Study On Unequal-thickness Billet Optimization For Isothermal Local Loading Forming Of Titanium Alloys Large-scale Rib-Web Component

The large-scale Ti-alloy rib-web component(LTRC)can achieve the high-performance and light-weight requirements owing to its structural and material aspects,which is an inevitable choice for the high-end equipment of the aviation and aerospace domains.Isothermal local loading forming(ILLF)technology combines the advantages of isothermal forming and local loading,which can significantly decrease the forming load,enlarge the forming size of the component and the capability of the equipment.Thus,ILLF is an effective solution to integrate manufacturing of shape and performance of LTRC and becomes an advanced plastic forming technology urgently to be developed.However,due to the complexity of the material flow in local loading forming process and the large-sized component with irregular and multi-ribs structure,optimizing the preformed billet and controlling the forming defects become great challenges.To this end,a systematic and thorough investigation on the optimization of the preformed billet in ILLF of LTRC has been carried out using finite element(FE)simulation combined with theoretical analysis and physical simulation test.A brief introduction to the work and its main achievements obtained are as follows.By analyzing the material flow rule and strain field evolution during the local loading,the short range transfer effect has been revealed under the interaction of the different loading steps.Due to the reciprocating material transfer,the local composite deformation is emerged from the die partitioning line to the first rib of the not loaded region.That is,the warping and the rib shifting are produced in the first and second-loading step,respectively.And they have the positive correlation with the transferred material.The excessive billet volume nearby the die partitioning line can increase the quantity of transferred material,which is the key factor to cause the folding.By comparing the strain field to that of the integral loading,it is observed that an additional strain field is produced between the die partitioning line and the first rib of the not loaded region.Howover,the strain field which is far away from the die partitioning line presents similar feature between the two types of loading.Meanwhile,alteration of the loading sequence has little influence on the strain of the that region.On this basis,the boundaries between the transitional region and first/second-loading region are determined,which provides the basis for satisfying the accuracy and efficiency of the billet optimization process.To address the die underfilling of the first and second loading region,an unequal-thickness billet(UTB)optimization design thought is proposed by combining the regulation of the neutral layer location with slight compensation of the billet volume.Three optimization ways and corresponding application condition are determined,i.e.,(I)adjust the partition location of the UTB,(II)increase the number of regions in the UTB,(III)increase the thickness of the regions in the UTB.Accordingly,the UTB optimization method based on forward step modification is determined.By adopting optimization scheme I and II,as well as scheme III three iterations,the die underfilling rate is decreased from 7.21% to 0.05% by comparing to the initial UTB.Owing to the proposed methodology,not only the filling is improved,but also the optimization efficiency is increased significantly.The effect of billet volume distribution on the percentage of material transfer and forming defects of the transitional region has been investigated.The correlative relationship between the geometric parameters of the UTB of the transitional region and the percentage of material transfer is established.In order to avoid the folding in transitional region,the critical value of the percentage of the material transfer is searched by stepwise decreasing the reduction amount in ETB.By combining the two aspects mentioned above,a determination method for acquiring the feasible region of the geometrical parameter of UTB is proposed.The reliability of the feasible region for avoiding the folding is verified by simulation and experiment.The effect of the fluctuation of the friction factor,the manufacturing tolerance of the billet,stroke length and forming temperature on the forming quality of the transitional region is revealed by significance analysis.The results show that the die underfilling and additional strain field can be significantly affected by the fluctuation of the friction factor,the manufacturing tolerance of the billet and stroke length.The fluctuation of friction factor is a major factor to affect the material transfer and folding defect while the effect of the other uncertainty factors is negligible.The forming temperature has little influence on the filling,strain and folding.On this basis,by both considering the effect of certainty factor(the geometric parameters of the UTB)and uncertainty factor,the robust optimization design of the UTB in transitional region is obtained based on dual-RSM and NSGA-II multi-objective optimization algorithm.The forming results of the optimized UTB with robust design in transitional region show that the folding defect induced by uncertainty factors can be effectively avoided.Moreover,the die underfilling and the average strain of the additional strain field can be decreased.Meanwhile,the variation ranges of the die underfilling rate and deformation homogeneity are relatively small.In conclusion,the UTB is optimized based on forward step modification in the first and second loading region,while the UTB is optimized based on multi-objective robust control,the overall optimization of the UTB can be revealed.The above billet optimization method provides an important foundation for the integrated forming of shape and performance during isothermal local loading forming of titanium alloy large-scale rib-web component.