3.2. Formation mechanism of process porosity
Figure 6(a) illustrates the morphology of the process porosities whose sizes range from 130 µm to 380 µm, and the shapes are full ellipses. The microstructure of the inner wall of the process porosity and the EDS results detected at the marked zone 1 and zone 2 are shown in Fig. 6(c-d). According to the EDS results, the substance in Zone 1 should be an oxide or other impurity. Besides, Fig. 6(d) reveals that the content of the Al element was reduced, while the content of the V element increased compared with the base metal.
Given the above analysis of the morphology, element characteristics of process porosity, the formation mechanism of process porosity formed during laser welding of the bottom-locking joint is illustrated in Fig. 7. It is worth pointing out that the oxidation film on the bulge is hard to be polished thoroughly, and the presence of oxides will affect the stability of the molten pool and the keyhole. Under the action of the high-energy laser heat source, the metal on the surface of the workpiece will melt and produce metal vapour. The keyhole is formed under the continuous induction of the reaction force generated during the continuous injection of metal vapour, and it will be inclined relative to the welding direction during the welding process. Besides, because the laser is continuously reflected and absorbed on the keyhole wall, the diameter of the keyhole will gradually decrease along the plate thickness direction.
Figure 7(b-g) shows that the formation of process porosity is the result of the combined effect of two mechanisms in the process of laser welding the Ti-6Al-4V bottom-locking joint. As described in Fig. 7(b-d), one is that the tip of the keyhole is closed by the strong molten metal vortex, which causes the inner wall of the keyhole to stick, thereby the process bubble is separated; the other is that the local laser energy concentration causes the metal element at a certain position on the front wall of the keyhole to evaporate, forming high-speed metal vapour streams. The back wall of the keyhole is dented under the impact of the metal vapour stream, and involves the shielding gas, then collapses under the dynamic behaviour of the molten metal vortex, forming a process bubble. Figure 7(e-g) illustrates the whole process of this formation mechanism.
Whether the bubbles can turn into porosities depends on the escape velocity of the bubbles and the solidification velocity of the molten metal. When the escape velocity of the bubbles is greater than the solidification velocity of the molten metal, the bubbles can escape and no porosities will be formed in the weld seam. Nevertheless, the fast welding speed leads to a fast solidification velocity of the molten metal, and the bubbles are easily captured by the solidified metal and remained in the weld seam. The Stokes Law is expressed as follows:
\({\text{V}}_{E}=\frac{2}{9}\times \frac{\left({{\rho }}_{\text{L}}\right.-\left.{{\rho }}_{\text{G}}\right){{\rho }}_{\text{G}}\text{g}{\text{R}}^{2}}{{\eta }}\) (1)
where VE is the escape velocity of the bubble, ρL is the density of the molten metal; ρG is the density of the gas in the molten pool, g is the acceleration of gravity, R is the radius of the bubble and η is the viscosity of the molten metal.
Process porosity is large in scale and therefore has a large escape velocity. However, the solidification process of molten metal during laser welding is also very rapid. These two factors together lead to the distribution characteristics of process porosities, which process porosities mainly distribute in the weld seam centre.
Additionally, the morphology and element characteristics of process porosity can also be explained. The process bubble is essentially a cavity formed by the collapse of the keyhole, so it has the characteristic of large size. Since the formation of process porosity is the result of the joint action of two mechanisms, some shielding gas or metal vapour, which makes the process bubble maintain a high and stable saturated vapour pressure, is always involved when the process bubble is formed. Meanwhile, under the action of the surface tension of the surrounding molten metal, the process bubble tends to shrink into a spherical shape. When the produced process bubble is long strips, the bubble will shrink into several spheres in series, and finally forms chain-spherical process porosity, as shown in Fig. 8(a).
Figure 8(b) illustrates the microstructure of the process porosity and the EDS results in Zone 1. It is worth noting that there are many micropores with a size of 2–5µm, and the EDS results of the micropore reveal that there is no obvious difference in element contents compared with the inner wall of the process porosity. The element characteristics and the formation mechanism of the micropore are explained in Fig. 8(c-f). For the element characteristics, that is, the decrease of Al element and the increase of V element in the inner wall of the process porosity. This is because the process bubble contains V and Al vapour inside, and the freezing point of the V element is relatively high causing V vapour to condense on the inner wall of the process bubble during the cooling stage. The condensation of V vapour causes the vapour pressure in the process bubble to drop. At the same time, the Al vapour has not yet solidified because of its low freezing point, so it quickly escapes under the action of the pressure difference between the inside and outside of the process bubble. This difference in the freezing point of metal elements leads to the accumulation and loss of elements in the inner wall of the process porosity.
In view of the above analysis of element characteristics of the process porosity, the formation mechanism of these micropores can be explained in depth. The formation process of the micropore goes through three stages, equilibrium stage, negative pressure stage, burst stage. When the molten pool has not been cooled and solidified, the vapour pressure inside and outside the bubble is roughly equal, and it is in the equilibrium stage; thereafter, as the V vapour condenses, the vapour pressure inside the bubble decreases, the pressure difference between the inside and outside of the bubble causes the Al vapour to compress, and it is in the negative pressure stage; compressed Al vapour is ultimately formed strong streams, which break through the bubble causing the gaps on the surface of the bubble, it is in the burst stage. Because the surrounding molten metal is about to solidify at this time, the process bubble has not yet shrunk into a regular shape under the action of surface tension, and it is finally captured by the solidified metal and forms process porosity with a large number of micropores.
In addition, Fig. 9. shows the microcrack on the surface of the micropore and its formation mechanism. The tensile stress generated during the solidification and shrinkage of molten metal is the main factor leading to microcracks. In this process, the micropores are sensitive sources of microcracks, which are subjected to different degrees of tensile stress, then the microcracks form and extend on the surface of the micropores.
3.3. Formation mechanism of metallurgical porosity
Figure 10. shows the morphology of the metallurgical porosity and the EDS results. The inner wall of the metallurgical porosity is smooth with a few impurity particles, and there is no obvious accumulation or loss of elements. The formation of the metallurgical porosity for Ti-6Al-4V titanium alloy is mainly owing to the H2O, which derives from the oxidation film and the surrounding atmosphere. In the high-temperature molten pool, H2O will directly decompose or react with metal elements to generate hydrogen atoms.
In the nucleation stage, supersaturated hydrogen atoms nucleate at the solid-liquid interface and the suspended impurities. The nucleation energy required for nucleation satisfies the following equation:
\(\text{W}=-\left({\text{P}}_{\text{G}}-{\text{P}}_{\text{L}}\right)\text{V}+{\sigma }\text{S}\left[1-\frac{{\text{S}}_{\text{a}}}{\text{S}}(1-\text{c}\text{o}\text{s}{\theta })\right]\) (2)
where W is the energy required for bubble nucleation, PG is the pressure generated in the bubble, PL is the pressure generated by the external liquid, V is the volume of the bubble, σ is the tension between the phases, Sa is the active area of the adsorption force, S is the bubble nucleus surface area and θ is wetting angle.
During the laser welding process of the Ti-6Al-4V bottom-locking joint, the metallurgical bubbles are more likely to nucleate at the suspended impurities and the solid-liquid interface in the molten metal at the crystallization front. At this time, θ is any value, the ratio of Sa/S is large, and the nucleation energy required for metallurgical bubbles to attach to these surfaces is the smallest.
Figure 11. shows the formation mechanism of metallurgical porosity. The solubility of hydrogen in titanium decreases with the increase in temperature. Consequently, in the high-temperature molten pool, the hydrogen is supersaturated and precipitated on suspended tiny impurities or solid-liquid interface, forming a metallurgical bubble to flow in the molten pool. As described in Eq. (1), the metallurgical bubble is small which results in a low escape velocity for it. In the process of rapid cooling of molten metal, the metallurgical bubble is easily captured, thereby forming metallurgical porosity at the bottom of the weld seam. Figure 11(c-d) shows the process of metallurgical bubbles being captured by solidified metal.