Experimental and numerical study of temperature field and molten pool dimensions in dissimilar thickness laser welding of Ti6Al4V alloy

Journal of Manufacturing Processes 49 (2020) 438–446

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Experimental and numerical study of temperature field and molten pool dimensions in dissimilar thickness laser welding of Ti6Al4V alloy

T

Zhixiong Lia,b, Khashayar Rostamc, Afshin Panjehpourc, Mohammad Akbaric, Arash Karimipourc, Sara Rostamid,e,* a

State Key Laboratory of Hydraulic Engineering Simulation and Safety, Tianjin University, Tianjin 300350, China School of Mechanical, Materials, Mechatronic and Biomedical Engineering, University of Wollongong, NSW 2522, Australia c Department of Mechanical Engineering, Najafabad Branch, Islamic Azad University, Najafabad, Iran d Laboratory of Magnetism and Magnetic Materials, Advanced Institute of Materials Science, Ton Duc Thang University, Ho Chi Minh City, Vietnam e Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Vietnam b

A R T I C LE I N FO

A B S T R A C T

Keywords: Laser welding Molten pool Ti6Al4V alloy Heat transfer Different thicknesses

Laser study is an important consideration in the present century with advances in laser technology. Titanium alloys are of great importance to the defense industry, aerospace and other industries due to its properties, including strength to weight. In this research, experimental and numerical study are investigated for laser welding on sheets of Ti6Al4V alloy with different thicknesses. Analysis of the temperature distribution around the molten pool and dimensions of the depth and width of the molten pool are performed by changing the parameters of laser such as focal length, speed of laser welding and power. The results show that the heat affected zone (HAZ) and molten pool is diverted to the thinner sheet. Also, by decreasing the focal length, the temperature of the workpiece and the dimensions of depth and width of the molten pool are increased. In addition, with enhancing the laser speed, the laser beam contact time with the workpiece surface reduces and the temperature decreases, resulting in a decrease in the dimensions of the depth and width of the molten pool. Eventually, as the power increases, the dimension of the melt pool increase and at both 180 W and 240 W powers, the thinner sheet experiences higher temperatures compared to the thicker sheet. In this study, the results of numerical simulation are matched with the experimental results and can be applied to obtain the temperature and geometry of the melt pool in other cases to reduce the cost and time.

1. Introduction

welding processing. Acherjee and Kuar [9] surveyed the black carbon effect on the rate of heat transfer during laser welding of polymer substances. In this study, the determination the temperature field for polycarbonate with different ratios of black carbon was performed. In a similar three-dimensional solution, Acherjee and Kuar [10] investigated the black carbon effect on the heat transfer process and the calculation of the temperature gradient and dissipation during laser welding of polycarbonate with different ratios of black carbon. Nekouie Esfahani and Coupland [11] developed a model for investigation the molten pool in laser welding applying the CFD. Using the developed model, the precidting the molten pool properties using a diferent process parameters of laser welding, is matched with the emprical results. The presented model is applied to obtain the suitable welding properties for industrial applications. By simulating and using the finite volume method, Shaibu et al. [12] surveyed the heat transfer of the molten pool, microstructure of fusion zone for the stainless steel and copper

In the new century, laser discovery has revolutionized the industry and new sciences. Laser has wide applications in industry and medicine, which reveals the need for extensive research to obtain the new technologies [1–8]. Due to its physical and mechanical properties, Ti6Al4V alloy is of great interest to the industries, especially the aerospace, shipbuilding, nuclear and military industries. As a result, its analysis is very important. In most cases, evaluation of the temperature distribution has a high importance under laser processing operations. The same property of all these issues is the existence of an interface that separates the solid and liquid areas. The expansion of this surface to the liquid or solid region depends on the temperature gradient of its both sides, and the rate of heat dissipated from the solid-liquid interface determines the rate of release of mentioned surface. Many studies have been conducted on the investigation of heat transfer in the molten pool under laser

⁎

Corresponding author at: Ton Duc Thang University, Ho Chi Minh City, Vietnam. E-mail addresses: [email protected] (Z. Li), [email protected] (S. Rostami).

https://doi.org/10.1016/j.jmapro.2019.11.024 Received 5 September 2019; Received in revised form 13 November 2019; Accepted 22 November 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.

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values of laser parameters. The yield stress was surveyed at different temperatures and pressures and the results showed that the temperature had more effect on the shear stress than the pressure. Tan et al. [26] studied laser welding-soldering process for magnesium alloy and stainless steel with variation the process of parameters such as speed of the welding; wire feed speed and inlet heat. The microscopic and mechanical structure of the welds were studied applying the SEM and TEM. Yildiz et al. [27] studied the joining of pure titanium and ferritic stainless steel plates using a nickel layer at several temperatures (800, 825, 850 and 875 °C) under a constant pressure (3 MPa). The Argon gas was used as means of shielding the melting zone area. Hardness testing was performed to determine the surface strength. Zhang et al. [28] presented a method for 3D determination the shape of molten pool during the process of laser welding for two dissimilar metals of low carbon steel and stainless steel using the triple regression equations. These equations prerents the relation between the parameters of laser welding (power and speed) or even geometrical parameters of the molten pool. Shen et al. [29] performed a research on laser welding of different titanium alloys. Both the mechanical properties and microstructure of the joints were analyzed. Kumar et al. [30] experimentally studied the laser welding of Ti6AL4V alloy 5 mm sheet thickness. The laser Parameters such as power, speed of welding and defocus positions of the laser beam on the molten pool geomery were investigated. Blackburn et al. [31] performed a high quality welding on a 3.25 mm thickness of Ti6Al4V alloy using a laser heat source. They conducted a welding with very low porosity. Comparison of Ti6Al4V alloy properties in pulsed laser welding and tungsten welding with argon shielding gas has been done by Gao et al. [32]. They found that comparing with the argon welding, the laser welded specimens had a lower distortion, smaller heat affected zone, better microstructure and higher hardness. In other words, pulse laser welding of Ti6Al4V is much better than argon welding. Kong et al. [33] introduced a numerical model for prediction of the width and depth of HAZ in Ti6Al4V laser heating and showed that the width and depth of HAZ reduced with increasing the speed of laser. Pulsed laser welding of Ti6Al4V alloy was performed by Akman et al. [34]. Their findings demonstrated that the penetration depth and molten pool dimensions can be monitored by accurate control of the laser parameters. They found that the ratio of pulse energy to pulse width is the formost factor in describing the penetration depth. They also found that at constant peak power, variations in pulse width had no effect on penetration depth. The geometry of HAZ and weld fusion zone are known as important critera for assessing of the Ti6Al4V weld quality. Akbari et al. [35] presented the temperature field and dimensions of melt pool in pulsed mode laser welding for Ti6Al4V sheet through experimental and numerical study. The results showed that the numerical simulation outputs has had a clear agreement with the results obtaine from experiments. In this section, some experimental and numerical researches on different types of laser welding with different materials were discussed. Investigation of molten pool and HAZ, temperature distribution, various laser parameters measurement to obtain a suitable welding was studied. In all previous researches, the laser welding was performed numerically and experimentally for similar or dissimilar materials with the same thicknesses. But laser welding for joining the Ti6Al4V alloy with dissimilar thickness has not been performed. Investigation of temperature distribution in different thicknesses and thereby the efficiency of the plate thickness related to melt pool deviation toward different thicknesses is a major aim of this study.

workpieces and used Co2 as a shielding gas. The formed melt pool is asymmetric and the experimental results confirm the simulation results. Kubiak et al. [13] obtained the temperature field during the combined welding process of the laser and the arc using a mathematical and numerical model. The finite volume method was used for numerical solution. The metal used in this research is steel. Experimental methods have been used to determine the applied parameters in the laser welding. Numerical simulation of thermo mechanical phenomenon in combined welding of steel sheets using the laser beam and electrical arc heat sources was done by Saternus et al. [14]. Casalino et al. [15] proposed a simple model for analysis the process of laser welding for lightweight sheet metal. The aim of this research is to predict the welding method using the laser welding parameters. The metals used in this study were pure industrial aluminum and titanium alloy. Shayganmanesh and Khoshnoud [16] studied the effects and properties of laser beam in laser welding of silicon by numerical solution. For modelling the process of welding, heat transfer equations were solved. The effect of different pulsed laser parameters (e.g pulse duration, pulse energy, frequency and welding speed) were studied. The findings demonstrated that the values of thermophysical parameters are important in laser welding modeling. Faraji et al. [17] presented a numerical model using a finite volume method for process of laser welding. An electromagnetic model based on Maxwell's equations was used to obtain the electromagnetic forces in the molten pool. The results of simulation including temperature, density of current, electromagnetism and melting rate of materials are presented. They found that fluid flow had important effects on the shape and dimensions of the molten pool. Kumar [18] presented a numerical solution of the fusion zone and the rate of the cooling which significantly affects the melt pool and the structure of workpiece. A 3D heat transfer model in dissimilar materials laser welding has been proposed by Isaev et al. [19]. In this study, an algorithm is proposed to gain both the temperature field and dimension of the melt pool in titanium and stainless steel laser welding by placing the copper as the intermediate metal. The melting and solidification process during welding highly depends on the beam strength, its velocity and the focal point position that have been studied. Liang and Luo [20] studied a transient heat transfer numerical model for pulsed laser dissimilar welding of metals. The model has composed of melting and evaporation and the effects these phenomena on the formation of the molten pool and welding width have been investigated. The results of the model used for pulse laser welding simulation showed that the temperature distribution, molten flow rate and surface waves are asymmetric because of clear descrepancies of materials physical properties. The obtained resulrs of the experiments have an agreement with the simulation results from the proposed model. Sun et al. [21] performed the tension and hardness tests for workpieces made of aluminum and stainless steel which were welded by laser with different laser parameters. The final tensile strength for optimum welding was about 120 MPa. The extraordinary properties of the NiTi alloy and its properties such as environmental compatibility and good mechanical properties make this alloy useful for a range of medical supplies and industries. Pouquet et al. [22] investigated the melt pool and NiTi alloy and austenitic stainless steel structure which were bonded by laser welding. Gui et al. [23] studied the laser welding of different Ti6AL4V and BTi6431S titanium alloys with different material’s thickness. The relationships between depth of penetration and laser power were proposed for various speeds. Tensile strength was more than 1090 and 750 MPa at room and high temperature (500 °C) orderely. Always, joining the two dissimilar metals in the industries has been important to obtain the better properties. Miranda et al. [24] experimentally conducted the laser welding of NiTi and Ti6Al4V plates with 1 mm thickness and different input thermal energy to control the cooling rate. Microstructural observations on butt joints of these alloys indicated the presence of a good dendritic structure in the fusion zone. Balasubramanian [25] has done a research on laser welding of Ti6Al4V alloy with stainless steel plate with silver foil as the middle layer in different

2. Experimental procedure In this study, laser welding was performed to measure the the adjacent temperature of molten pool of the Ti6Al4V alloys sample with 60 × 15 mm & 1, 1.5 and 3 mm thickness (different thickness). According to the Fig. 1, two grooves with a depth of 1.1 mm were created on each workpiece with a 3 cm spacing to mount the thermocouples. A titanium sheet in motion was then exposed to laser energy, and the asymmetric 439

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grid system was used for the weld pool and the area was far enough from this zone to clearly diminish the time of computation. 3.1. Governing equations In the numerical simulation, governing equations were derived according to the following assumptions:

• The laser beam was fixed. • The system of coordinate was motionless. • The velocity of workpiece in the x-direction was selected invariant (v ). • The thermophysical properties of material are function of tem-

Fig. 1. A piece of 3 mm thickness titanium sheet with two grooves. F.

welding

perature.

thermal field was obtained by changing the different laser parameters. By conducting the metallography tests on welding specimens, the molten pool size and HAZ were measured. Table 1 illustrates the chemical properties of Ti6Al4V alloy. The pulsed Nd: YAG laser (model IQL-20) was utilized that has the 750 W maximum averaged power and 1.06 μm wavelength. the laser pulse duration has the range from 0.2–25 ms and frequency from 1 to 250 Hz with 0 pulse energy up to 40 J. To measure the output average power, Ophir W-Lp 400 power meter was applied. To prevent of oxidation the molten pool, Argon gas was applied from a coaxial nozzle. The flow rate of shielding gas was 15 (lit/min). The setup schematic configuration is seen in Fig. 2. In this research, flexible type-K thermocouples with 10 cm length, 1 mm tip diameter, −40 to 1260 °C operating temperature, and ± 1 % accuracy were used to measure the temperature. Since most metals have a melting point higher than this range, the thermocouple cannot be placed at the welding line as the thermocouple’s tip would melt. For that reason, to obtain the variations of temperature of the molten pool, the temperature of its vicinity is measured. On the top of the surface, thermocouples were installed at a 2 mm lateral spacing from the molten pool centerline. Since the two welded pieces had different thicknesses and thermal fields, for increased precision, two thermocouples were attached on each metal piece to record the changes in temperature. Fig. 3 shows the size of workpieces and the positions of the thermocouples. In this research, butt welding was used for welding the metal pieces. After polishing, the pieces were placed on a CNC table capable of motion in three coordinate axes. The data was grabbed and recorded on PC using the Advantech USB 4718 data acquisition card. The sampes preparation for metallography ething process was performed applying the standard techniques. The Olympus SZ-X16 stereoscopic microscope was applied to measure the dimensions of the melt pool. Table 2 shows a series of the experiment which was performed in this study.

The governing equations is presented as following [36]: Mass conservation:

∂ρ ∂ (ρw ) ∂ (ρv ) ∂ (ρu) =0 + + + ∂t ∂z ∂y ∂x

(1)

Equation of X -momentum:

∂ (ρu) ∂ (ρuw ) ∂ (ρuv ) ∂ (ρuu) ∂ ∂u ∂ ∂u = (μ ) + (μ ) + + + ∂t ∂x ∂x ∂y ∂y ∂z ∂y ∂x μ ∂ ∂u ∂P + − (u − vw ) (μ ) − K ∂z ∂z ∂x

(2)

Equation of Y -momentum:

∂ (ρv ) ∂ (ρvw ) ∂ (ρvv ) ∂ (ρvu) ∂ ∂v ∂ ∂v ∂ ∂v = (μ ) + (μ ) + (μ ) + + + ∂t ∂x ∂x ∂y ∂y ∂z ∂z ∂z ∂y ∂x μ ∂P − − v K ∂y (3) Equation of Z -momentum:

∂ (ρw ) ∂ (ρww ) ∂ (ρwv ) ∂ (ρwu) ∂ ∂w ∂ ∂w = (μ ) + (μ ) + + + ∂t ∂x ∂x ∂y ∂y ∂z ∂y ∂x ∂ ∂w ∂P (μ ) − + ∂z ∂z ∂z μ − w + ρgβ (T − Tref ) K

(4)

Energy equation:

∂T ∂T ∂T ∂T ∂ ∂T ∂ ∂T + (u − vw ) +v +w )= (k ) + (k ) ∂t ∂x ∂y ∂z ∂x ∂x ∂y ∂y ∂ ∂T + (k ) + S ∂z ∂z ∂ ∂ ∂ − (ρuΔH ) − (ρvΔH ) − (ρwΔH ) ∂x ∂y ∂z ρCp (

(5)

3. Numerical simulation The Eqs. (1)–(5) were used simultaneously for theliquid and solid phases. At the top surface, the boundary conditions are presented in the following: For liquid region of the molten pool

The aim numerical modeling in this study is investigation of temperature distribution of the area near the fusion zone and prediction of melt pool dimensions in laser welding of dissimilar thickness materials. Moving the workpiece in x direction will create asymmetric temperature field and the shape of melting pool. So, the a transient three-dimensional problem ough to be solved. The calculation domain were 60(mm) × 15(mm) × 1(mm) , 60(mm) × 15(mm) × 3(mm) 60(mm) × 15(mm) × 1.5(mm) and (length × width × thikness ). A non-uniform (finer and courser mesh)

−

∂γ ∂T ∂γ ∂T ∂v ∂u =μ ; w=0 =μ ; − ∂T ∂y ∂z ∂T ∂x ∂z

(6)

In Eq. (6), the coefficient of surface tension temperature was illustrated by ∂γ . ∂T For the solid region:

Table 1 Chemical composition of Ti6Al4V alloy. Ti%

Al%

V%

Cu%

Mn%

Fe%

Cr%

Mo%

Si%

Sn%

Zr%

Nb%

Base

6.5

4.0

< 0.02

0.02

0.04

< 0.01

< 0.03

0.03

< 0.05

0.02

0.02

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Fig. 2. Setup configuration of the laser welding.

beam intensity as follow [39],

P (x , y, t ) = P0 h (t ) e −[2r 2/ rb2]

(11)

where P0 is the intensity of laser beam at the beam center and r = x 2 + y 2 which is known as the radial distance from the beam center is presented as,

P0 =

Table 2 The process parameters of laser welding.

1 2 3 4 5 6 7 8 9 10 11 12

Welding speed (mm/ sec)

Laser Power (W)

Pulse frequency (Hz)

Pulse duration (ms)

Focal length (mm)

4.3 2 4.3 6.2 4.3 4.3 4.3 2 2 2 4.3 4.3

150 180 180 180 180 180 150 180 180 240 225 240

15 20 20 20 20 20 15 20 20 20 15 20

6 6 6 6 6 6 6 6 6 6 8 8

3 3 3 3 2 4 4 2 4 4 4 4

u = vwelding ; v = 0; w = 0

4. Result and discussion In this section, the temperature field and melt pool area is presented according to the variations of the parameters of laser including focal length, speed of welding and peak power. 4.1. The effect of focal length variation In the first series of experiments (No. 8, 9), the variation in focal length is observed in Table 4 while the other parameteres remained fixed. Generally, enhancing the beam spot size via increasing the nozzle gap from the surface results in decreasing the total energy density of laser beam. Not only it is expected that the surface temperature reduces significantly but also the beam penetration in the substances is decreased. Fig. 4 showed the temperature variations versus time for the 3 and 1.5 mm thicknesses at 2 and 4 mm focal lengths using the

(7)

3.2. Initial and boundary conditions The initial condition is given as

T (x , y, z , 0) = T0

Table 3 Ti6Al4V thermophysical properties [40–42].

(8)

Property

Boundary conditions at the top surface is as follow,

z = 0 q − h (T − T∞) − ε (T 4 − T∞4 ) = −k

∂T ∂z

(9)

Symbol

Value

Density

ρ

4420 kg m3

Specific heat

c

c = (540 + 0.176T ) J kgC T ≤ Tm

Thermal conductivity

k

k = (7 + 0.0156T ) W mC T ≤ Tm

Latent heat

L

418680 J kg

Melt temperature Boiling temperature Dynamic viscosity

Tm Tb μ

1650 °C 3290 °C

c = 830.4 J kgC

where σ is the constant of Stefan–Boltzmann and is equal to 5.67 × 108 W m2K 4 , ε is the emissivity coefficient and h is the coefficient of convective heat transfer. Frewin and Scott [37] presented the following correlation for these problems as follow,

h = 2.4 × 10−3εT1.61

(12)

where Ptot is the overall absorbed power, Ptot = ηPincident , Pincident is the incident laser beam power, rb is the radius of laser beam, and η is the mean value of material absorption. h (t ) in the Eq. (11), indicates the temporal intensity changes which takes the values 1 and 0 when the pulse is active or inactive, respectively. The Ti6Al4V Thermophysical properties of is observed in Table 3.

Fig. 3. Schematic model of sample for laser welding.

Test No.

2Ptot πrb2

k = 32.74 W mC

(10)

The temperature-dependent emissivity values was derived from the study of Yang et al. [38]. A Gaussian distribution was considered for 441

5.2 × 10−3 N . s m2

T > Tm

T > Tm

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Table 4 Laser welding parameters in tests 8, 9. Welding speed (mm/s)

Power (W)

Focal length (mm)

Frequency (Hz)

Pulse duration (ms)

Test number

2 2

180 180

2 4

20 20

6 6

8 9

Fig. 4. Numerical and experimental results of temperature distribution versus time as a function of focal length (a) F.L = 2 mm, (b) F.L = 4 mm.

Fig. 5. The cross sectional area of the samples at different focal length, (a) F.L = 2 mm, (b) F.L = 4 mm. Table 5 Comparison the melt pool geometry and the peak temperature for different focal lengths. Sample

Focal length (mm)

Welding Width (mm)

Welding Depth (mm)

HAZ Width (t = 1.5 mm) (mm)

HAZ Width (t = 3 mm) (mm)

T max (t = 1.5 mm) (ºC)

T max (t = 3 mm) (ºC)

8 9

2 4

0.94 0.9

1.12 1.02

0.58 0.56

0.52 0.5

187.01 177.77

74.38 99.88

Table 6 Laser welding parameters in tests 8, 9. Welding speed (mm/s)

Power (W)

Focal length (mm)

Frequency (Hz)

Pulse duration (ms)

Test number

2 4.3 6.2

180 180 180

3 3 3

20 20 20

6 6 6

2 3 4

temperature change in the thinner sheet increases more quickly. In Test 9, the temperature of the 1.5 mm sheet increased from 25 to 177.77 °C, whereas, in the 3 mm sheet, the temperature rose from 25 to 99.88 °C. The higher temperature indicates that the thinner piece acted as a

experimental and numerical results. As seen in this figure, when the laser beam moves across the area in front of the thermocouple, the temperature of the thinner (1.5 mm) sheet is higher than that of the thicker (3 mm) sheet. Also, the rate of

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Fig. 6. Experimental results of temperature versus time as a function of welding speed (a) t = 1 mm, (b) t = 1.5 mm.

the sizes of the molten pool and HAZ are presented. According to the molten pool comparison presented in Table 5, when the focal length decreases, the laser is irradiated from a shorter distance, and the temperature of the metal further increases, subsequently increasing the temperature and dimensions of the HAZ and molten pool.

4.2. The effect of welding speed As seen in Table 6, in the second series of the experiments (2, 3, 4 test No), the welding speed was changed while another parameters remained constant. The thickness of samples in these tests are 1 and 1.5 milimeteres. By comparing the results, it can be noted that by increasing the speed of welding, the measured temperature by thermocouples was clearly decreased. According to the Fig. 6, enhancing the welding speed from 2 to 6.2 mm/s, caused to sharply reducing the temperature adjacent area of the melt pool from 280.17 °C to 98.24 °C and from 224.32 °C to 68.57 °C for samples with 1 and 1.5 mm thickness, respectively. When the laser motion speed is increased, the laser beam is projected onto the piece for a shorter duration, resulting in a decreased temperature of the molten pool and its vicinity. This also has a remarkable effect on the microstructure, quality of weld, and size of the molten pool. generally, the molt pool size grows as the temperature increases. By comparing Fig. 6 (a) and (b), it is found that for a constant laser speed, the thinner piece experiences a higher temperature. It shows that the thicker piece serves as a larger heat sink and the heat input to the piece remains in it for a shorter duration, resulting in a higher conductive heat transfer to the vicinity of the welding position and a lower temperature. In other words, the cooling rate of the thicker piece was higher than, the thinner piece. Fig. 7 shows the top view of the welded surface in Test 3. It shows that the molten pool has inclined towards the thinner piece with relative to the contact point of the 1 mm and 1.5 mm pieces. Table 7 compares the sizes of the molten pools and the maximum temperature in Test 2, 3 and 4. Fig. 8a presents the changes of the melt pool with varing the

Fig. 7. Weld bead surface appearance of Test 2.

smaller heat sink and retained the heat input in the workpiece for a longer duration. As a result, conduction heat transfer to the vicinity of welding area was smaller and a higher temperature was occured. In other words, the cooling rate of the thinner piece was lower than the thicker piece. This is evident in the comparison between temperature variations of the thin piece at the two focal lengths of 2 and 4 mm (Fig. 4a and b) where an increase in the focal length and lower heat absorption led to a drop in the maximum temperature of the thicker piece, whereas for the thinner piece this parameter did not show a significant change. Also, the the numerical simulation results were match with the experiments. Fig. 5 demonstrated the images of molten pool at 2 mm and 4 mm focal lengths. As seen in Fig. 5, the molten pool has deviated towards the thinner sheet indicating a higher volume of molten metal in the thinner sheet. The heat-affected zone is also larger in the sheet with less thickness. The reason is that the thicker sheet serves as a larger heat sink compared to the thinner sheet, thereby providing a higher cooling rate. Based on the above explanations, the temperature distribution in the 3 mm sheet occurs at a lower rate and the molten pool becomes smaller. In Table 5,

Table 7 Comparison the geometry of the molten pool and peak temperature for various welding speeds. Sample

Welding speed (mm)

Welding Width (mm)

Welding Depth (mm)

HAZ Width (t = 1 mm) (mm)

HAZ Width (t = 1.5 mm) (mm)

T max (t = 1 mm) (ºC)

T max (t = 1.5 mm) (ºC)

2 3 4

2 4.3 6.2

1.51 1.26 0.93

Full penetration Full penetration 0.89

0.86 0.74 0.59

0.79 0.7 0.48

280.17 147.18 98.24

224.32 93.91 68.57

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Fig. 8. Results of numerical and experimental data (a) width as a function of speed of welding, (b) maximum temperature as a function of speed of welding and (c) maximum temperature as a function of width of melt pool. Table 8 Laser welding parameters in tests 9, 10. Welding speed (mm/s)

Power (W)

Focal length (mm)

Frequency (Hz)

Pulse duration (ms)

Test number

2 2

180 240

4 4

20 20

6 6

9 10

Fig. 9. numerical simulation and experimental results of the temperature distribution versus time as a function of power (a) P = 180 W, (b) P = 240 W.

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Table 9 Comparison the geometry of the molten pool and peak temperature for various focal lengths. Sample

Power (W)

Welding Width (mm)

Welding Depth (mm)

HAZ Width (t = 1.5 mm) (mm)

HAZ Width (t = 3 mm) (mm)

T max (t = 1.5 mm) (ºC)

T max (t = 3 mm) (ºC)

9 10

180 240

0.9 1.73

1.02 1.41

0.56 0.92

0.5 0.73

177.77 325.51

99.88 264.47

• The sheet thickness has had the highest impact on the weld devia-

welding speed achieved by performing tests 2, 3 and 4. The variation of speed of welding was between 2 and 6.2 mm/s. Looking at the results, by enhancing the laser speed of welding, the molten pool width was decreased. In other words, welding speed and welding width have an inverse effect on each other. Fig. 8b shows the relation between the welding speed and experimental and numerical maximum temperature around the molten pool (2 mm far from the molten pool center). It can be concluded that the temperature was diminished by enhancing the welding speed. Fig. 8c illustrates variation of the peak temperature with a molten pool width. This figure was obtained from the results of Fig. 8a and b. The results indicated that a smaller width is related to smaller temperature. This trend can be reasonable and numerical results are completely match with experimental findings.

• • • •

4.3. Effect of power As shown in Table 8, in the third series of the experiments (910 test No), the power was changed while another parameters remained constant for samples with 1.5 and 3 milimeteres thickness. Commonly, enhancing the electrical current causes to the increasing the peak power. By rising the electrical current, the laserpower was increased from 180 W to 240 W. Usually, increasing the laser peak power has a direct effect on the penetration depth in laser welding and the amount of heat input to the work piece. The most important effect of increasing peak power is increasing the pulse energy. In Fig. 9, the comparison of the temperature near the molten pool in experiments No. 9 and 10 indicates a significantly enhance in temperature with rising the power. Table 9 compares the sizes of the molten pool and the maximum temperature in Tests 9 and 10. As seen in Fig. 9 and Table 9, it is concluded that as the laser power rises, the maximum temperatures experienced by 1.5 mm and 3 mm thickness pieces also increase. Further, at both 180 W and 240 W powers, the thinner sheet experiences higher temperatures compared to the thicker sheet. Similarly, the width and depth of the molten pool grew significantly by increasing the power of laser. This is normal due to the higher heat absorbed by the workpiece at higher powers.

Funding No funding was received for this work. Declaration of Competing Interest We wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome. Acknowledgements This research is partially supported by NSFC (51979261), State Key Laboratory of Hydraulic Engineering Simulation and Safety-Tianjin University (No. HESS-1901) and Australia ARC DECRA (No. DE190100931) References

5. Conclusions

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In this study, pulsed laser welding experiment were conducted on Ti6Al4V alloy plate with 60 × 15 mm area and 1, 1.5 and 3 mm different thicknesses. To predict the temperature, HAZ and molten pool dimensions through the laser welding. The key results are as follow,

• According • •

tion because of creating different cooling rate. Lower cooling rate induced by thineer plate created higher temperature gradient on thinner plate and thereby the laser beam absorbtion remarkably increased and the melt pool deviated toward the thinner plate. By decreasing the the laser power, the melt volume and the temperature of the molten pool boosted remarkably compared to changing the other parammeters. The numerical simulations temperature gradients were adequately match with the empirical findings. Enhancing the welding speed from 2 to 6.2 mm/s, sharply reduced the temperature of the adjacent area of the melt pool from 280 °C to 98 °C for and 1 mm thickness. A reduction from 224 °C to 68 C was observed for 1.5 mm thickness. Although the depth of melt pool was identical for both thicknesses (1and 1.5 mm), the width of the melt pool for 1 mm thickness was almost towice bigger than 1.5 mm thickness because of inclination the melt pool toward the thiner sheet.

to the temperature measured by thermocouples, the thinner plate showed the higher temperature gradient and the melt pool has been deviated from the center of the butt joint toward the thinner plate due to lower rate of cooling. The results showed that the thinner plate determines the sets of parameters selection in order to achieve appropriate both penetration depth and width of the melt pool. Changing the pulsed laser process parameters has a considerable effect on the temperature and dimensions of the molten pool. According to selecting appropriate process parameters composed of laser power, pulse frequency, pulse duration and peak power, acceptable quality of weld bead will be produced. 445

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