Study Of Strength And Fracture Toughness Of A7N01 Aluminum Alloy

7N01 aluminum alloy is one kind of Al-Zn-Mg series alloys,which can be enhanced by heat treatment.It has high strength,good extrusion properties and good weldability,and is widely used in China’s high-speed trains manufacturing.It is well known that the fracture toughness is an important factor for materials and the heattreatment affect this factor largely.So,it is necessary to study the fracture toughness of 7N01A1-alloy and its welded joint.In the first part,A7N01 aluminum(Al)alloy is treated by five heat treatment methods as peak aging(T6),over aging(T74),high temperature and subsequently low temperature aging(HLA),retrogression and reaging(RRA)and double retrogression and reaging(DRRA).The strength and fracture toughness of the five samples are tested,and the microstructure is investigated by optical microscopy,transmission electron microscopy,and scanning electron microscopy.The results show that A7N01 treated at T6 condition has high strength but low fracture toughness.Compared with T6 treatment,T74 and HLA treatments increase the fracture toughness by 67%and 90%respectively,while the strength decrease by 9%and 17%.RRA process is a proper treatment method for A7N01 which improves the fracture toughness without sacrificing strength.The fracture toughness of DDRA treated alloy is much lower than that of treated by RRA.Quantitative analysis through TEM images shows that the heat treatment affects the mechanical properties of A7N01 Al alloy highly through changing the precipitates in grains and on grain boundaries,which can be explained by the coherency strengthening mechanism and Orowan mechanism.Secondly,A7N01-T5 Al alloy plates are welded by hybrid laser-MIG welding technology.The mechanical properties of the base metal(BM),heat affect zone(HAZ),and welding zone(WZ)of the hybrid welding joint werestudied.The yield strength of the samples from different zones decreases in the order of BM>HAZ>WZ,the UTS follow the order of BM≈HAZ>WZ,the elongation order is WZ>HAZ>BM,and the fracture toughness decreases in the order of HAZ>BM>WZ.To analyze the different mechanical properties,the microstructure was observed by optical microscopy and electron back scattered diffraction,whilethe fracture surface and precipitation were observed usingscanning and transmission electron microscopy.The microstructure,grain size,and distribution of precipitatesformed under the effect of hybrid welding heat can explain the different yield strength and UTS with Orowan mechanism.The as-cast microstructure and second phase particles segregating on grain boundaries and between dendritic branches made the WZ possess the lowest yield strength and UTS.The different strength between matrix and grain boundaries is affected by the distribution and size of precipitates,width of precipitation free zone(PFZ)and the anisotropy of grains.These are the main factors that affect the fracturemode and toughness of the samples from different zones.