Influencing mechanism of pre-existing nanoscale Al₅Fe₂ phase on Mg-Fe interface in friction stir spot welded Al-free ZK60-Q235 joint

R.Z.Xu^1,2，Q.Yang^3,4，D.R.Ni¹，B.L.Xiao¹，C.Z.Liu²，Z.Y.Ma¹

1. Institute of Metal Research, Chinese Academy of Sciences2. College of Material Science and Engineering, Shenyang Aerospace University3. Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign4. Fredrick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign

摘 要：Al-free ZK60 magnesium（Mg） alloy sheet was selected as substrate material of Mg-steel pinless friction stir spot welding（FSSW）,avoiding the effect of the Al element in the substrate on the alloying reaction of Mg-iron（Fe） interface.The sound FSSW joint of ZK60 Mg alloy and Q235 steel with a hot-dipped aluminum（Al）-containing zinc（Zn） coating was succes s fully realized.The detailed micro structural examinations proved that Al₅ Fe₂ phase at the Mg-Fe interface came from the pre-existing Al₅ Fe₂ phase in the coating and acted as the transition layer for promoting the metallurgical bonding of Mg and Fe.The interfaces with well-matched lattice sites among Fe,Al₅ Fe₂ and Mg were formed during FSSW.A low energy interface with good match of lattice sites(（002）_{Al5 Fe2}//（110）_Fe,[110]_{Al5 Fe2}//[113]_Fe) between Al₅ Fe₂ and Fe was identified.For the interface between Al₅ Fe₂ and Mg,an orientation relationship of（622）_{Al5 Fe2}//（3112）_Mg and[158]_{Al5 Fe2}//[2423]_Mg was observed.The tensile^-shear load of the ZK60^-steel joint could reach 4.6 kN.Moreover,the joint fracture occurred at the interface between the Al₅ Fe₂ layer and the Mg alloy substrate,suggesting the brittle fracture characteristic.

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