| This article takes two types of AZ91 materials with and without Ca as the research objects,and uses a Gleeble-1500D thermal simulation testing machine to conduct hot compression experiments.The high-temperature rheological stress behavior characteristics of these two materials during hot compression deformation are studied,and four high-temperature constitutive models are established for experimental prediction and fitting accuracy comparison.Finally,the most suitable constitutive model for the characteristics of these two alloy materials is determined.The hot working diagrams of the above two alloy materials were constructed and drawn,and the best process windows of the two materials were proposed by comparing and observing the microstructure.The main research findings are as follows:(1)The addition or absence of Ca does not significantly affect the high-temperature rheological curve characteristics of AZ91 alloy materials.Experiments show that AZ91 alloy is a temperature and deformation speed sensitive material,because its flow stress value decreases with the increase of deformation temperature and increases with the increase of strain rate.At the same time,the flow stress increases rapidly at the initial stage of processing deformation,and as the strain increases,the flow stress value reaches a peak value and then decreases,Therefore,it is a typical dynamic recrystallization feature.However,the high temperature strength of alloy 1 with Ca added is higher than that of alloy 2 without Ca added,and the peak strain of alloy 1 is smaller than that of alloy 2,indicating that Ca has the effect of weakening the work hardening phenomenon,which also leads to differences in the accuracy of the same constitutive model.(2)In this paper,based on the experimental values of high-temperature rheological stress and strain in thermal compression experiments,four high-temperature constitutive models have been established to more accurately predict and describe the stress-strain relationship.By comparing the calculation amount,fitting accuracy,applicability,and other aspects,it is found that Model 1 has a large calculation amount and limited fitting accuracy;Although the calculation amount of Model 2 is small,which can reflect the characteristics of work hardening during processing,the fitting accuracy is still low;Although the fitting accuracy of models 3 and 4 is higher than that of the first two models,model 4 has the highest fitting accuracy,indicating that model 4 is the best model.Represent the specific results of applying the optimal model 4 to alloy 1 and alloy 2.The curve of the first stage of alloy 1#AZ91 can be represented as:According to equations(3-25)-(3-28),the softening stage after the peak stress can be expressed as:The first stage curve of 2#AZ91 alloy can be represented as:The second stage nonlinear function is expressed as:(3)By calculating the strain rate sensitivity index m and dissipation rate factor of the hot working zone at strains of 0.1,0.15,and 0.2 η,Instability criterion ξ,The hot working drawings for materials 1#and 2#have been determined.A comparative analysis of the instability zone and safety zone of two alloy materials was conducted.It was found that when the strain was 0.2,the 1#material added with Ca had fewer instability zones than the 2#material,indicating that Ca was beneficial to improving the processing ability of AZ91 alloy material.Secondly,by comparing the applicable range of deformation temperature and the unstable region of processing rate of the two materials,the machinability region of the two materials is determined,namely,the suitable range of temperature for the process window of 1#material:300～335℃ region,and the suitable range of processing rate:10 s-1 region;The suitable temperature range for the process window of material 2#:330～375℃,and the suitable processing rate range:0.1 s-1. |
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