Multiscal Approach For The Damage Analysis Of An Aluminum Casting Alloy With Pore Stochastic Character

Casting defects are still unavoidable and challenging in the foundry industry.The randomness of the spatial distribution of the micro-defects results in the local inhomogeneous mechanical behavior of materials related to significant stress state,which makes the deformation and failure analysis of the casting become more complicated.In this work,the experiment and numerical simulation for cast aluminum alloys AlSi9Mn were carried out.The multi-scale research was conducted on the casting aluminum components considering the randomness of the shape and distribution of the voids,furthermore,the influence of the voids on the damage process of casting components under complex loading conditions was discussed in detail.The main conclusions and achievements obtained as follows:(1)The failure test of the cast aluminum components and metallographic analysis of the specimens cut from the components under the complex loading were performed.The effects of the stress state and the randomness distribution of the void on the failure mode of the castings were investigated.The meso-random characteristics of the castings caused by the casting process were considered to be a factor influencing macro-micro mechanical behavior.The results revealed that the deformation behavior and failure modes of specimens and components highly dependent upon the loading conditions and specimen type,and which have obvious dispersion.In additition,it was found that the microporous void was the main casting defect,and its size and distribution in the specimens and components were random.This randomness was the major reason for the large discrepancy of the test results.(2)The normal hexahedron representative volume element(RVE)containing mesoscopic voids was furtherly studied.The computationally tractable calculation method of macroscopic Cauchy stress tensor and logarithmic strain tensor of RVE was established.The loading in arbitrary stress state along different stress paths was successfully implemented by using such RVE embedded into the finite element software.On this basis,we can use the RVE to conduct the deformation and failure simulation of materials under complicated stress state and different stress loading paths.(3)A sufficiently large number of numerical investigation on the RVE with spherical voids in the center were carried out.The effects of initial porosity,stress triaxiality and Lode parameters on the mechanical behavior of materials were investigated.The results showed that the influence of initial porosity on the elastic modulus,Poisson's ratio and yield can be described by exponential function.Stress triaxiality has a significant influence on the deformation and failure of materials and the evolution of pores.However,the influence of the Lode parameter was slight.Based on the influence of initial porosity,stress triaxiality and Lode parameters on the macroscopic mechanical response of representative volume elements,considering porosity and stress state,a constitutive model of ductile damage was developed and a material failure criterion was proposed,and a user-defined material subroutine contained damage criterion was improved.(4)Based on the results of pore CT for cast aluminum components and specimens,using Markov Random Field to describe the shape and distribution of voids,a simulation software program to accurately account for the pore generation process was compiled and a new model was developed to be able to capture the random distribution of porosity for castings.The statistical results of void shape on average obtained by the model were consistent with data produced by the real casting process.The simulation confirmed that the model developed here can be used as describe inhomogeneous microstructure and random distribution of the pores in cast aluminum components.(5)In this work,we have proposed a damage constitutive model,where the real voids in the mesoscopic model were replaced by the porosity,and macro-micro computationally efficient simulations of the specimen and the component based on the proposed model were achieved.Using the model containing random voids and combined with the failure criterion proposed in this paper,defining the material parameters of the new model by the void ratio of specimen and component in different positions,a damage and failure method of simulation for cast aluminum at macro-micro scale considering random distribution of voids was established.The simulation results showed that the load-displacement curve,the failure mode and the influence of the pore distribution were well consistent with the experimental results.