Research On Optimization Of Hot Working Process For Deep Blind Hole Forgings

For the forming process of large deep blind hole forgings,there is great drawback on the mechanical properties or damage of die by direct machining or one-step extrusion forming,and it is difficult to operate and of low feasibility.Therefore,this research proposed a two-step forming scheme of "backward extrusion + mandrel drawing".It has advantages of improving mechanical properties of forgings,reducing production costs,equipment loss and the machining cycles.However,there are also many problems in this process,such as "stuck cars" and cracks during the backward extrusion,and inhomogeneous deformation of the forging,poor forging penetration of the head,"draw-back" of the mandrel,and metal folding at the bottom of the hole during the mandrel dra wing.Therefore,based on the actual production conditions,this study established a new material database of G31 L steel on the DEFORM.The basic forming rules during the backward extrusion and the mandrel drawing were studied by numerical simulation to optimize the hot working process and improved the quality of deep blind hole forgings.G3lL steel was an independently developed new type of steel,and there was no corresponding material data in the DEFORM database.Therefore,this paper systematically tested the flow stress-strain curve,elastic modulus,Poisson's ratio,thermal expansion coefficient,thermal conductivity,and specific heat capacity of G31 L steel to establish the material model.Furthermore,the reliability of the material model was verified by Gleeble thermal simulation tests.The basic forming rules and the influence of key process parameters during the backward extrusion of the forging were revealed.The research showed that there were close coupling relationships between metal flow,load ing-stroke curve,damage distribution,and effective-strain field distribution.The extrusion process could be divided into three stages,namely the initial extrusion stage,the stable extrusion stage,and the end extrusion stage.It was illustrated by the key parameters study that the optimal process was 1200? of the initial temperature of forgings,80 mm/s of the extrusion speed,0.2 of the friction coefficient,200 ? of the preheating temperature of the mold.Based on the present using drawing process,the influencing rules of key process parameters such as reduction rate,turning angle,and feed rate were analyzed,and 45°/60° rotation process were proposed that to reduce the deformation disparity between head and tail of the forging and to improve the penetration of the head.The head and tail of the forging were compared and studied respectively,the results showed that the new process can obviously improve the circumferential uniformity of the forging,and the combination of large reduction ratio ??10% and 60° rotation on the head could improve the penetration,while the combination process of small reduction rate ??8% and 45° rotation on the tail could effectively control the distortion rate of the inner ho le.Strain field homogeneity of the head and tail was improved after three-time-drawing out of the whole length of the forging based on this process,but the head strain remained low.The tapered head forging process was further proposed that to improve the penetration of the head,and it was found t hat the low strain region of the head was improved with the head taper slope was 5°.Finally,upper and lower U-shaped anvils were used to in the spheronizating and finishing process.Based on the optimized process by the numerical simulation,industrial trial production was applied and the forging obtained excel ent forming effect.In this paper,the G31L steel material data model was established to effectively fill the gaps in the DEFORM database.The process optimization research on the backward extrusion and the mandrel drawing provided theoretical guidance and reference for the actual production of large deep blind hole forgings.