Process Development And Deformation Characteristics Analysis On Feature-oriented Incremental Sheet Metal Forming Of Aluminium Alloy

Incremental sheet forming(ISF),aimed at small batch and customized products,with universal tools and equipment under designed toolpath,is capable of forming freeform surface without dedicate dies in several hours comparing to traditional stamping.Besides high flexibility and high efficiency as well as short process,the ISF process has other characteristics as low forming force,localized deformation and incremental forming.Nevertheless,with the blank fixed on its boundary,traditional ISF happens as bulging,in which the material formability is highly improved while its application is limited due to the severe thinning.How to improve deformation uniformity and ease the thinning degree have become kernal issues in ISF for sheet metal parts with deep cavity.For parts with edge feature formed by ISF,deformation uniformity is also a challenge.In this study,for aluminium sheet metal parts with different features,experiments and simulations have been done systematically,new methods have been proposed and deformation characteristics of aluminium sheet metals have been further revealed,which have great importance for extending the application of ISF technology.Main researches and results are briefly presented as follows:To optimize the thickness uniformity of sheet metal parts with deep cavity,a ‘zig-zag' multistep strategy is proposed with repeated outside-in and inside-out toolpath,by which the unformed area ratio is reduced and thus makes the material fully participated in the deformation.Experiments and simulation results show that the standard deviation of thickness on the cross section of hemisphere made with AA 1100-O is reduced by 20.9%,which provides a new multistep toolpath design method for sheet metal parts with deep cavity.To solve the severe thinning problem of sheet metal parts with deep cavity,a hybrid process,including multi-point drawing and robot ISF,is proposed.In the preforming step,with proper pressing strength of the blank holder,boundary material,under the radial tension and circumferential compression strain state,flows into the forming zone to compensate the severe thinning caused by bi-axial tension strain state in the subsequent ISF step.A fairing that can not be formed with traditional ISF,is achieved successfully with maximum thinning measured as 38%.Process efficiency is improved with the developed flexible hybrid forming system with two working stations and transition between two working stations is simplified by the rotatable blank holder.To have a quick evaluation of the designed preforming shape,a 3D thickness prediction model is built.Non-Uniform Rational B-Spline(NURBS)surface is utilized for preform shape control and optimization.Compared to simulation,the developed model could predict the thickness distribution in a very quick way.Taking a hemisphere and a fairing made with 2A12-O as researching objects,the thickness prediction model shows reasonable accuracy compared with experiments' results.The preform shape optimization includes height optimization and shape optimization.While height optimization improves the general part thickness,shape optimization aims for local thinning.Taking a hemisphere as the research object,the maximum thinning is reduced from 58% to 50% by height optimization and the maximum thinning is further reduced to 41.7% by shape optimization.With optimized preform shape,the fairing is formed with maximum thinning as 24.7%.Aimed at sheet metal parts with edge feature formed by ISF,a cumulative double-sided incremental flanging method is proposed to effectively control bulging in the non-deformed area and thus improves geometric accuracy.Deformation characteristics and formability of stretch flanging and shrink flanging are revealed accordingly.The strain state is dominated by the circumferential strain,which also determines the forming limit,instead of radial strain in traditional ISF.Reasons for enhanced formability in incremental flanging is analyzed.While the forming limit could be enhanced with reduced radial strain in stretch flanging,enlarged radial strain could improve the forming limit in shrink flanging.Compared to the fracture forming limit curve(FFLC)predicted by funnel cone and funnel pyramid,incremental flanging forming limit is revealed to be less than the prediction value.Aimed at the up and down fluctuation of the non-deformed area caused by lack of support during the process,a pair of tools with complementary-shape cross section is designed to control the material flow,lower the force normal to the blank surface and thus improve the geometric accuracy in the non-deformed area.