| With the increasingly strict laws and regulations on energy saving and emission reduction,higher requirements are put forward for the lightweight of cars.As a kind of typical lightweight material,advantages of aluminum alloys include high specific strength and desired structural properties,which can meet the economic benefits and ensure automobile safety at the same time.However,the aluminum alloy has poor formability at room temperature and severe springback,which results in its limited application in automotive structural parts.Aluminum alloy Hot Form Quench forming process(HFQ?)has been developed over years,which combines the features of hot stamping and heat treatment.By using this process,not only the formability of the material can be improved,but also the thermal distortion of the parts can be avoided.In order to shorten the production cycle,a rapid hot stamping process(Pre-hardened Al Hot Forming,PAHF)has been proposed.In this work,the hot stamping forming quality of 7075 aluminum alloy B-pillar is taken as the object.First,the thermal deformation behavior of 7075 aluminum alloy both in HFQ?and PAHF will be studied.Then,the microstructure evolution under different forming process parameters will be examined,and the influence of process parameters on the mechanical properties of forming parts will be explored.Consequently,the model which can predict the yield strength of forming parts will be established.This work is supposed to provide powerful theoretical guidance for the establishment of hot stamping process parameters in practical production.Hot tensile tests of 7075 aluminum alloy has been carried out to investigate the effects of deformation temperature and strain rate on deformation behaviors during HFQ?and PAHF.In HFQ?process,the 7075 sheet was firstly subjected to full solution heat treatment at 475?/30min,and then hot tensile tests were performed at different deformation temperatures(300??450?)and strain rates(0.01 s-1?1 s-1).In PAHF process,the 7075 sheet was directly heated to different deformation temperatures(300??450?)and stretched at different strain rates(0.01 s-1?1 s-1).The characteristics of the heat flow behavior and the elongation at break of the studied material were analyzed both in the HFQ?and PAHF process,and the constitutive models which can accurately describe the flow behavior of these two kinds of thermal deformation were established respectively.The peak stresse in both HFQ?and PAHF process was decreased with the increase of the deformation temperature and the decrease of the strain rate.The elongation at break of the HFQ?and PAHF expressed the opposite tendency with the increase of the deformation temperature.In HFQ?,the elongation after solid solution decreased generally with the increased deformation temperature(300??450?),and reached its lowest when the deformation temperature is 450?.Whereas the elongation at break increases with the increase of the deformation temperature(200??350?)in PAHF.The microstructure evolution of different process parameters(such as deformation temperature,strain rate,deformation ratio,aging time,etc.)on HFQ?thermal deformation and PAHF thermal deformation was studied by OM,SEM,TEM and DSC.It was found that only dynamic recovery occurred in both HFQ?and PAHF,and the grain structure was almost the same under different process parameters.For HFQ?,with the increase of deformation temperature(300??450?)and the decrease of strain rates(0.01 s-1?1 s-1),the fracture mechanism gradually switched from ductile fracture to intergranular fracture,but for PAHF,as the deformation temperature increased(200??350?),the fracture mechanisms changed from intergranular fracture to ductile fracture.The results showed that the deformation after solid solution promoted the precipitation during artificial aging.The precipitation activation energy of?'and?under different deformation ratio were 102.7 k J·mol-1?120.8 k J·mol-1 and89.7 k J·mol-1?102.7 k J·mol-1,respectively.The influence of deformation process parameters on the final mechanical properties of hot stamping parts was also studied.For HFQ?,the Vickers hardness and yield strength of the formed parts increased first and then decreased slowly with the aging time increase(0?28 h)under different deformation ratios.For PAHF,the Vickers hardness and yield strength of formed parts decreased with the increase of deformation temperature.The results indicated that the strain rate and deformation ratio have little influence on Vickers hardness and yield strength of formed parts.Based on the knowledge of precipitation kinetics and material sciences,the relationship between the process parameters and yield strength of 7075 aluminum alloy was studied.The model of the precipitation volume fraction and size evolution with deformation ratio and aging time were established,and then the yield strength model which is suitable for 7075aluminum alloy hot stamping was developed.At last,the process parameters of 7075 aluminum alloy B-pillar were optimized by using artificial neural network(Back Propagation,BP)and genetic algorithm(Genetic Algorithms,GA).The optimum process parameters that suitable for HFQ?and PAHF were obtained,and the 7075 aluminum alloy B-pillar with good forming quality and satisfactory performance could be produced under the optimized forming process conditions.The results showed that the two process are all suitable for the forming of complex-designed automobile components with strength requirement.The advantage of the HFQ?is that the blank temper can be optional and the strength of the formed part is perfect,but the disadvantage is that the forming cycle is too long.Whereas in PAHF,the production cycle can greatly shortened and the efficiency is greatly improved,but the strength of the PAHF formed part is slightly lower than that of HFQ?. |
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