Theoretical Formulation
4.3 Laser microwelding
4.3.1 Macro/microstructural characteristics
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Figure 4.41(a) shows the feasible domain for a successful weld joint of welding speed and energy for a constant pulse width of 5ms and frequency of 20Hz. It is found that as the speed is increased, more energy is required to melt the metal. Beyond a certain limit of the laser power, burn out of the material occurs. The stable welding is obtained under the suitable combination of weld energy and speed within the range of the current study. Figure 4.41(b) shows the front and back surface appearance of a full penetration welded joint. It is observed that both bead formations are smooth and in a bright sliver color which indicates no oxidation in the joints.
Fig. 4.41 (a) Process map for laser microwelding of Ti6Al4V butt joint configuration as a function of welding energy and speed; (b) Appearance of the weld surface.
Figure 4.42 presents the macrographs obtained at different welding conditions. Different types of profiles with variation of heat input per unit length are reflected. The macrograph reveals elliptical shape of the fusion zone at low heat input (Fig. 4.42a). As the heat input increases, the shape changes from trapezoidal to an hourglass shape. Macrographs reflecting the effect of pulse width are shown in Fig. 4.43 and Fig. 4.44. It is seen that the fusion area at higher pulse width is more. Also misalignment of the sample is observed when the heat input is very low. Thus, the optimum choice of heat input is important to decide to produce the full depth of penetration along with better quality.
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Fig. 4.42 Macrographs: (a) 33.33 J/mm; (b) 43.33 J/mm; (c) 53.33 J/mm; (d) 63.33 J/mm.
Fig. 4.43 Macrographs: (a) 26.6, 5ms; (b) 26.6, 3ms.
Fig. 4.44 Macrographs at 35.5 J/mm: (a) 5 ms; (b) 3 ms.
Figure 4.45 shows the grain refinement in the three zones due to laser micro welding corresponding to heat input of 53.5 J/mm. The HAZ zone is small which is typical, due to the high energy delivered in a short period. It is observed that a significant grain growth in the HAZ where the average prior-β grain size is ~ 35 μm. The evolution of the grain size of the prior-β phase in the HAZ and the FZ with respect to heat input is represented in Fig. 4.45 (b). From Fig.
4.45(b) it can be observed that very large prior-β grains in the FZ with an average size of 153 μm exist.
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Fig. 4.45 (a) Macrographs showing different zones for Ti6Al4V weldment at heat input of 53.5 J/mm; (b) Prior-β grain size in the FZ and HAZ for the different welding conditions.
The rapid cooling rate in the weld pool, above the critical value for a full martensitic transformation, promoted the microstructural evolution to𝛼' martensitic structure though the microstructure. The microstructures of Ti–6Al–4V alloy weld metal with different parameters are shown in Fig. 4.46. There is no essential distinction among the four macrographs, and the martensitic phase transformation of all joints occurs to a certain degree. However, the percentage of martensite decreases with increase in the heat input.
Fig. 4.46 Microstructure of fusion zone for Ti6Al4V weldment at welding conditions of :(a) 8 J, 5 ms, 6mm/s; (b)9.5 J, 5 ms, 6 mm/s; (c) 9.5 J, 5 ms, 4.5 mm/s; (d) 9.5 J, 5 ms, 3 mm/s.
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Fig. 4.47 Weld bead dimensions with (a) pulse energy, and (b) welding speed.
Figure 4.47 (a) shows the variation of weld dimensions with pulse energy (proportional to heat input per unit length). The weld penetration increases with increase in heat input and reaches in full depth at a threshold value of pulse energy between 8 J to 9.5 J. Once the molten pool extents to full depth, the heat transfer is enhanced in thickness direction that essentially reduces the weld width. The optimum (minimum) condition of weld width avails when full depth of penetration just reaches. With further increase in heat input the size of weld pool increases and becomes steady at pulse energy of 12.5 J. In case of very low heat input, misalignment is prone to occur that reduces the strength of the weld joint. Thus full penetration welds are always desirable. Fig. 4.47(b) indicates that with increase in pulse energy and decrease in welding speed is having similar effect on weld dimensions [Kou, 2003]. There is marginal difference in weld dimensions at low pulse energy viz. low pulse width. In laser welding, sufficient penetration with an acceptable weld surface (width) may be regarded as a signature of good weld joint. Low depth/width ratio as well as low heat input (less than 64 J/mm) by laser source are not enough to produce a keyhole and suggests the conduction mode of welding for the selected range of parameters. Also the macrostructures reveal no defects such as underfill, undercut, porosity and solidification cracking. Thus the weld bead obtained is of acceptable quality.
Energy dispersive X-ray (EDX) analysis is performed to find out the elemental distribution before and after welding in order to estimate the oxidation rate. EDX analysis confirms the presence of the alloying elements (Ti, Al, and V). The parent metal shows the presence of very little amount of oxygen (Fig. 4.48a). At high temperature during welding,
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oxygen concentration increases which might lead to the decrease in usability of the fabricated sample in the industry. It is found to be the highest for lower scanning speed (3mm/s) as well the maximum heat input. The maximum oxygen percentage is ~ 11.4% which can be considered as in the acceptable range. This ascertains the robustness of the current laser micro welding method for titanium alloy.
Fig. 4.48 EDX analysis of Ti6Al4V alloy:(a) at welding speed 6 mm/s, 5 ms, 8 J; (b) at welding speed 6 mm/s, 5 ms, 9.5 J; (c) at welding speed 4.5 mm/s, 5 ms, 9.5 J; (d) at welding speed 3 mm/s, 5 ms, 9.5 J .