Process,and,Equipment,of,Double-Sided,Sheet,Hydroforming,for,Large-Sized,Al-Alloy,Tailored,Shell

LIU Wei ,XU Yongchao ,YUAN Shijian ,ZHANG Zhichao ,DU Tongliang ,LI Ying ,HU Lan

1 National Key Laboratory for Precision Hot Processing of Metals,Harbin Institute of Technology,Harbin 150001

2 Shanghai Aerospace Equipments Manufacture Co.,Ltd,Shanghai 200245

Abstract:In the industrial field,tailored blank forming with aluminum alloy (Al-alloy) has developed fast to meet the demands for large size integrated components with curved surfaces of high precision and with uniform mechanical properties.Traditional forming methods for tailored blank components faced challenges with uneven deformation behaviors and coexistence of rupture and wrinkling defects occuring during the forming process.In this paper,a new manufacturing procedure is proposed with advanced welding and forming technologies for forming integrated shell components.Friction stir welding with post-weld heat treatment was employed to prepare the tailor welded blank and improve its formability prior to forming.A double-sided pressure sheet hydroforming process was introduced to fabricate the Al-alloy tailored blank into a curved surface shell.Finite element modeling was established to analyze the effect of the weld line position and loading paths of stress distributions during the double-sided sheet hydroforming (DSHF)process.A large double-action CNC sheet hydroforming press with tonnage of 150 MN and high pressure liquid volume of 5 m3 was developed in China.As an application case of the proposed process and equipment,a full-scale tank dome with a diameter of 3 m was successfully hydroformed with a large size Al-alloy tailored blank.It was shown that the DSHF process has the advantages in controlling rupture and wrinkling defects with an Al-alloy tailored blank,and the novel manufacturing procedure enables the production of integrated thin-walled component more competitively than traditional methods.

Key words:sheet hydroforming,aluminum alloy,tailored blank,liquid pressure

In recent decades,integrated shell component fabrication with a tailor welded blank (TWB) was looking as a promising substitute for the multi-piece welded part produced using conventional forming methods with merits of improved service reliability,closely matched shape and increased manufacture efficiency.However,traditional forming technologies face huge challenges on forming of complex surface parts with TWB,especially for high strength and with low plasticity aluminum alloys(Al-alloys).

TWBs are typically heterogeneous materials with uneven mechanical properties and varied local microstructures.A local weakened or strengthened zone of the weld joint compared to base material may lead to large a stress or strain gradient distribution and localized necking.Although,post-weld heat treatment is applied to enhance the formability of a tailored blank,the stress state of the weld seam under complicated loadings has great influence on its deformation behavior.For deep drawing of a curved surface shell,the absence of normal constraint and under the circumferential compressive stress,results wrinkling of the unsupported surface more easily,especially in the case of very small thickness to diameter ratio (t/d).As a result,this poor formability and uneven deformation behavior would be prone to induced ruptures and wrinkling simultaneously during the integrated forming process during the complex stress state.Hence,it’s very hard to manage an appropriate process to avoid these problems successfully using traditional forming technologies.

In view of the above,efforts were made to improve the formability of TWB and control the deformation behavior under complicated stress states for curved surface shells.The forming limit of Al-alloys is largely increased at an elevated temperature,but the deformation process of the welded blank at high temperature is inhomogeneous,and the strength of the weld joint is obviously decreased.Although,a complicated process composed of friction stir welding (FSW),warm spinning and NC machining was used to manufacture large curved shells with thickened plates for aerospace applications,there were still several shortcomings in this process including acomplicated process,difficult to control microstructure and low material utilization for mass machining.

Compared with conventional methods,the sheet hydroforming process is an efficient and reliable technology which improves formability and stability of TWBs by backward liquid pressure.However,for TWBs with t/d lower than 0.5%,there are still great challenges in eliminating wrinkles and ruptures.The backward pressure cannot be increased all the time,meaning that too high a liquid pressure may lead to rupture defects,especially at the weld joint.

Focusing on this point,a new double-sided sheet hydroforming process was proposed with two pressurized liquids loaded on both surfaces of the blank.Some studies showed that the maximum thickness strain could be improved,and it was found that double-sided pressure was beneficial for supression of reverse bulging fractures in a cylindercal cup.However,no previous studies had shown that the DSHF process was effective for large curved shells,especially for tailored blank forming.

In this study,the DSHF process was introduced to enable an integrated forming of a large curved shell with TWB.The deformation behaviors of TWB during the DSHF process were studied by FE simulations and experimental works on a full-scale tank dome.The sheet hydroforming equipment designed by Harbin Institute of Technology was introduced and a tank dome with diameter of 3 m was successfully produced by employing the equipment with an optimized DSHF process.

The proposed forming procedure of Al-alloy TWB was divided into five stages including the preparation of a tailored blank by FSW,post-weld annealing heat treatment,solution treatment,hydroforming using double-sided pressure and artificial aging treatment,as indicated in Figure 1.Compared with the traditional method using multi-piece foming and joining procedures,the technical advantages of this new procedure were a short production cycle,low materials cost and high service reliability.

Figure 1 Forming procedure of large-sized Al-alloy tailored blank

In this study,two 2219 Al-alloy blanks with a thickness of 8 mm were tailored under the FSW process into annealed treatment state using optimal welding parameters.Then,the tailored blank was further annealed by an air furnace at a temperature of 300°C for 2 hours for improved uniform formability.The solution treatment was conducted on the whole tailored blank at a temperature of 535°C for 40 minutes according the blank thickness,then the tailored blank was quenched into cool water quickly within 5 seconds.Finally,the tailored blank was formed using the proposed DSHF process into a curved shell within 2 hours.The component was then subject to artificial aging treatment for 18 hours at 175°C.

The blanks were tailored on both sides by using a CNC 3D friction stir welding machine with double driving motors.The shoulder diameter,pin diameter and pin length of the friction stir tool were 20,6 and 7 mm,respectively.The tool was tilted at an angle of 2.8° from the stir pin to the normal direction of the blank.The welding parameters were optimized after several welding trails within a speed from 800 − 1200 rpm with a feed speed of 200 − 400 mm per minute.The advancing direction of FSW was parallel to the rolling direction of the blanks.After welding,the tailored blank was cut into a circular geometrical profile according to the forming process,and the weld seam quality was determined by using a X-ray inspection device to detect flaw defects in the weld joint.Figure 2 shows the large sized TWB of 2219 Al-alloy.

To characterize the grain size in welded zone of the welded joint,microstructure examination were carried out by optical microscopy (OM,LEICA DMI 3000).Figure 3 shows the cross-sectional microstructures of the as-welded AA2219 FSW joints.The welded joint incorporates four obvious micro-zones in terms of the varied thermo-mechanical states during the FSW process,that is the base material (BM),heat-affected zone (HAZ),thermo-mechanical affected zone (TMAZ) and nugget zone (NZ).Meanwhile,The NZ is nonhomogeneous with three sub-zones: the shoulder affected zone (SAZ),the pin-driven zone (PDZ) and the swirl zone (SWZ).As far as the as-welded joint be concerned,BM and HAZ show smaller changes in the grains.The grain growth of the HAZ do not occur under heat cycle during the welding.The initial grains in TMAZ were curved and elongated around NZ.There was no discernible interface between TMAZ and NZ.The distribution of grain shape exhibits obvious onion rings in NZ.

Figure 2 Large-sized FSW blank of 2219 Al-alloy

Figure 3 Optical micrographs of the as-welded AA2219 FSW joints

A post-weld annealing heat and solution treatment was carried out to overcome the poor formability of the tailored blank after FSW due to inhomogeneous severe deformation,as shown in Figure 4.To evaluate the mechanical properties of the weld joint,samples cut from the FSW blank along the transverse direction (TD) and longitudinal direction (LD) were both prepared for uniaxial tensile tests according to the standard GB/T 2651-2008.The stress-strain curve of base material (BM),weld joint as-weld and post-weld heat treatment (PWHT) are shown in Figure 5 (a).In general,the weld joint shows reinforcement mechanical behavior and lower formability than BM after FSW.The elongation of the weld joint along TD and LD retain 76.5% and 82.1% of that of BM,while the strength of both specimens show a reinforcement effect after welding,because the grain sizes in the weld joint are significant fine in the FSW process.The strength of welded joint along TD and LD were both lower than that of the BM after annealing and solution heat treatment,especially for the TD specimen.Additionally,the weld joint elongation of LD was 28.2%,almost equal to that of BM.However,there was a slight reduction of elongation in TD.The strength and elongation coefficients of the weld joint in TD compared to BM are 97.3% and 78.6% respectively,which means higher stress and strain transverse in the weld line should be avoided.These percentages were used as a guide to determine the maximum allowable stress and strain in the weld line.

Figure 4 AA2219 TWB after heat treatment for improved formability

Figure 5 Mechanical properties of AA2219 FSW-joint in as-welded and PWHT tempers

The microhardness tests along transversal cross-section of the weld joint showed that the hardness value distribution of the whole specimen was at a relatively consistent level (Figure 5b).Thus,the uniformity in mechanical properties of the weld joint was greatly improved and the formability of the tailor weld blank had increased after the post-weld heat treatment.

5.1 Equipment and Setup

To satisfy the requirements for manufacture of large-sized components,a double-action hydroforming press was developed with 150 MN in tonnage and 5 m3in volume of high pressure liquid,as shown in Figure 6.The tonnage and high pressure liquid volume were 1.5 times and 10 times larger than the previous world’s largest hydroforming machine.The difficulty in the development of the huge equipment was how to apply the mass volume of liquid with high pressure synchronously,within a very short response time,hence multiple high pressure intensifiers of large volume were adopted in parallel for this purpose.Furthermore,a complex CNC system algorithm was also a key technology to enable the precision control of parameters during the hydroforming process,such as the clamping force,liquid pressure,liquid volume and punch stroke.

5.2 Double-sided Sheet Hydroforming Process

In the DSHF process,two liquid pressures are applied separately on both sides of the blank surface with one forward pressure and one backward pressure.The tools for the process constituted a punch,a blank holder and a die with liquid cavity.Multi-intensifiers were used to provide forward and backward liquid pressures during the punch stroke,with each one capable of being controlled separately.To achieve the proposed forming process,a complex liquid pressure loading strategy and PLC control system was designed for the precise numerical loading of the double pressures synchronously,as shown in Figure 7.

Figure 6 CNC sheet hydroforming press

Figure 7 Control strategy in DSHF

6.1 FE Simulation Model

A hemispherical tank dome with diameter of 3 m was modeled as an industrial application using the DSHF process,a half 3D numerical simulation was performed using Abaqus/Explicit,as shown in Figure 8 (a).The weld seam was considered a special mesh with a grid of tiny size,and both the centre-weld and the off-centre-weld were studied,as shown in Figure 8(b).User Defined Subroutine-Fortran Codes were coupled into the Abaqus software to add liquid pressures on the double sides of blank.Hill90 yield criterion was applied to describe the anisotropy plastic deformation behavior of the alloy sheet in the numerical simulation.Four types of liquid pressure loading paths were compared with different pressure ratios of 0,0.25,0.5 and 0.75,namely Path A,B,C and D,respectively,as shown in Figure 8 (c).

6.2 Weld Line Position

The stress distribution of typical points on the weld line corresponding to centre weld and off-centre-weld dome at 100%punch stroke are shown in Figure 9.For the centre-weld dome,the stress along the parallel direction is all tension stress,and the maximum value is located at point A1 with a value of 230 MPa.However,the stress value changes from positive to negative along the perpendicular direction,which means the stress state is not coincident along the weld line from the bottom central region to the flange region.The maximum tension stress and compressive stress were about 230 MPa and −330MPa,which are lower than the allowable maximum value of the weld joint.In contrast,the situation for the off centre weld was much different.It was found that the maximum stress was located at point B1 with a biaxial tension stress of 228 MPa and 395 MPa in parallel and perpendicular directions,which exceeded the limit stress of the weld joint.

Figure 8 FEA model of DSHF process for tank dome with FSW-blanks

Figure 9 Stress distribution of centre-weld and off-centre-weld dome

6.3 Double Sided Pressure Ratio

The stress distribution of typical points on the centre-weld dome at different liquid pressure loading paths are shown in Figure 10.With the increasing of the pressure ratio from 0.25 to 0.75,an obvious decreasing can be found both on radial stress and circumferential stress for the region of point 3,and the stress state changes from biaxial-tension state to equal tension-compression state when pressure ratio is above 0.5.This means the proper stress state of the weld joint can be optimized under a pressure ratio of about 0.5,and rupture and wrinkling can be controlled with good quality.

Figure 10 Stress distribution on centre-weld dome at different liquid pressure ratio

6.4 Industrial Trails

Industrial trials were carried out using a full-scale Al-alloy tank dome of 3 m in diameter.Using an optimized loading path with a pressure ratio of 0.5,the dome was fabricated successfully with a thickness to diameter ratio lower than 0.2%,and both wrinkles and ruptures were suppressed effectively,as shown in Figure 11.Additionally,the thickness distribution and mechanical properties of the hydroformed shell was measured,as shown in Figure 12.The maximum thinning rate was lower than 7.8% and the tensile strength was above 405 MPa with 8.6% of enhancement than the design required,indicating that DSHF process has a positive impact on improving uniform plastic deformation and mechanical performance.It should be noted that the largesized tank dome was integrally hydroformed for the first time ever with an Al-alloy tailored blank near to the finial thickness of the end component.Compared with the traditional spinning and machining method,the production cycle decreased from 3−6 months to 1 week,with a reduction in the manufacturing cost of 50%.

Figure 11 Large-sized integrated tank dome with Al-alloy tailored blank

Figure 12 Thickness distribution and mechanical properties of the hydroformed shell

To satisfy the requirement of integrate component forming,a novel manufacture procedure was proposed through the combination of FSW,heat treatment and a double-sided sheet hydroforming process to fabricate a large curved shell with an Al-alloy tailored blank.The large-sized tailored blank of high strength Al-alloy was prepared by FSW using optimized welding parameters,and the mechanical properties,uniformity and plastic formability of the tailored blank were significantly improved after proper post-weld heat treatment.The effect of weld line position and double-sided pressure ratio on deformation behavior were revealed by finite element analysis according to the stress state of the tailored blank.As an innovation of manufacturing technology,a double-action CNC hydroforming press with tonnage of 150 MN was built in China,and a high performance Al-alloy tank dome with 3 m diameter was integrally formed by this press for industrial applications.For the future with the increasing demand on large shell components,the process and equipment will demonstrate great application potential in the fabrication of space launch vehicles,aircraft panels,highspeed train bodies and so on.

Acknowledgements

This work was supported by the Project of National Science Foundation of China (No.U1637209) and Project of National Key Research and Development Program (No.2017YFB0306304),the authors would like to thank them for the financial support during the programs.