Improving thermal efficiency and stability of laser welding process for magnesium alloy by combining power modulation and subatmospheric pressure environment ?

Jie Ning,Suck-Joo N,?,Lin-Jie Zhng,Xing Wng,Jin Long,Won-Ik Cho

aState Key Laboratory for Mechanical Behavior of Materials,Xi’an Jiaotong University,Xi’an 710049,China

b BIAS-Bremer Institut für angewandte Strahltechnik GmbH,Klagenfurter Stra?e 5,28359 Bremen,Germany

Abstract The laser welding(LW)process of highly reflective materials presents low thermal efficiency and poor stability.To solve the problem,the effects of subatmospheric environment on LW process,technological parameters in subatmospheric environment on weld formation and welding with sinusoidal modulation of laser power on the stability of LW process in subatmospheric environment were explored.The AZ31 magnesium(Mg)alloy was used as the test materials.The test result revealed that the weld penetration in subatmospheric environment can increase by more than ten times compared with that under normal pressure.After the keyhole depth greatly rises,significantly periodic local bulge is observed on the backwall surface of the keyhole and the position of the bulge shifts along the direction of the keyhole depth.Eventually,the hump-shaped surface morphology of the welded seam is formed;moreover,the weld width in local zones in the lower part of the welded seam remarkably grows.During LW in subatmospheric environment,the weld penetration can be further greatly increased through power modulation.Besides,power modulation can inhibit the occurrence of bulges in local zones on the backwall of the keyhole during LW in subatmospheric environment,thus further curbing the significant growth of the weld widths of hump-shaped welding beads and local zones in the lower part of welded seams.Finally,the mechanism of synchronously improving the thermal efficiency and stability of LW process of highly reflective materials through power modulation in subatmospheric environment was illustrated.This was conducted according to theoretical analysis of recoil pressure and observation results of dynamic behaviors of laser induced plasma clouds and keyholes in the molten pool through high speed photography.

Keywords:Laser welding;Subatmospheric environment;Power modulation;Highly reflective materials;Thermal efficiency;Stability.

1.Introduction

In recent years,new energy vehicles point out a new development direction for the future automobile industry as environmental pollution and energy crisis are increasingly concerned across the world.The high-quality and efficient welding of lightweight materials including magnesium(Mg)and aluminum(Al)alloys and conductive material pure copper has been the research focus.These materials show a low absorptivity for infrared(IR)laser which is the most widely used in industrial fields[1-3].The absorptivity of magnesium alloy for IR laser with a wavelength of about 1μm is about 15%,which is about 2/5 of steel.Scholars try to obtain magnesium alloy welded joints with high quality in an efficient way by using various welding methods such as brazing,arc welding,laser welding,electron beam welding,friction stir welding and laser-arc hybrid welding[4–8].In the field of new energy vehicles,it is required to have small weldingtraces,as well as both body tightness and aesthetics.Therefore,laser welding is one of the ideal methods for magnesium alloy welding[9–10].During laser welding(LW)of the highly reflective materials,the following problems are likely to occur:(1)the critical laser power required to generate the keyhole effect significantly rises;(2)after laser beams experience multiple reflections within keyholes,a large amount of laser energy is still not absorbed but escapes,thus leading to low thermal efficiency and weld penetration during welding;(3)low absorptivity and focusing effect on wall surface result in the reducing proportion of the absorbed energy by the upper part while the growing proportion of the absorbed energy by the lower part of the internal surface of keyholes.Hence,the distribution of laser energy in the direction of the keyhole depth is highly imbalanced,thus triggering the decrease of the stability of the keyhole[11].

The boiling point of metal materials is closely related to ambient pressure.A decrease of ambient pressure corresponds to a reduction of the boiling point of metal materials.Therefore,in subatmospheric environment,same laser energy density can generate stronger evaporation and lead to larger recoil pressure,which is favorable for forming keyholes with large depth and improving the thermal efficiency during welding.Katayama et al.[12]welded 304 stainless steel under normal pressure(0.1MPa)and vacuum environment by using a YAG laser.The result showed that the weld penetration greatly increases on vacuum conditions.Youhei et al.[13]found that the weld penetrations of laser welded SUS304 stainless steel and A5052 Al alloy under 10kPa are about 1.6 times of those under normal pressure.By investigating the causes for the growth of the weld penetration during LW in subatmospheric environment,Fabbro et al.[14]and Pang et al.[15]suggested that with the decreasing of ambient pressure,the temperature required to generate the recoil pressure induced by metal evaporation is lower,which further makes it easier to form a keyhole.Therefore,the weld penetration increases with the reduction of ambient pressure.However,the phenomenon was also influenced by other factors and explained by other mechanisms.For example,Luo et al.[16]showed that ambient pressure greatly affects the laser induced plasmas.Compared with welded joints under normal pressure,the size of the plasmas significantly declines and the weld penetration of the joints is multiplied during the welding under ambient pressure of 0.025MPa.The recent research result by Jiang et al.[17]also indicated that the plasma clouds are greatly inhibited and weakened on vacuum conditions.Katayama et al.[18]found that the refractive effect of plasma clouds on laser beams is weakened with the reduction of ambient pressure,and thus the weld penetration gradually rises.Apart from the boiling point and laser induced plasmas,numerous nano-and micron-sized metal particles in plasma clouds also presented an important effect on the weld penetration during LW.By surveying high-power fiber laser welding,Zou et al.[19]found that numerous nano-and micron-sized metal particles occur in plasma clouds during welding.Scholz et al.[20,21]revealed that the generation of nano-sized metal particles is closely related to the evaporation behaviors of metal materials.Na et al.[22,23]explored the shielding effect of metal particles in plasma clouds on laser energy under normal pressure.The result showed that the presence of a great number of nano-and micron-sized metal particles in plasma clouds will decrease the transmission efficiency of laser beam energy,which results in the loss of laser beam energy by about 12%and the reduction of the weld penetration.It can be seen that the boiling point of metals reduces and the shielding effect of plasmas and nano-sized metal particles on laser beams during LW in subatmospheric environment decreases,which both contribute to enhancing the thermal efficiency and growing the weld penetration during welding.However,existing research on LW in subatmospheric environment mainly concentrated on common materials representative of steel while the LW process of highly reflective materials in subatmospheric environment was hardly studied.Reisgen et al.[24]found that the laser welded joint of pure copper with a large weld penetration can be attained under ambient pressure of 0.2kPa;however,depressions still appears on the surface of the welded seams.It can be found that the stability problem during LW of highly reflective materials in subatmospheric environment has not been completely solved,and also related mechanisms need to be further explored.

In recent years,the method of power wave modulation has been validated to be effective in improving the thermal efficiency and stability of the LW process of highly reflective materials.Andreas et al.found that when performing 400～600Hz sinusoidal modulation on laser output power,the stability of the LW process of pure copper significantly strengthens,accompanying with regular shape of welded seams and remarkable reduction of the number of pores in welded seams[25,26].Heider et al.[26]and Zhang et al.[11]observed the dynamic evolution behaviors of keyholes during LW of copper by separately using X-ray and high speed photography.They found that excessive expansion and growth of the bottom of keyholes is the main cause for the poor stability during LW.Ning et al.increased the weld penetration of the laser welded joint of Mg alloy to be about 1.56 times that of the previous weld penetration by performing sinusoidal modulation on laser power[27–29].They further applied the power modulation to laser-arc hybrid welding of highly reflective pure copper.On this basis,the stability of the welding process was improved by forming fully penetrated keyholes to avoid the excessive concentration of energy at the bottom of the keyhole[30–32].

The low melting point and low boiling point of magnesium alloy lead to the easy evaporation during laser welding,resulting in the burning loss of alloy elements,spatter and uneven appearance.Compared with other high reflectivity materials,it is more difficult to control the stability of welding process for magnesium alloy.The lower boiling point of metal under subatmospheric pressure may also aggravate the instability of its welding process[33].In the study,the LW test was carried out on AZ31 Mg alloy at subatmospheric pressure conditions.Furthermore,the influence of technological parameters and ambient pressure on weld formation was ob-tained;the transient behaviors of keyholes and molten pools during LW without and with power modulation in subatmospheric environment were separately observed by applying high speed photography.The mechanism of synchronously improving the thermal efficiency and stability of LW process of highly reflective materials based on power modulation in subatmospheric environment was analyzed.The research result will exert universal signicance for solving low thermal efficiency and poor stability of the IR LW process of highly reflective materials.

Fig.1.Laser welding system for realizing subatmospheric pressure conditions.

2.Materials and methods

The as-rolled plate made of AZ31 Mg alloy with the thickness of 8mm was used for the test and its compositions are shown in Table 1.

Table 1Chemical compositions of AZ31 Mg alloy.

The welding process was carried out by using an IPG YLS-4000 fiber laser working at the maximum power of 4300W,with the wavelength of 1070nm and focal length of 150mm.The sinusoidal modulation of laser power was realized by employing an UTG9000C function signal generator.The laser head was inclined by 10° backwards during welding to prevent reflected light from damaging the optical elements.Owing to oxygen in the welding atmosphere would greatly influence the absorptivity of highly reflective materials for laser energy,the shielding gas pipeline was reserved in the suction device in order to avoid the effect of oxygen in air on the result[34].The subatmospheric environment was realized by using the self-designed sub-pressure chamber and laser beams were radiated onto the test plate after going through the quartz glass window on the upper surface of the chamber.Before the welding,99.99% of pure argon was first injected into the chamber for 60s at the flow rate of 20L/min to replace the air therein;afterwards,the argon was extracted from the chamber by applying a vacuum pump until the pressure reached the ambient pressure required in the test.The plasma morphologies during welding were observed through the transparent glasses by using a Phantom M250 high speed camera,at the frame rate of 1052 frames per second for the exposure time of 1μs.The schematics and setup of the laser welding device are shown in Fig.1.The test parameters during LW of Mg alloy in subatmospheric environment are displayed in Table 2.

Additionally,the influences of ambient pressure and power modulation on the transient behaviors of the keyhole and molten pool during LW of Mg alloy were further explored by applying the half-sandwich specimens of high-temperature glass–Mg alloy.The welding was performed by scanning of laser beams,with the central line at the interface between high-temperature glass and Mg alloy(half-sandwich specimens).By employing the high speed camera,the evolution behaviors of keyholes and molten pools during welding were observed through the quartz glass windows at the flanks of the sub-pressure chamber,as shown in Fig.2.In order to investigate the transient evolution behaviors of keyholes,it was necessary to apply a large frame rate of 4254 frames per second and keep the exposure time of 1μs when photographing the keyholes with high speed photography.The transient behavior was the main concern when shooting the keyhole,while the overall shape of the plasma was paid attention to when shooting the plasma,so different frames were selected to shoot the keyhole and plasma.By utilizing Matlab software and difference method,the differences of two adjacent frames of pictures were observed in order to explore the dynamic evolution of morphologies of the keyhole and molten pool,as shown in Fig.3(a)and(b).The profiles of the keyhole and molten pool could be drawn according to the images processed by Matlab software,as shown in Fig.3(c).The test parameters during LW of the half-sandwich specimens of high-temperature glass–Mg alloy are shown in Table 3.

Table 2Laser welding parameters.

3.Results

3.1.The effect of ambient pressure on the morphologies of welded seams

Fig.4 shows the surface appearances and cross section morphologies of the welded seams of AZ31 Mg alloy with a thickness of 8mm under different ambient pressures.It canbe seen from Fig.4(a)that the surface appearance of the welded seam greatly varies when ambient pressure reduces from 10kPa to 1.0kPa.According to Fig.4(b),it can be seen that at ambient pressure of 101 and 10kPa,the weld seams present a low weld penetration while the weld width is significantly larger than the weld penetration,in which the former is about 3.3 times of the latter.The welding process is typical thermal conduction welding.After ambient pressure drops to 1.0kPa,the weld penetration suddenly increases to be about 7.1 times of the original value while the weld width of the upper surface is lowered to be about one third of the original value.In this case,the welding process changes from thermal conduction welding to deep penetration welding.The weld penetration marginally varies with further reduction of ambient pressure.That is,on the research conditions,1.0kPa is the critical ambient pressure under which the weld penetration of Mg alloy significantly increases in subatmospheric environment.Moreover,it can be also found from Fig.4 that after ambient pressure is lower than 1.0kPa,the upper surfaces of the welded seams are found to be hump-shaped and the weld width of the lower part is greatly larger than that of the upper part of the cross section.The weld width of the welded seam is nonuniformly distributed in the thickness direction.

Fig.2.Schematic description of the half-sandwich specimen configuration for observing keyhole behaviors.

Fig.3.Image-processing procedure for obtaining the profiles of a keyhole and molten pool from an original HCDD image.

3.2.The effect of technological parameters on the morphologies of the welded seams

At constant ambient pressure of 0.1kPa,the effects of laser welding parameters on the surface appearance and cross sec-tion morphologies of the welded seams of laser welded Mg alloy in subatmospheric environment are explored.The result is shown in Fig.5.As shown in the figure,the weld penetration significantly increases with growing laser power;an increase of the welding speed corresponds to a reduction of the weld penetration.With the growth of the defocusing amount,the weld penetration rises at first and then declines,in which the weld penetration is the largest,about 6.98mm,when the defocusing amount is+1mm.It can be seen that it is possible to further improve the thermal efficiency during the LW of Mg alloy in subatmospheric environment by optimizing welding parameters.As shown in the figure,a hump-shaped welding bead occurs on the upper surfaces of the welded seams generally under all welding parameters and the weld widths of the lower part are all larger than those of the upper part on the cross sections of the welded seams.It means that it is hard to improve the stability of LW process of Mg alloy in subatmospheric environment by optimizing the welding parameters.Therefore,three problems are mainly concerned:(1)the mechanism by which the thermal efficiency during LW of Mg alloy increases in subatmospheric environment;(2)the reason why the stability of LW of Mg alloy decreases in subatmospheric environment;(3)regulation measures and mechanism on the stability of LW of Mg alloy in subatmospheric environment.

Fig.4.The effect of ambient pressure on the surface appearance and cross section morphologies of the laser welded joint of AZ31 Mg alloy(AP=4000W,v=4m/min,f=0,A=0 and F=0).

3.3.The influence of ambient pressure on laser induced plasmas

Fig.6 displays the morphological change of laser induced plasmas generated during LW of Mg alloy under different ambient pressures.It can be seen from Fig.6 that strong laser induced plasmas are generated during welding under normal pressure and their morphologies are quite instable;moreover,lots of spatters occur above the molten pool.The laser induced plasmas above the molten pool deliver a shielding effect on laser energy and metal particles in optical paths present absorption and scattering effects on laser energy,which both lead to the reduction of energy reaching the workpiece surface.When ambient pressure decreases to 10kPa,the laser induced plasmas have basically disappeared while some spatters still appear above the molten pool.As ambient pressure further drops to 1.0kPa,the laser induced plasmas and spatters in optical paths nearly completely disappear.Combined with the results in Fig.4 that welding in the subatmospheric environment could increase the weld penetration depth,it can be concluded that the reduction of laser-induced plasma and spatters in optical paths was one of the reason for the improvement of thermal efficiency of laser welding in subatmospheric environment.

3.4.The influence of ambient pressure on keyholes

By using high speed photography,the evolution behaviors of keyholes during LW of half-sandwich specimens under normal pressure and 0.1kPa of ambient pressure were separately observed.The results are shown in Fig.7.It can be seen from Fig.7(a)that under ambient pressure of 101kPa at the laser power of 4000W,a cone-shaped keyhole shows a low depth and its shape is stable during welding,having no significant fluctuation;under ambient pressure of 0.1kPa at the laser power of 2800W,a blind hole(Fig.7(b))with a large depth is formed;when the laser power rises to 4000W,the fully penetrated keyhole is generated(Fig.7(c)).

As shown in Fig.7(b)and(c),local bulges are observed at the backwall of the keyholes during LW in subatmospheric environment,and also the positions of bulges gradually shift downwards during welding.As shown in Fig.7(b),blind keyholes are formed at low power.On this condition,the bottom of the keyholes is excessively expanded and the size of keyholes in the upper part dramatically declines and keyholes even collapse when bulges on the backwall shift to the bottom.In the case that fully penetrated keyholes are formed at large laser power,the local bulges on the backwall of the keyholes will disappear after they shift to the opening at the bottom of the fully penetrated keyholes,as shown in Fig.7(c).It is similar to the result obtained during laser-arc hybrid welding of highly reflective red copper with power modulation[32].According to the cross section morphologies of the welded seam in Fig.5(b),it can be also found that after forming a fully penetrated welded seam at a low welding speed,the phenomenon that the growth of weld width in thelower part of the cross section of the welded seam grows is significantly inhibited.Based on the aforementioned results,it can be seen that poor stability during LW in subatmospheric environment is related to local bulges formed on the backwall of keyholes;in addition,the phenomenon during nonpenetrating welding in subatmospheric environment will remarkably influence the stability of welding process and weld formation.

Fig.5.The effect of welding parameters on the surface appearances and cross section morphologies of the laser welded joint of AZ31 Mg alloy in subatmospheric environment(Pamb=0.1kPa,A=0 and F=0).

3.5.The influence of power modulation on the weld appearance

Two cross sections were intercepted at different positions of each weld seam.Fig.8 displays the cross section morphologies in subatmospheric environment under different modulation frequencies.The weld penetrations on the cross sections of the welded seams were measured and averaged to attain the change of the weld penetration with the modulation frequency,as shown in Fig.8(i).

It can be seen from Fig.8 that no matter what level the modulation frequency is,the weld penetrations on the cross sections of the welded seams with power modulation are all greatly larger than those without modulation.During welding without power modulation,the weld penetration is about 2.0mm while it is the largest(about 5.6mm)at the modulation frequency of 100Hz;as the modulation frequency increases to 1000Hz,the weld penetration is gradually lowered to be about 3.3mm;afterwards,the weld penetration fluctuates at about 3.8mm as the modulation frequency continues to grow.The weld penetration was taken as the standard for measuring the thermal efficiency during welding.It can be seen that after the weld penetration gradually stabilizes withthe change of the modulation frequency,the weld penetration of laser welded Mg alloy in subatmospheric environment with power modulation is about 1.9 times of that without modulation.That is,it is feasible to further improve the thermal efficiency during LW of Mg alloy in subatmospheric environment with power modulation.

Fig.6.Comparison of the shapes of laser-induced plasma in LW of Mg alloy according to the change of ambient pressure.(AP=4000W,v=4m/min,f=0,A=0 and F=0).

The welded seams obtained at different modulation frequencies were subjected to X-ray nondestructive detection.The results are shown in Fig.9.As shown in the figure,a small pore is found at the position close to the initial welding point during welding without power modulation,where depressions also appear.With power modulation at 100Hz,consecutive keyholes are formed along the central line of the welded seams,which is consistent with the result shown in the image of the cross section,as shown in Fig.9(b).At the modulation frequencies of 500 and 700Hz,no significant porous defects are found while the weld width near the initial welding point greatly decreases,and the image for defect detection is obviously darkened.By comparing the images of surface appearances,it can be seen that obvious depressions are formed,as shown in Fig.9(c)and(d).At the modulation frequency of 1300Hz,there are no significant porous defects and depressions.As the modulation frequency continues to grow,the pores and depressions both greatly increase.That is,performing LW on Mg alloy in subatmospheric environment with optimized modulation parameters can reduce the pores and depressions in,and further improve the quality of,the welded seams.

4.Discussion

4.1.Shielding effect of plasmas in subatmospheric environment

The adsorption and scattering of laser induced plasmas and the metal particles therein for laser beams will lead to the re-duction of laser energy reaching the workpiece surface.When performing welding by applying an IR laser with the wavelength of about 1μm,the inverse bremsstrahlung absorption coefficient is low and thus can be ignored owing to the low ionization degree of plasmas caused by their low temperature[14].Mie scattering and Rayleigh scattering are the main absorption and scattering mechanism of metal particles in laser induced plasmas for laser beams[33,35].The Rayleigh scattering coefficient is inversely proportional to the fourth power of the wavelength and therefore Rayleigh scattering is quite important during LW with short wavelength.On condition that the size of metal particles is equivalent to the order of magnitude of the wavelength,the effect of Mie scattering is remarkable.The scattering effect of metal particles on laser beams is not closely related to the particle size but also the density of metal particles.As the ambient pressure decreases,the metal evaporation temperature decreases,and the metal vapor increases with the same laser energy.There are two positions of metal vapor in laser welding process:above the molten pool and inside the keyhole.As can be seen from Fig.6,with the decrease of ambient pressure,the metal vapor above the molten pool significantly decreases,indicating that there’s more metal vapor inside the keyhole.On the one hand,the metal vapor inside the keyhole can be used as a second heat source to melt more metals.On the other hand,it will have a complex influence on the propagation direction of laser beam,which will lead to the decrease of the stability of welding process.It can be also seen from Fig.6 that as ambient pressure gradually declines,the brightness of plasmas greatly reduces and also metal particles agglomerated in optical paths obviously decline.Furthermore,the absorption and scattering of laser induced plasma plumes and metal particles for laser beams decrease so the laser beams reaching the workpiece surface can increase,which contributes to growing the weld penetration.However,Shcheglov et al.[36]found that the total attenuation of laser beams that reach the workpiece surface after going through laser induced plasma plumes during welding with the IR laser was only about 10%.Jiang et al.suggested that plasma plumes were inhibited in vacuum environment and thus their attenuation effect on laser energy was weakened.Therefore,the laser energy reaching the surface of the base metal during welding in vacuum environment rises by about 15%[17].The growth of laser energy reaching the surface of the base metal contributes to increasing the weld penetration.However,the maximum weld penetration in subatmospheric environment is far larger than the weld penetration during LW under normal pressure,about ten times of the later.That is,the reduction of the scattering and absorption of laser induced plasma plumes for laser beams is the cause for the increase of the thermal efficiency during welding in subatmospheric environment;however,it is insufficient to explain the great growth of the weld penetration.

Fig.7.Comparison of the shapes of keyhole and molten pool in LW of Mg alloy according to the change of ambient pressure.(v=4m/min,f=0,A=0 and F=0).

Fig.8.Effect of modulation frequency on fusion zone shape at subatmospheric pressure.(AP=2800W,Pamb=0.1kPa,f=0,v=4m/min and A=1200W).

Fig.9.Effect of modulation frequency on defect formation at subatmospheric pressure.(AP=2800W,Pamb=0.1kPa,f=0,v=4m/min and A=1200W).

4.2.Recoil pressure in subatmospheric environment

As we all know,the evaporating temperature is lowered with the reduction of ambient pressure.It is feasible to calculate the evaporating temperatures of Mg alloy under different ambient pressures according to Clausius–Clapeyron Equation(Eq.(1)),as shown in Fig.10.

Fig.10.Relationship between the boiling point of Mg alloy and ambient pressure.

where,P1andP2refer to different ambient pressures(Pa);T1andT1stand for the boiling points(K)under ambient pressuresP1andP2;ΔHVAPand R represent the latent heat of vaporization(J?mol-1)and gas constant(8.314 J?K-1?mol-1),respectively.

The keyhole wall during welding is mainly affected by recoil pressure and surface tension,in which recoil pressure promotes the expansion of keyholes while surface tension drives their closure.In the welding process,the recoil pressure and surface tension are in a dynamic equilibrium state to enable keyholes to stabilize.The recoil pressure depends on the surface temperature of liquid metals.When the surface temperature of liquid metals is lower than or equivalent to the boiling point,the metal vapors are restricted by ambient pressure.Liquid metals cannot be greatly evaporated unless the surface temperature of liquid metals is higher than the boiling point.Therefore,Pang et al.modified the numerical model for recoil pressure[15]:

where,Pr(Ts),PambandTVdenote the recoil pressure(Pa)induced by metal vapors when the surface temperature of liquid metals isTs,ambient pressure(Pa)and the boiling point(K)under normal pressureP0,respectively;βRrefers to the condensation coefficient(fraction of recrystallized particles in evaporated particles),which is related to Mach number at the outlet of Knudsen layer.The value ofβRis correlated with the boundary conditions of steam flow,generally ranging from 0.18 to 1.At a high evaporation rate(vacuum environment or high power density of laser),the condensation coefficient can be 0.18;at a low evaporation rate(hyperbaric environment or low power density of laser),the coefficient can be set as 1[37-39].According to the model,the values ofβRunder normal pressure and ambient pressure of 1kPa are taken as 1 and 0.18.

Fig.11.The changes of recoil pressure with the surface temperature under normal pressure and in subatmospheric environment.

The changes of surface pressure(the sum of ambient pressure and recoil pressure)of Mg alloy with the surface temperature of liquid metals under normal pressure and in subatmospheric environment(ambient pressure of 1kPa)can be calculated through Eq.(2).The result is shown in Fig.11.It can be seen from the figure that the recoil pressure under normal pressure is still equal to 0 when the surface temperature of liquid metals reaches 1363K(the boiling point under normal pressure);however,the recoil pressure induced by metal vapors in subatmospheric environment has increased to be over 50 times of that under ambient pressure.Therefore,the critical temperature at which the thermal conduction welding changes to deep penetration welding is lower in subatmospheric environment.As a result,it is easier to form keyholes with a large aspect ratio,thus improving the weld penetration and thermal efficiency.

4.3.Stability of keyholes during LW in subatmospheric environment

Fig.12(a)shows the three-dimensional surface appearances of the welded seams during LW without and with power modulation at the modulation frequency of 1300Hz.It can be seen from Fig.12 that during welding without power modulation,four slight bulges are shown on the surface of the welded seam corresponding to the zone with stable welding,which are separately found at positions about 6,10,16 and 18mm along the direction of the length of the welded seam.The highest height of bulges fluctuates in the range of 1.5～2.4mm;during welding with power modulation,the highest height of the bulges within the stable welding zone of the welded seam varies within 2～3mm,with the fluctuation amplitude slightly higher than that of the welded seam without power modulation.There only exists a bulge at the position of about 18mm within the stable welding zone ofthe welded seam and the formation of the bulge is caused by the effect of the initial welding point.However,within the zone in the range of 4～12mm along the direction of length,the height of the welded seam nearly does not fluctuate,which is superior to the welded seam obtained without power modulation.Overall,performing welding with power modulation in subatmospheric environment can improve the surface formation of welded seams.

Fig.12.The effect of power modulation on the surface appearance of the welded seams and dynamic behaviors of keyholes during LW of Mg alloy in subatmospheric environment(AP=2800W,Pamb=0.1kPa,v=4m/min,f=0).

Fig.12(b)compares the dynamic evolution processes of longitudinal planes of keyholes during LW of Mg alloy without power modulation and with power modulation at 1300Hz both in subatmospheric environment.As shown in Fig.12(b),during LW without power modulation in subatmospheric environment,the bottom of the keyholes is obviously expanded.As the laser energy incident into the internal wall of the keyholes continues to increase,the bottom of keyholes continues to grow and finally collapses to eject liquid metals outwards.The result is shown in the high speed charge-coupled device(HCCD)image at the timet=t0+0.52ms.After modulating the laser power,the bottom of keyholes around the peak power is slightly expanded;afterwards,the instantaneous power starts to reduce,inhibiting the constant expansion of the bottom of keyholes.The result is displayed in HCCD images att=t0+0.52ms andt=t0+0.78ms in Fig.12(b).Therefore,it is inferred that it is the main reason why power modulation can improve the stability during welding of highly reflective materials that power modulation can inhibit the excessive expansion of the bottom of keyholes.

The expansion of the bottom of keyholes is mainly related to disequilibrium distribution of energy along the direction of keyhole depth during deep penetration welding of highly reflective materials.During deep penetration welding,laser energy will converge at the bottom of the keyhole after laser beams are subjected to multiple reflections within the keyhole.Highly reflective materials show a low absorptivity for laser energy.Therefore,compared with materials of low reflectivity,the problem that laser energy is concentrated at the bottom of the keyhole is more significant during the deep penetration welding of highly reflective materials.It causes that the bottom of the keyhole is easier to be excessively expanded during deep penetration welding[11].Besides,the keyhole depth during welding in subatmospheric environment is significantly larger than that at normal pressure,which will further increase the distribution nonuniformity of energy along the direction of the keyhole depth.Moreover,the gravity of liquid metals delivers a more significant effect,thus leading to the reduction of the stability of the keyhole.

According to the previous research results[29],the keyhole depth grows step by step during LW of highly reflective materials.Through analysis,it can be found that the further growth of the keyhole depth needs accumulation of high energy density at the bottom of keyholes.That is,to further increase the keyhole depth,it requires that the laser energy absorbed at the bottom of keyholes surpasses a high energy level.It is supposed that the total energy absorbed by a keyhole isEtotal;the required energy for maintaining the present situation of a keyhole isEmain;the energy level required to surpass so as to further grow the keyhole depth isΔEand the energy consumed due to radiation and convection during two adjacent increases of the keyhole depth isEexh.Thus,thecondition allowing the keyhole depth to further grow is

Fig.13.Multiple reflections of laser beams at the bottom of keyholes during laser deep penetration welding of highly reflective materials;(a)without power modulation;(b)with power modulation[4].

During LW without power modulation,the laser energy converges at the bottom of keyholes.However,owing to the laser power is constant,the laser energy absorbed by the bottom of keyholes is hard to surpass the energy levelΔEto further increase the keyhole depth,only leading to the excessive expansion of the bottom;finally,keyholes collapse,as shown in Fig.13(a).After performing wave modulation on laser power,the energy density absorbed by the bottom of keyholes constantly increases with the constant growth of the instantaneous power within the semi-period when the instantaneous power is larger than the average power.The keyhole depth grows when the net energy density is larger than the required energy levelΔE.In this case,the energy absorbed by the bottom of keyholes after multiple reflections is mainly used to grow the keyhole depth and thus the bottom of keyholes is not obviously expanded in the width direction.As the instantaneous power rises and approximates to the peak power,the growth amplitude thereof gradually declines.The laser energy absorbed by the bottom of keyholes is hard to surpass the energy level required to further increase the keyhole depth.On this condition,similar to LW without power modulation,the energy absorbed by the bottom of keyholes will result in the expansion and growth of the bottom of keyholes in the width direction.However,the instantaneous power starts to attenuate afterwards,which can relieve the excessive concentration of laser energy at the bottom of keyholes.Thus,it avoids the excessive expansion and growth of the bottom of keyholes,thus improving the stability of keyholes,as shown in Fig.13(b).Above all,during LW with power modulation,the attenuation of instantaneous power after the peak power relieves the excessive concentration of laser energy at the bottom of keyholes and alleviates the greatly nonuniform distribution of laser energy in the direction of keyhole depth.Therefore,the stability of keyholes is improved.

5.Conclusions

(1)LW in subatmospheric environment can obviously promote the formation of keyholes during LW of Mg alloy,allowing the transformation of thermal conduction welding at normal pressure into deep penetration welding in subatmospheric environment,however,the stability during welding is lowered.

(2)The reduction of the evaporating temperature of metal materials and the decrease of the shielding effect of laser induced plasmas in subatmospheric environment are important causes for formation of keyholes with large depth during LW in subatmospheric environment.

(3)The excessive concentration of energy at the bottom of keyholes causes the expansion and growth of the bottom of keyholes,finally leading to the collapse of keyholes,which is the reason why the stability of LW process in subatmospheric environment decreases.

(4)Performing power modulation on LW of Mg alloy in subatmospheric environment contributes to improving the surface formation of welded seams and strengthening the stability of the welding process.Under the condition in this study,the thermal efficiency and process stability can be improved simultaneously when welding at the optimized parameters(i.e.,power of 2800W,ambient pressure of 0.1kPa,welding speed of 4m/min,defocusing amount of 0,amplitude of 1200W and frequency of 1300Hz).

(5)The mechanism of power modulation improving the stability of LW process is that after forming keyholes with large depth within the positive semi-period,the reduction of instantaneous power avoids the energy concentration at the bottom of keyholes.

Acknowledgement

This work is supported by National Natural Science Foundation of China(Grants No.52005393,51275391)and National Thousand Talents Program of China(Grant No.WQ2017610446).