Investigation of the phase composition and mechanical characteristics of laser welded joints of aluminum-lithium alloys

A study of laser welding of modern aluminum-lithium alloys has been carried out. Optimization of post heat treatment of laser welded joints has been carried out. The change in the structural-phase composition of welded joints was investigated. The strength of welded joints after heat treatment was equal to the strength of the base alloy.


Introduction
Increasing the weight efficiency is traditionally considered one of the main tasks of modern aircraft and rocketry. One of the possible solutions to this problem is the use of aluminum-lithium alloys in welded structures.
Modern high-strength thermally hardened aluminum-lithium alloys are considered one of the most promising for use in the aerospace industry, due to their high mechanical characteristics: strength, stiffness, ductility, machinability and corrosion resistance. This allows them to compete with traditional aluminum alloys and polymer composite materials. High mechanical properties of these alloys are provided due to special thermomechanical treatment, which results in the formation of strengthening phases δʹ(Al3Li), T1(Al2CuLi), T2(Al6CuLi3), θʹʹ, θʹ (Al2Cu), θ (Al2Cu), R (Al5CuLi3), TB(Al7Cu4Li). Depending on the ratio of the concentrations of the main alloying elements Cu, Mg Li, the role of these phases involved in the hardening mechanism is different.
When creating structures and products from these alloys in aircraft construction, the technology of riveted joints is usually used, the main disadvantages of which are high labor intensity, negative impact on the environment and humans. The use of riveted joints is largely due to the fact that the ultimate strength of various types of welded joints of these alloys is 0.6-0.85 of the strength of the base material. A decrease in the mechanical properties of welded joints is associated with a sharp change in the structural-phase composition of the weld in comparison with the original alloy. The aim of the work is to obtain high-strength laser welded joints of aluminum-lithium alloys with different concentrations of alloying elements by optimizing the operating modes of laser during welding and post heat treatment of the obtained samples.
At the initial stage, the laser welding process was optimized in order to obtain welded joints without external defects in the form of open porosity, undercuts, non-penetrations, cavities, discontinuities, cracks.
The criterion for the quality of the internal microstructure and morphology of the laser welded seam of butt joints was the minimum porosity, as well as equality of the width of the upper and root parts and obtaining an X-shaped butt weld with two curved bevels of two edges [8]. An X-shaped weld can have advantages in mechanical properties due to a more symmetrical centerline weld [9].
The range of variation of the laser power W was 2-3.5 kW, the position of the focal spot of the laser radiation relative to the surface of the workpiece was -3 to +3 mm, the welding speed V was 2-5 m / min (33.3-100 mm / s), the consumption gas in the nozzle was 3-15 l / min. Laser was focused using a ZnSe lens with a focal length f = 254 mm. The diameter of the laser radiation incident on the lens is D = 30 mm. The focused beam diameter at the focus is ≈168 µ for this type of CO2 laser. Figures 1 and 2 show typical optical photographs of the macrostructure of the weld cross-section when optimizing the welding speed at constant laser power for alloy 1420 of the Al-5.2Mg-2.1Li system and V-1469 of the Al-3.9Cu-0.3Mg-1.2 system. Li. A laser power was 2.7 and 3.3 kW for alloy 1420 and V-1469, respectively, focus deepening was -3 mm.  The minimum value of the coefficient k for the weld of alloy 1420 is achieved at two values of the laser power, 2.7 and 2 kW. At a power of 2 kW, there is no X-shape of the weld, and undercuts and sagging of the weld are also observed. For a welded seam of alloy B-1469, the coefficient k decreases with increasing laser power. The X-shape of the welded seam is achieved at a power of 3.3 kW.
At a welding speed of 4 m / min and a laser power of 2.7 kW for alloy 1420 and 3.3 kW for alloy V-1469, optimization was carried out according to the position of the laser radiation focus relative to the upper boundary of the sheet. Figures 6 and 7 for alloy 1420 and V-1469, respectively, show the microstructure of the weld at different focal positions ∆f relative to the upper border of the sheet. According to Figures 6 and 7, when the focus is on the surface, the porosity of the welded joint is maximum; when the focus is deepened into the material, the porosity decreases. The optimal focus position, when the porosity is minimal, was 3 mm from the upper border of the sheet being welded.  [10]. At very low welding speeds, the gas-vapor channel is unstable because the laser beam irradiates the front wall of the channel, which causes the molten metal to move to the bottom of the weld. When the laser beam moves, a collapse process occurs, at the same time, a high welding speed leads to a melt flow along the side walls of the vapor-gas channel, but at high welding speeds, no penetration may occur. It is needed to keep the speed balance while welding. The influence of the focus position in the mode of dagger penetration and thermal conductivity during laser welding of aluminum alloys is theoretically considered in detail in [11]. The profile of the weld in the dagger penetration mode was calculated based on the energy balance on the wall of the steam-gas channel, where the temperature was taken to be equal to the boiling point of the alloy. The three-dimensional temperature field of the weld was calculated taking into account the thermal conductivity. As a result, it was shown that deepening the focus leads to an increase in the penetration and, thereby, to a decrease in the porosity of the weld. It can be assumed that in our case, the optimal balance of power, speed and focus position leads to a decrease in the porosity of the weld.
Optimization in terms of shielding gas consumption showed that an unoxidized weld seam is obtained with a helium shielding gas flow rate of more than 4 l/min. Welding was carried out at a Helium flow rate of 5 l /min. Based on the research and analysis performed, Table 1 presents the optimal modes for obtaining laser welded joints without external defects in the form of undercuts, discontinuities, cracks, pores, and sagging of the weld. Table 1 also presents estimates of the energy conditions for obtaining a high-quality weld under optimal welding conditions for a given sheet thickness: P is the heat input equal to the ratio W / V, E is the energy per unit of volume of the molten material equal to W/Vth, where t is the thickness of the alloy, h is average width of the weld. It can be seen from Table 1, for copper-containing alloys, the energy per unit volume of the weld is higher than for alloys containing magnesium, while the heat input is, on the contrary, higher.
As a result, the parameters of the laser welding process of the studied aluminum alloys were optimized: welding speed, radiation power, diameter, depth and location of the focal spot, as well as the consumption of a protective neutral gas in order to obtain welded joints without external defects. Table 2 shows the main mechanical characteristics of samples with a welded seam of the investigated heat-strengthened alloys after tensile tests, where σUTS is the ultimate strength, σYS is the yield strength, δ of plasticity limit.  The decrease in strength is caused by the peculiarities of the structure of the material. Fig. 8 shows the diffraction patterns of the initial alloy 1420, V-1469 and the laser weld obtained by X-ray phase analysis in transmission using synchrotron radiation. The diffractograms are vertically displaced relative to each other for ease of comparison.  In the diffraction pattern of alloy 1420, in addition to intense reflections of the Al phase, additional reflections are also presented. These reflexes can correspond to the phases δ '(Al3Li); S1(Al2MgLi). In the welded joint only the metastable phase S1(Al2MgLi) was recorded.
In the initial V-1469 alloy, the most intense reflections of impurity phases correspond to the T1 and θ (Al2Cu) phases. Strong reflections of the T1 phase (Al2CuLi) and the θ(Al2Cu) phase are observed in the welded joint. In the process of solidification of the melt in the weld of alloy 1420 (Al-Mg-Li system), a triple phase S1(Al2MgLi) was formed, randomly located in the solid solution; there is no strengthening phase δ'(Al3Li) in the weld (see Fig. 9 a). For the V-1469 alloy (Al-Cu-Li system), the Cu solid solution is depleted and concentrated at the dendrite boundary with the formation of copper-containing phases T1(Al2CuLi), θ (Al2Cu). (see Fig.9b). а) b) Figure. 9. Nano structure of the weld. a) alloy 1424, b) alloy V-1469 To achieve the maximum strength of thermally hardened alloys, it is necessary to obtain some intermediate nonequilibrium structure, which corresponds to the initial stages of the decomposition of a supersaturated solid solution, during which strengthening phases are formed in the solid solution, using regulated heating due to the dissolution of impurity phases [12].
For welded joints of alloy 1420, the heat treatment modes were: quenching at a temperature of 490 ° C and a time of 30 min, followed by artificial aging at a temperature of 170 ° C and the duration of 16 hours. For laser welded joints of alloy V-1469, the heat treatment modes were quenching at a temperature of 560 ° C with a holding time of 30 min, followed by artificial aging at a temperature of 180 ° C and the duration of 32 hours.
The hardening of alloy 1420 of the Al-Mg-Li system made it possible to significantly dissolve the metastable S1(Al2MgLi); the hardening phase δ '(Al3Li) is nearly absent in it. Alloying components in intermetallic phases (in particular, in the triple phase S1(Al2MgLi)) completely or partially dissolved in aluminum, and an extremely nonequilibrium state was formed -a supersaturated solid solution of alloying elements Mg and Li in aluminum. The hardening of samples of the Al-Cu-Li systems increases the concentration of copper in the solid solution, which is determined by the dissolution of coppercontaining phases at the boundaries of dendritic grains.
Artificial aging of alloy samples of the Al-Mg-Li system makes it possible to ensure the precipitation of the main strengthening intermetallic phase δ '(Al3Li) in the weld, which is confirmed by the XRD analysis. For alloys of the Al-Cu-Li system, copper-containing phases are formed in a solid solution. Optimization of artificial aging for redistribution of strength depending on time and The results of optimization of the modes of heat treatment of laser welded joints made it possible to obtain mechanical characteristics close to or equal to the original alloy.

Conclusions
The technology of laser welding of aluminum-lithium alloys has been developed, based on the optimization of the laser welding process and post heat treatment.
For laser welded joints and optimal post-processing, the mechanical characteristics of the aluminumlithium alloys are comparable to those of the alloys as delivered.
It was found that for alloy 1420, the strengthening phase δ '(Al3Li) dissolves in the solid solution of the weld. For alloy V-1469, copper-containing phases T1 (Al2CuLi), θ (Al2Cu) are formed at the dendrite boundary.
For the first time, for welded joints of aluminum-lithium alloys obtained by laser welding and optimal post-processing, mechanical characteristics are achieved that are comparable to those for alloys in the state of delivery.