Research On Mechanical Behavior Of Mg/Mg Composites Fabricated Via Insert Molding And Co-extrusion

Magnesium is widely used as structural materials,due to the advantages of lightweight,high specific strength,and high specific stiffness.However,the conventional magnesium alloys have many undesirable properties,such as poor absolute strength at ambient temperature and low creep resistance and poor corrosion resistance.Mg-based alloys containing rare earth(RE)elements are achieving ultra-high strength,due to the combination of solid solution strengthening and precipitation hardening.however,the high cost of rare-earth-containing Mg alloys and their poor toughness greatly limit their applications.Fabricating the bimetal composites such as Mg/Mg may be the most effective way to meet the demands for lightweight and high-performance components since the composites can integrate the advantages of their base alloys.Here,the conventional magnesium alloys,such as AZ31,AZ91,and ZK60,with good plasticity as the matrix and WE43 as the enhanced structure were selected to realize the formation of high strength Mg/Mg composite materials.Firstly,three types of Mg alloy composite ingots were prepared by the Insert Molding method.They are AZ31/AZ91,AZ31/WE43,and ZK60/WE43 ingots.The interfacial bonding mechanism for the insert molding method can be explored based on understanding the microstructure formation and the evolution rule of the oxide layer at the interface.Secondly,the interfacial evolution rule during the heat deformation and heat treatment can be studied based on constructing the relationship among parent metal selection,processing parameters,microstructure,and properties.At last,high strength AZ31/WE43 and interface-reinforced ZK60/WE43 composite rods were fabricated by co-extrusion.Interfacial microstructure,mechanical behavior and interfacial reinforcement mechanism the Mg/Mg composite rods were systematically addressed.The rule of mixtures(ROM)for mechanical behavior of bimetal composite rods was systematically studied by tensile and compressive tests of two types of bimetal rods,AZ31/WE43,ZK60/WE43.The influence of the interface characteristic,interfacial microstructure evolution,interfacial reinforcement mechanism and the strain hardening behavior of two components on the ROM was addressed.The results of this thesis show as follows:1)Microstructural characteristics and formation mechanism of the as-cast interface.Due to the similar chemical element types of AZ31 and AZ91 alloys,the bonding can be mainly realized by the elemental diffusions such as A1 and Zn and no interface reaction occurred during the bonding.However,in the experiment of the different series alloys,cast bonding is mainly realized by the elemental chemical reactions and the formation of intermetallic compounds such as A12RE,A13Zr,and Y-rich phases can be observed.Thus it seems that during the compound casting process of Mg/Mg bimetal composites,the interface bonding can be divided into two types,i.e.without chemical reactions and with chemical reactions from elemental mixing and diffusions.That is,for the preparation of bimetal composites from the same series of magnesium alloys,the possible interfacial bonding can be realized only by elemental diffusions and no intermetallic compounds can be formed.However,if fabricating the bimetal composites by bonding different series of magnesium alloys such as AZ-series and WE series,ZK-series and WE-series and so on,the possible interface bonding mechanism can depend on elemental chemical reactions at the interface.2)The study on the interfacial characteristics,mechanical properties and interfacial evolution of composite ingots during heat treatment.After heat treatment,the AZ31/WE43 bimetal composites were found that the high melting-point Y-rich phases and Mg-RE(rare earth)phases at the interface were distributed more uniformly,so the interfacial strength was improved from 108 MPa to 120 MPa.Interfacial characterization revealed the presence of a(Mg,Zn)3RE phase(RE:Y,Nd,Gd),stacking faults,and variations in chemical composition at the as-cast ZK60/WE43 bonding interface.After heat treatment,interfacial LPSO structures,characterized as 14H and 24R types,were formed.The novel composites exhibited an interfacial shear strength(159 MPa)superior to that of base metals and other light bimetal composites due to the generation of LPSO structures and kink-deformation bands3)The investigation of the microstructure and mechanical properties of AZ31/WE43 bimetal composite rods.Compared to extrusion of a monolithic Mg billet,co-extrusion of the AZ31/WE43 bimetal billets does not change the typical extrusion texture of Mg sleeve(basal pole largely perpendicular to the ED).The component ratio is of the composite was stable and The lamellar phases at the solution interface were dispersed during extrusion,which can prevent the growth of grains.The interface of the AZ31/WE43 bimetal composite rod was well-bonded without any cracking.During the whole deformation process of the tensile and compression along the extrusion direction.The experimental flow curves consist with the predicted ones.When the WE43 fraction is 33.3%,the CYS increases by about 154%from 59 MPa to 154MPa.When the WE43 fraction is 50%,the TYS increases by about 40%,from 161 MPa to 225 MPa.At the same time,the elongation was increased by more than 30%.The improvement of mechanical properties mainly results from the containing of a hard WE43 core and the fine grains in the interface.4)The investigation of the interfacial reinforcement mechanism and tension-compression yield asymmetry behavior of the ZK60/WE43 bimetal composite rods.The LPSO phases(interfacial reinforcement structures)were arranged along the direction of extrusion after co-extrusion.The strengthening mechanism is similar to that of staple fiber in composite materials.The experimental curve of the ZK60/WE43 composite rods was higher than that predicted ones by ROM.When the fraction of the interfacial reinforcement structure is 10%,The experimental tensile yield strength is higher than that from the ROM prediction.The experimental one shows the greatest hardening effect of 57 MPa.When the fraction of the interfacial reinforcement structure is 6%,The deviation of the compressive yield strength from the ROM prediction is 43 MPa.The ZK60/WE43 rod also possesses a much lower tension-compression yield asymmetry than the monolithic ZK60 matrix.The much lower yield asymmetry mainly results from the containing of a hard WE43 core and interfacial reinforcement structures.