Study On The Coefficient Of Linear Expansion Of Fixture Steel For Aluminum Alloy Brazing

Al （6063）alloy have accepted considerable attention and been used in many industries,such as aerospace, automotive, construction aluminum windows, power electronic equipmentand other industries because of their low density, high intensity, lower cost and fine corrosionresistance properties. In the Al（6063）alloy processing technology, brazing is one of theimportant processing technology, fixture involve the whole brazing process, therefore thedesign of fixture can directly affect the quality of weldment.The thermal expansion between fixture and weldment should be considered, to ensurethe fixture can prevent and decreases the deformation of weldment, and while ensure areasonable groove gap. The fixture must have certain degree of mechanical properties toensure the fixture without deformation and no loss of strength during brazing process. Thebrazing temperature of Al（6063）alloy is620℃which near eutectoid transformationtemperature of1Cr18Ni9Ti which using as fixture material by now, therefore may occurphase transition after long time using, then different thermal expansion between the fixtureand the weldment can lead to stress, it will affect the manufacturing precision and theperformance seriously.It is well known that coefficient of linear expansion of steel depends on characteristics ofconstituent phases, the coefficient of linear expansion of austenite is more larger than otherphases, Fe-13Mn-C alloy were heated to austenitic temperature followed by isothermal heattreatment for a certain time in order to carbide solution to the austenitic followed by quenchedinto water, so γ can be obtained, Ni helps to extend and stable austenite zone. This experimentwas designed to study novel Fe-Mn-C-Ni alloy with high expansion as fixture material, tunedcoefficient of linear expansion and mechanical properties for a better balance to meet theproduction requirements and improve the quality of weldment. Microstructure、mechanicalproperties and coefficient of linear expansion evolution of eight kinds of Fe-Mn-C-Ni alloywere studied during isothermal heat treatment for300h at620℃, effect of main element onmicrostructure、mechanical properties and coefficient of linear expansion were discussed, inorder to develop Fe-Mn-C-Ni alloy with high performance.The experimental results indicate that Fe-13.78Mn-0.095C-2.27Ni has betterperformance which is more suitable fixture steel for Al（6063）alloy brazing. For theFe-13.5Mn-xC alloy containing0.107%C、0.238%C、0.379%C, average coefficient of linearexpansion、impacting toughness and dilatation will increase by the content of C reduce. Thehardness and average coefficient of linear expansion increased first and decreeased later asholding time prolonging. Dilatation and the average coefficient of linear expansion ofFe-13.37Mn-0.107C is more close to dilatation and average coefficient of linear expansion of Al（6063）alloy during isothermal heat treatment. For Fe-xMn-0.1C containing13.37%Mn、16.96%Mn、19.62%Mn, average coefficient of linear expansion and dilatation will increaseand hardness is markedly depressed by Mn addition, dilatation and average coefficient oflinear expansion is greater than dilatation and average coefficient of linear expansion of Al（6063）alloy. However, impacting toughness is significantly increased by Mn addition. Forthe Fe-13.5Mn-0.1C-xNi containing2.27%Ni、4.78%Ni、6.89%Ni, average coefficient oflinear expansion, dilatation and hardness will decrease by Ni addition, impacting toughness issignificantly increased by Ni addition. Average coefficient of linear expansion and dilatationof Fe-13.78Mn-0.095C-2.27Ni alloy is more close to average coefficient of linear expansionand dilatation of Al（6063）alloy. Fe-13.5Mn-0.1C-xNi containing2.27%Ni、4.78%Ni havelinear increase as holding time prolonging, average coefficient of linear expansion ofFe-13.5Mn-0.1C-6.89Ni is basically invariant as holding time prolonging. The results indicatethat eight alloys used in experiment is composed of γ, ε-M and α′-M after solution treatment,the content of γ will decrease and the content of ε-M will increase as holding time prolonging,impacting toughness is decreased as holding time prolonging.