African Fusion June 2018

Welding of thick titanium alloys

Figure 9: Fracture modes of the joints welded at (a): E=2.2×10 5 J/m, (b): E=3.2×10 5 J/m, (c): E=5.5×10 5 J/m

Figure 10: SEM fractographs at low magnification of the joints welded at (a): E=2.2×10 5 J/m, (b): E=3.2×10 5 J/m, and (c): E=5.5×10 5 J/m.

Figure 11: SEM fractographs at high magnification of the joints welded at (a): E=2.2×10 5 J/m, (b): E=3.2×10 5 J/m, and (c): E=5.5×10 5 J/m.

input was 2.2×10 5 J/m, the fracture propagated is in a straight line, perpen- dicular to the tensile direction. With the increase of heat input, the fracture path became serratedand irregular, as shown in Figure 9(b) and Figure 9(c). Figure 10 displays the SEM im- ages at lowmagnification of the fracture surfaces. As shown in Figure 10(a), a large amount of uniform dimples were observed, indicating that fracture mor- phology had the characteristic of ductile fracture. However, when the heat inputs were high, many stripe-like planes were presented as shown in Figure 10(b) and Figure 10(c). The high magnification SEM images of the fracture patterns are illustrated in Figure 11. Obviously, the morphologies of dimples significantly changed with

tensile strength of the joints welded at different heat inputs are depicted in Figure 8. For comparison, the tensile strength of base material is also given in Figure 8. It could be concluded that the ten- sile strength of the joints was decreased with increasing heat input. When the heat input reached 5.5×10 5 J/m, the tensile strength of the joint was even lower than the base material. To identify the fracture mode and the relationshipbetweenmicrostructure and mechanical properties, in situ ten- sile testswere conducted and the results are discussed below. Figure 9 illustrates the low magni- fication images of the fracture propa- gation paths of the joints produced at different heat inputs. When the heat

intracrystalline, which is accompanied by stress generation and release. When the heat input is low, the transition stress in the α '-martensite nucleation and growth is small and the dislocation density is also low. As the heat input increases, the formation of a large amount of α '- martensite clusters cause large phase transition stress and high dislocation density. Mechanical performance According to the above analysis, dif- ferent heat input resulted in various morphologies of platelet α martensite, β phase and dislocations. Meanwhile, the morphologies of platelet α '-martensite and β phase have a significant effect on the tensile strength of the joints. The

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June 2018

AFRICAN FUSION