Observations on impact toughness of electron beam welds of an α+β titanium alloy
Introduction
Electron beam welds, due to their low heat input are subjected to high cooling rates, as cooling rate is inversely proportional to heat input. Mechanical properties of α+β titanium alloys are sensitive to cooling rate 1, 2, 3, 4. High cooling rates produce thin α′ and α. These thin α′ and α plates provide poor medium for energy absorption and hence exhibit low toughness [5]. The fracture toughness of electron beam welds of α+β titanium alloys are composition sensitive. Alloys containing intermediate β stabilizer content exhibit low toughness [6]. Electron beam welds of alloy VT 9 are reported to have low impact toughness 7, 8. In the transformed β structures of β treated α+β titanium alloys 9, 10and in metastable β alloys [11]impact toughness is reported to increase with increase in the β grain size.
The present study is aimed to investigate the effect of starting base metal heat treatment history on fusion zone impact toughness of electron beam welds in an α+β titanium alloy (Ti–6.8Al–3.42Mo–1.9Zr–0.21Si). The actual composition of the alloy is given in Table 1. The alloy was obtained from Mishra Dhatu Nigam, India in the form of 100-mm square cross-section billets in the α+β forged condition. Effect of post-weld heat treatment in the supertransus and subtransus regions and the effect of heat input also forms part of the study. The observed toughness trends are explained on the basis of effective crack path length, crack path deviation and ductility. The positive effect of β grain size on toughness, as has been reported in respect of base metals 9, 10, 11, has not hitherto been reported in the case of welds, although it has been reported in titanium castings [12]. This study therefore assumes significance.
Section snippets
Experimental procedure
The base metal was subjected to two types of heat treatment namely, α+β (1233 K/1 h/AC/803 K/6 h/AC) and β (1303 K/15 min/FC (360 K/h)/803 K/6 h/AC). Butt joints of 10 mm thickness with allowance for post-weld machining were obtained. The electron beam parameters were: beam voltage, 150 kV; and beam current, 56 mA. Heat input was varied by varying the speed of welding. Beam focus was adjusted to achieve full penetration in all the cases. Details on speed of welding, post-weld heat treatment and
Results and discussion
In Fig. 2, the base metal microstructures and corresponding fusion zone grain structures are given. The base metal in the α+β heat-treated condition consists of equiaxed primary α and transformed β phases (Fig. 2a). The β treated base alloy consists of colonies of aligned α laths with a thin layer of β sandwiched between the α laths. This structure also consists of grain boundary α phase (Fig. 2b). The prior β phase grain size in the α+β condition is 35–50 μm, and in the β condition 300–350 μm.
Conclusions
(i) Increase in heat input resulted in improved toughness due to crack arrest effects of thicker α/α′ and wider fusion zone β grain width along the crack path and consequent crack path deviation at larger angles resulting in comparatively more tortuous crack path. (ii) β base welds exhibiting wider fusion zone β grain width along the crack path exhibited higher toughness due to mainly crack path deviation at large angles at the β grain boundaries prior to PWHT, and a combination of crack arrest
Acknowledgements
The research programme is funded by the Defence Research and Development Organisation, and we are grateful to the Gas Turbine Research Establishment, Bangalore for extending help in electron beam welding. We are thankful to one and all who have been either directly or indirectly responsible for the successful completion of the programme.
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