2.1

Classification of lock-bottom structures

A vital step is seam welding for pressure vessels used in aerospace. The joint is frequently welded by the lock bottom construction without wire filling and groove opening in order to reduce the production of welding residues and maintain compatibility between the weld metal and the medium in the vessel. As illustrated in Figure 1., the four types of bottom locking structures that are currently most frequently employed are the lapped L-type bottom locking structure, the butt U-type bottom locking structure, the butt T-type bottom locking structure, and the butt T-type substrate structure. The above four structures belong to the non-penetration welding structure, but the design penetration depth is required to be greater than the base plate thickness. The technical difficulty of a laser-welded titanium alloy lock bottom structure is to ensure the porosity problem with a width-thickness ratio of less than 3. Compared with penetration welding, the bottom-locking structure blocks the overflow channel of the bubble root, and the difficulty of pore suppression increases. Compared with the flat surfacing weld, there is an additional transverse gas entry channel, which increases the porosity sensitivity [5–6].

Figure 1.

Type of lock bottom structure

2.2

Porosity inhibition state

Process pores and metallurgical pores are the two basic categories of pores in titanium alloy laser welding. The generation of porosity is mainly due to the instability of the keyhole during laser welding. Metallurgical pores are pores produced in the welding environment and metallurgical process. The types of pores are divided into low-temperature insoluble gases such as hydrogen, nitrogen, and oxygen. Metallurgical porosity is due to the large solubility of hydrogen, oxygen, and nitrogen in titanium alloys at high temperatures, and laser welding has the characteristics of being extremely cold and extremely hot. Therefore, the solubility of this kind of gas decreases and precipitates in a large amount during the cooling process, and the gas that cannot escape before the solidification of the molten pool is left in the weld to form porosity defects [7–8]. Therefore, the inhibition of metallurgical pores is achieved by cleaning the weld before welding and adding protective gas during welding. At present, the titanium alloy lock bottom weld of an aerospace pressure vessel is usually treated by pickling and scraping before welding. The results show that there are only small pores in the weld. Based on the current laser welding technology scheme combining pulse, galvanometer, and hybrid heat source, it is not difficult to find that it mainly focuses on solving the stability of keyhole-type pores. According to literature reviews conducted both domestically and overseas, the keyhole’s instability causes the weld to have pores in the form of tiny holes. According to Figure.2 [9–10], this type of pore has irregularly shaped pores that are large in size and have an uneven inner wall. There are two sources of keyhole-type pores: one is the involvement of protective gas, and the other is the formation of bubbles by metal vapor eruption, which generally exists in the middle and lower parts of the weld, as shown in Figure. 3a [11].

Figure.2

Pore defects of titanium alloy process type

The following describes how the keyhole-type pore forms: First, the bubble’s speed of floating is slower than the metal in the molten pool’s rate of solidification. Under the acceleration of gravity, the upper port of the molten pool closes before the bottom half, trapping the bubble in the weld. The second is to keep the keyhole stable while being subjected to the combined forces of surface tension, the gravity of the liquid metal in the molten pool, and the recoil force of the metal vapor. As seen in Figure 3b [12–13], when this equilibrium is upset, the molten metal will collapse and the bubbles or shielding gas will wrap around the lowest portion of the molten pool to generate pores. Additionally, titanium alloy has a low density, a tiny floating pressure difference between the liquid metal and the bubble, and a low bubble overflow rate. At the same time, the viscosity coefficient of the liquid metal is large, and the movement of the keyhole tip lags behind the backflow of the molten pool metal. Therefore, the tip of the unpenetrated keyhole makes it easy to wrap the unspilled bubble in the weld.

Figure. 3

The stress state of pore formation and the schematic diagram of pore formation

Therefore, to decrease the porosity of keyhole type, one must increase the stability of the keyhole, decrease bubble creation, and minimize protective gas absorption; the other must improve the waist shape of the inner diameter of the keyhole and increase the bubble’s capacity to escape.

3.1

Pulsed laser welding

A welding technique known as pulsed laser achieves fine energy management through the intermittent input of laser energy. The equipment is not much different from a continuous laser. The process parameters mainly include welding speed, pulse frequency, pulse width, pulse energy, defocusing amount, and so on. At present, scholars at home and abroad have applied pulsed laser welding to the research and application of high-strength steel, stainless steel, aluminum alloy, and titanium alloy. The findings demonstrate that, as shown in Figure. 4 [14], the welding speed has the greatest impact on the quantity of pores, followed by pulse frequency, pulse length, pulse energy, and defocus. In the laser non-penetration welding of titanium alloy structures, it is found that the porosity in the weld can be reduced by adjusting the process parameters of the pulsed laser. When the laser pulse frequency is consistent with the vibration frequency of the molten pool metal, the pores in the molten pool can be well suppressed [15]. In addition, it is found that the smaller the assembly gap and the higher the spot remelting rate, the better the inhibitory effect on the pores [3, 16].

Figure.4

Effect of welding parameters on porosity formation

3.2

Galvanometer laser welding

The galvanometer laser is a technology that uses the galvanometer system to make the beam output energy in the form of a scanning trajectory and perform dynamic welding. The fundamental distinction from a continuous laser is in the laser head. Galvanometer laser technology has recently come into use, which has the advantages of fast welding speed, high precision, and variable trajectory energy. It also enhances the tolerance of single laser welding to assembly gaps. The laser’s power, welding rate, degree of defocusing, scanning path, scanning amplitude, and scanning frequency are its primary process characteristics. The beam oscillation improves the temperature field distribution and heat and mass transfer characteristics of the molten pool. At the same time, the oscillation of the laser beam stirs the molten pool and stabilizes the keyhole to suppress the pores [17]. The scanning mode, scanning amplitude, and scanning frequency have all been thoroughly investigated so far. The findings demonstrate that the circular trajectory mode can successfully prevent the development of stomata and that both the scanning frequency and amplitude have a threshold value. The effect of suppressing stomata can only be realized when the threshold value is exceeded [18–19].

3.3

Laser-arc hybrid welding

Laser-arc hybrid welding is a processing technology that consists of two or more heat sources to achieve its functional effect of 1 + 1 > 2. The hybrid method mainly includes coaxial and side axes. There are various types of heat source hybrids, such as laser-TIG, laser-MIG, and laser-MAG. The primary process variables include heat source spacing, voltage, current, defocusing amount, laser power, welding speed, and others. Laser-arc hybrid welding provides the advantages of a high energy utilization rate, quick welding speed, and great bridging ability over single laser welding. Through the interaction of laser-induced plasma with arc plasma, laser hybrid welding improves the transmission efficiency of the beam energy and causes the arc to transition from a stable state to a periodic oscillation state. When the oscillation frequency is equivalent to the keyhole frequency, the process stability of hybrid welding is improved, so that the pores have sufficient time to overflow [20–21].

3.4

Effect of laser welding process parameters on porosity

Pore formation is significantly influenced by the laser welding process parameters. The method of one factor and one variable will be used to analyze the following: With the increase in pulse frequency, the remelting rate of the laser solder joint increases, the diameter of the keyhole opening increases, and the solidification rate of molten pool metal decreases, so the probability of pore overflow increases, but this is not the case. Some scholars have found that the pulse frequency has a threshold value for the inhibition of pores. When the laser frequency matches the laser welding keyhole oscillation frequency, the pores can be effectively suppressed [15, 22–23]. With an increase in laser peak power comes an increase in heat input, a broader weld pool, and a higher penetration depth, which have two effects on pore inhibition. First off, as metal vapor concentration rises, more pores may form; secondly, as heat output rises, the molten pool’s solidification is delayed, which promotes bubble overflow. Under the assumption of a single-factor variable, the increase in laser power has more drawbacks for non-penetration welding than advantages [24]. The pulse width has an important influence on the matching of the keyhole switch. The pulse width is large, the laser peak power lasts for a long time, and the pores have a long overflow. The larger the pulse width, the better the inhibition of pore formation. This requires adjusting the pulse width to coordinate the pore overflow time and the molten pool metal reflux time [25]. The primary element influencing the weld’s line energy is welding speed. The weld pool’s vortex motion is stable as welding speed rises, and the stability of the keyhole also improves. This is because the keyhole is getting smaller and the molten metal is cooling more quickly, which increases the viscosity of the molten metal at the back of the keyhole, increases the surface tension, and improves the stability of the keyhole, decreasing the likelihood of pores forming and decreasing the porosity. In addition, molten metal is difficult to collapse during welding at a fast speed because of the upward eruption of metal vapor that hinders the participation of shielding gas, as shown in Figure 5 [26–29].

Figure.5

Effect of welding speed on keyhole and weld pool morphology,

The key parameter used to regulate the energy density of the laser welding action zone is the amount of defocusing. The upper surface of the specimen is typically zero during welding. Defocusing turns out to be a successful method for suppressing the weld porosity. The reduction in the energy density of the laser action zone caused by an increase in positive defocusing promotes the reduction of metal vapor pressure, expansion of keyhole opening size, and slow keyhole closure rate, all of which are advantageous to pore overflow [30–32]. Secondly, the increase in negative defocusing amount reduces the ratio of waist diameter to root diameter and improves the condition of the bubble overflow channel. Protective gas is an indispensable guarantee for laser welding instead of vacuum electron beam welding. At present, the protective gas by laser welding is mainly divided into active gas and inert gas. Titanium alloys are easily oxidized under high-temperature conditions, so inert gas becomes an inevitable choice. In addition to preventing hydrogen, oxygen, and nitrogen in the atmosphere from entering the weld, the inert gas can also control the photo-induced plasma and maintain keyhole stability [10, 33]. The scanning mode is the trajectory of the spot, which makes the energy density of the laser action zone more uniform; the flow of the molten pool has directionality; and the diameter of the keyhole is increased. In a variety of scanning paths, the circular trajectory can effectively suppress the pores. This is due to the steady keyhole trajectory throughout, which minimizes bubble formation. The bubbles at the keyhole’s end will migrate simultaneously to the molten pool’s center, which encourages bubble overflow [34–35]. The scanning amplitude has an important influence on the diameter of the keyhole. As the amplitude increases, the diameter of the keyhole orifice increases, which is conducive to stomatal overflow [36]. The scanning frequency provides a high-speed beam movement, which reduces the blowhole overflow distance by changing the penetration depth. The study found that the larger the scanning amplitude, the smaller the scanning frequency threshold required to eliminate the pores [34].

Through the analysis of the influence of the above process parameters, it is not difficult to find that under the condition of constant laser power, the macro performance of solving the porosity problem is that the molten pool becomes wider and the penetration depth becomes smaller. However, this will affect the structural strength and leave the designer dissatisfied with the design requirements. In the single factor variable, each parameter has an effect on the pore inhibition, but it cannot be adjusted by a single variable to make the porosity of the weld reach the level I requirement.