The Oxidation And Ignition Mechanisms Of Magnesium Alloys During Resin-sand Differential Pressure Casting

In order to meet the requirement of weight reduction in the aerospace industry,the magnesium alloy components tends to be large and complicated.Resin sand technology,especially PEP-SET resin sand technology,was widely used to ensure the dimensional accuracy and surface quality of castings.However,as the PEP-SET resin binder was subjected to a high temperature enviroment during the casting process,it would pyrolyse and release an oxidizing atmosphere,causing the ignition of magnesium alloy melt during solidification.Counter-gravity casting process requires the castings to be solidified in a closed tank with high pressure,so the castings more easily get ignition and explosion than gravity casting.For solving the ignition problem of magnesium alloys during PEP-SET resin sand casting process,this research systematacially analyzed the influence of temperature rising conditions on the pyrolysis kinetics of PEP-SET resin sand and the evolution of gas products,and studied the oxidation behaviors of magnesium alloys in the PEP-SET resin sand mold.The fracture mechanism of the protective oxide layer on t he melt surface due to stress was proposed,and the connection between the ignition and the casting process parameters was clarified.The pyrolysis processes of PEP-SET resin sand were detected using thermogravimetric analysis coupled with Fourier transform infrared spectroscopy?TG-FTIR?and on-line FTIR methods.The non-isothermal pyrolysis of PEP-SET sand can be subdivided into three stages: emission of inherent moisture?below 215??,thermal decomposition at low temperature?215-315??and further cracking process at high temperature?315-432??.The main emission gaseous products in pyrolysis processes were H2 O,CO2,NH3,CH4,C₂H₆,NO and a little amount of CO,according to the FTIR spectrum analyses.PEP-SET resin sand mainly released H2 O in the first pyrolysis stage.CO appeared in the second stage with the increasing of N-containing products.The amount of N-containing pyrolysis products further increased in the third stage,which was generated from cracking of the C-N bond,reflecting the collapse of the resin binder.The estimated activation energy and pre-exponential factors for the equilibrium pyrolysis of the PEP-SET resin sand were 29.9k J/mol and 25.2s-1 respectivly at high temperature stage.Those for low temperature pyrolysis stage were 6.40 k J/mol and 0.017s-1,respectively.At temperatures below 500?,PEP-SET resin sand released gas slowly,and no peaks appeared during thermal shock.With the increasing of thermal shock temperature,the peaks of the released gas becomed more and more obvious.However,regardless of the thermal shock temperatures,these gases released follow a fixed sequence: concentrations of CO,CO2 and NO firstly increased to the peaks and then decreased with the rising of CH4 and C₂H₆.At last the CO2 concentration increased again.Using the self-built oxidation test device,the oxidation and ignition processes of the magnesium alloy melt in cavity were observed.The oxidation behaviors of ZM2,ZM5,ZM6 and WE43 alloy melts in resin sand atmosphere were systematically analyzed.Among them,the surface oxide layer of WE43 alloy showed the best protective effect to the melt.Although ZM2 contains a small amount of rare earth Ce,ZM2 most easily get ignition.The oxidation resistance of magnesium alloys was not only related to the amount of active elements,but also related to the solid solubility of the active elements in matrix and their influences on the solidification temperature of the alloys.The main component of the surface oxide layer of ZM6 alloy was a mixture of MgO and Nd₂O₃.Nd₂O₃ can effectively inhibit the outward diffusion of Mg and play a protective role for the melt.During oxidation,Nd and Zn elements were enriched at the interface of ZM6 oxide layer and the melt,while the Zr element diffused uphill into the melt.The growth of surface oxide layer on WE43 melt included three stages.In the early stage,a three-layer structured oxide layer was quickly formed: the outer layer was a mixture of Y₂O₃,MgO and Zr O2,the middle layer was MgO,and the inner layer was mainly Y₂O₃.The second stage was the growth of MgO in the middle layer,and this growth process was controlled by the diffusion rate of oxygen in Y₂O₃.The third stage was the reduction of MgO in the middle layer.The surface oxide layer was protective to the melt during these stages.The ignition mechanism of magnesium alloy melt under the coupling effects of mold atmosphere and thermal environment was studied.It was clarified that the fracture of the protective oxide layer is the direct cause of magn esium alloy melt ignition in mold cavity,and the fracture mechanism of oxide layer due to internal stress was proposed.The fracture process of oxide layer on WE43 caused by internal structural transformation during isothermal oxidation was discussed in detail.It was pointed out that the critical thickness of inner layer Y 2O3 before the fracture of oxide scale increased with the rising of oxidation temperature.This phenomenon was casued by the decreasing of growth strain parameter C of inner layer Y₂O₃ with temperature increasing.Based on the existing MgO mechanical property parameters,the fracture map of magnesium alloy oxide layer was established.It was found that the surface oxide layer on magnesium alloy mainly fractured through buckling mechanism during the cooling of the melt.The relationship between the surface oxide layer fracture on magnesium alloy and the casting process parameters was established,which provided guidance for the design of ignition-proof technology for complex magnesium alloy castings.Moreover,the ignition-proof technology for large magnesium alloys counter-gravity casting process was proposed based on the oxidation and ignition mechanisms of magnesium alloys in mold atmosphere.