different transition patterns (stepping pattern, rounding pattern, beveling pattern, etc.) and the different located positions (die partitioning boundary, web region, rib cavity, etc.) of VTRB are analyzed systemically. The results of the present study may provide a basis for determination of the transition pattern and transition condition of VTRB for large-scale unequal-thickness billet.     

Effect of tilted plate vibration on solidification and microstructural and mechanical properties of semisolid cast and heat-treated A356 Al alloy

To optimize the machining process, finding the minimum uncut chip thickness is of paramount importance in micro-scale machining. However, strong dependency of the minimum uncut chip thickness to the tool geometry, workpiece material, tool-work friction, and process condition makes its evaluation complicated. The paper focuses on determination of the minimum uncut chip thickness experimentally during micro-end milling of titanium alloy Ti-6Al-4V with respect to influences of cutting parameters and lubricating systems. Experiments were carried out on a CNC machining center Kern Evo with two flute end mills of 0.8 and 2 mm diameters being used in the tests for micro- and macro-milling, respectively. It was found that the micro-milling caused more size effect than macro-milling due to higher surface micro-hardness and specific cutting forces. The specific cutting force depended strongly on feed rate (f _z) and lubricating system, followed by depth of cut (a _p) and cutting speed (v _c), mainly in the micro-scale. All output parameters were inversely proportional to the specific cutting force. Finally, depending on different process parameters during micro-milling of Ti-6Al-4V, the minimum uncut chip thickness was found to vary between 0.15 and 0.49 of the tool edge radius.     

热矫形原理研究

热矫形是消除高弹性板金工件回弹畸变的工艺。本文论证了热矫形的定形原理,即材料软化与短时应力松弛综合效应。据此建立了弯曲回弹热矫形的理论规律。定量反映了材料性能、几何尺寸、温度和时间等主要因素对矫形过程的影响。理论值与实验结果吻合。可以将其用来估计热矫形工艺参数。此外还给出了Ti-6Al-4V和Ti-2Al-1.5Mn在规范成形及矫形温度下的热力学特性与回弹矫形实验曲线。     

Interfacial properties of diffusion bonded Ti-6Al-4V to AISI 304 stainless steel by inserting a Cu interlayer

The joining characteristics of Ti-6Al-4V with AISI 304 stainless steel by inserting a Cu interlayer was investigated in a vacuum-free diffusion bonding process. The diffusion bonds were carried out in the temperature range of 820, 850 and 870°C for 50, 70 and 90 minutes, respectively, under 1 MPa load in argon atmosphere. The joining performances of diffusion bonded Ti-6Al-4V to AISI 304 were studied experimentally. The influence of the insert layer on the microstructure-formed interface region, bonding quality and mechanical properties have also been estimated. The microstructures formed in the diffusion region were observed and determined by scanning electron microscopy (SEM). The microhardness across and perpendicular to the interface were measured and the strength of the joints were also determined with lap-shear test.