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propagation in the weld seam. The rea- sons for this result are listed as follows. • The β phase distributed in α ' mar- tensite plates possess low creep strength and plastic deformation, resulting in high tensile strength. • In addition, the high dislocation density is detrimental intracrystal- line crack propagation. Therefore, cracks prefer to start and expand along the grain boundary. Conclusions Based on the above experimental analysis, the following conclusions were drawn: (1) Themicrostructure of theweld seam was characterised by TEM analysis and the results indicated that the weld seam consisted mainly of α ' martensite and β phases. The grain

size of the cosmetic weld zone is much larger than that of backing weld zone due to the different cool- ing rates. (2) With increasing heat input, the morphology of α '-martensite trans- formed from acicular at low heat input to platelet-like at high heat input. A thick β phase and high dis- location density makes the cracks form more easily and expand along the grain boundary during tensile testing. (3) As the heat input increases, the frac- ture surfaces changed from stretch and equiaxial type dimples under lowheat input to stretch and tearing elongation type dimples under high heat input. Therefore, lowheat input was selected for LAMIG welding of thick-section TC4 alloy.

the increase of heat input. Under low heat input in Figure 11(a), the fracture surface dimples were of the stretch and equiaxial type. As the heat input increases, the fracture surface exhibits stretch and tearing elongation type dimples. The above fracture morphologies were also researched by Matsumoto et al [15]. It can be concluded that when there is relatively high heat input, the main fracture mode is that the microcrack is formed at the crystal boundary, predominantly under exogenic action, and visible microvoids are generated by aggregation of adjacentmicrocracks. Then the microvoids grow, proliferate and finally they connect to each other to form the fracture. According to above analysis, high heat input is beneficial for the crack

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andmechanical properties of a thick- sectionhigh-strength steel welded joint by novel double-sided hybrid fibre laser- arc welding”; Mater Sci Eng A, 582, 284-293 (2013). [9] B Ribic, TA Palmer, T DebRoy: “Problems and issues in laser-arc hybridwelding”; IntMater Rev, 54, 223-244 (2009). [10] M Brandizzi, AA Satriano, D Sorgente, L Tricarico: “Laser- arc hybrid welding of Ti6Al4V titanium alloy: mechanical characterization of joints and gap tolerance”; Weld Int, 27, 113-120 (2013). [11] RF. Li, ZG Li, YY Zhu, L Rong: “A comparative study of laser beamwelding and laser-MIG hybrid welding of Ti-Al-Zr-Fe titanium alloy”; Mater Sci Eng A, 528, 1138-1142 (2011). [12] J Mazumder, WM Steen: “Microstructure and Mechanical Properties of Laser Welded Titanium6Al- 4V”; Metall Mater Trans A, 13, 865-871 (1982). [13] IUygur,IDogan:“TheeffectofTIGweldingonmicrostructure and mechanical properties of a butt-joined unalloyed titanium”: Metalurgija, 44, 119-123 (2005). [14] VN Moiseev: “The martensitic transformation during deformation of titanium alloys with metastable β phase”: Met Sci Heat Treat, 14, 391-395 (1972). [15] H Matsumoto, H Yoneda, K Sato, S Kurosu, E Maire, D Fabregue, TJ Konno, AChiba: “Room- temperature ductility of Ti-6Al-4V alloywith α ′martensitemicrostructure”: Mater Sci Eng A, 528, 1512- 1520 (2011).

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