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Proceedings of the

1st International Conference on Engineering Materials and Metallurgical Engineering

22- 24 December, 2016 Bangladesh Council of Scientific and Industrial Research (BCSIR)

Dhaka, Bangladesh

OPTIMIZATION OF THE PROCESS PARAMETERS IN LASER WELDING OF TITANIUM TI-6AL-4FE-0.25SI ALLOY

Maksudur Rahmana,*, Yeong-Do Park b

a,bDepartment of Advanced Materials Engineering, Dong-Eui University, Busan, South Korea.

Abstract In this current study, Laser welding with different welding parameters of Ti-6Al-4Fe-0.25Si alloy was evaluated. Microstructure and mechanical properties was investigated at different welding conditions to find out the better welding parameter. Laser welding gives narrow weld zone and HAZ than other conventional welding process. In laser welding, lower welding power and higher welding speed give the higher cooling rate, hence the finer structure as well as increased hardness is possible to occur. It is noticed that the higher the welding speed, the finer the columnar structure. This is attributed to an increase in both solidification and cooling rates due high welding speed. Concerning the effect of laser power, the higher the laser power, the coarser is the columnar structure due to decreasing cooling rate. Hence lower laser power and higher welding speed gives the higher cooling rate and weld with increased hardness. Keywords: Laser welding, Solidification, Cooling rate, Columnar structure.

1. INTRODUCTION

Titanium alloys is having high strength to weight ratio, good corrosion properties. The conventional titanium alloys contain expensive molybdenum, vanadium etc. To reduce the cost of production iron, silicon etc. are currently used to make the alloy cheaper but maintaining good properties. The effect of aging heat treatment is investigated on Ti-6Al-4Fe-0.25Si alloys microstructure [1]. Effect of Fe and Si on the microstructure and tensile properties of Ti-6Al-4Fe-0.25Si is also investigated by the other researcher [2]. It is also shown that the strength and elongation is strongly depends on silicon contents. Solid solution hardening and precipitation hardening due to silicon content make the material more strengthen. But these materials need to be joined by welding to build structures. After welding the properties can be deteriorated due to change in microstructure which would cause failure lead to severe accident.Olabi et al. applied RSM to investigate the effect oflaser welding parameters on residual stress distributionover the depth, at three locations from the weld centerline of AISI304 butt joints [3].Casalino et al. investigated butt welding of Ti 6Al 4Valloy by using continuous CO2 laser [4]. Olabi et al. usedan ANN and Taguchi algorithms integrated approach tothe optimization of CO2 laser welding of medium carbonsteel [5]. In the current investigation, (1) Laser welding was performed in Ti-6Al-4Fe-0.25Si and the alloy was in surface milled condition, (2) Optimization of the Process Parameters LASER Welding.

2. EXPERIMENTAL CONDITIONS

Specimen of 150mm x 50 mm dimension is used to make bead on plate. A standard Yb:YAG Disk laser with 200um delivery fiber diameter, 220um beam spot diameter was used to weld. The Argon gas atmosphere was used as shielding. Before welding the alloys surface was cleaned by high purity solvent acetone. After cleaning, the surface is pickled with a solution of 4% hydrofluoric acid (52%) and 35%nitric acid (70%) in distilled water for 2 min and followed by cleaning with water rinse and air drying using nitrile gloves. The thickness of as received condition is 1.90 mm. The parameter settings for laser welding are shown in table 1. The welded parts were polished using emery paper, and finally using diamond suspension up to 3µm level; the polished surface was revealed by using etching solution containing 2ml HF, and 5ml HNO3 in 20ml of distilled water.

Proceedings of the 1st ICEMME, 22-24, Dec, 2016, Dhaka, Bangladesh

Table1: Experimental welding conditions

Parameters Experimental Laser type Yb:YAG Disk Laser

WeldingPower (kW) 1, 1.5, 2 Welding speed (m/min) 2, 3, 4

Focal length (mm) 220 BPP 8mm*mrad

Focal position Focus on base metal Beam spot diameter (µm) 220

Diameter of delivery fiber (µm) 200 Vickers micro-hardness profiles were carried out on the base metal, the HAZ and the fusion zone with a 500 g load and penetration time of 10 seconds using a digital machine. Mechanical tensile tests were used to evaluate the properties of the base metal and the welded joints from the two welding programs. Six test specimens were prepared for each type of welding and three for the base metal. The tests were developed in accordance to the ASTM E-8sub size standard.

3. RESULTS& DISCUSSIONS

In this current study, Laser welding with different welding parameters of Ti-6Al-4Fe-0.25Si alloy was evaluated. Microstructure and mechanical properties was investigated at different welding conditions to find out the optimum process parameter.

3.1. BASE METAL

The base metal microstructure for Ti-6Al-4Fe-0.25Si (α-β alloy) is shown in Fig. 1. The base metal microstructure contains grain boundary with the black spot (beta [β]).

FIG. 1: BASE METAL AS RECEIVED CONDITIONS (500X) The hardness of base metal is measured as received conditions. There are some variations in hardness in different positions at as received conditions. The average hardness is 375HV. Tensile tests were taken along the rolling direction as well as transverse to the rolling direction. The specimen standard size was followed according ASTME E-8 sub size. The average tensile strength of this alloy is 1100MPa along the rolling direction and 1114MPa at transverse to the rolling direction. Hence it can be stated that the average tensile strength is similar at the both direction and there is no effect of rolling is case of tensile strength.

3.2. BEAD APPEARANCE

The surface and back bead are shown in table 2. The color of the appearance is silver, which means the welding quality is very good.

Proceedings of the 1st ICEMME, 22-24, Dec, 2016, Dhaka, Bangladesh

Table 2: Bead Appearance after Laser welding

In case of 1.0kw welding power, more than 2.0 m/min welding speed didn’t give any back bead. At 1.5kw welding power and 4 m/min welding speed, discontinuous back bead was occurred. The bead width measurements are shown in the Table 3.

Table 3: Bead width measurements

Speed (m/min)

Power (1.0 kW) Power (1.5 kW) Power (2.0 kW) Front Bead

(mm) Back Bead

(mm) Front Bead

(mm) Back Bead

(mm) Front Bead

(mm) Back Bead

(mm) 2.0 1.84 0.75 1.92 1.49 1.71 1.85 3.0 1.37 - 1.53 1.13 1.43 1.22 4.0 1.18 - 1.36 0.91

(Discontinuous) 1.16 1.20

As the welding speed increases the bead width decreases. But in case of 3kw power the back bead width increases after 3m/min speed. When the welding power increases the back bead width increases but in case of 2kw power, the front bead is lower than the back bead. The heat input decreases with increasing the welding speed and increases with increasing the welding power. Higher heat input gives the larger bead size. The differences between the front and back bead widths are given in the Fig. 2. In case of 2kw welding power, the back bead width became larger at 2 m/min and 4 m/min welding speed. From lower heat input to higher heat input, the welding mode changes from convection to keyhole mode. At the keyhole mode, it is possible to get larger back bead width.

2 3 4

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

1 kw

2kw

FBW

- BB

W (m

m)

Welding speed, m/min

1.5kw

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FIG. 2: THE DIFFERENCES BETWEEN THE FRONT AND BACK BEAD WIDTHS

The above results have shown that laser power and welding speed should be optimized in order to minimize heat input, then a satisfactory weld with reliable quality could be obtained. This reflects one of the most notable features of laser welding compared with other welding processes, which are small heat input [6].

3.3. HARDNESS

Vickers hardness was measured across the welded part which in distance of 0.1mm with a load of 0.5kg.The hardness measurement is shown fig 3. It can be seen that the weld zone shows much higher hardness at 2 kW & 2 m/min, 1.5 kW & 2 m/min and 2 kW & 3 m/min welding conditions respectively.

(a) Higher Hardness (b) Lower Hardness

FIG. 3: HARDNESS MEASUREMENT ACROSS THE WELD

Different welding power and different the welding speed cause the different heat input and cooling rate. For this reasons the hardness profile is different in different samples. From the Table 4, it is clear that with the increasing the heat input the hardness is increases. At the same heat input with different welding conditions, the hardness at the center of the weld is similar.

Table 4: Hardness at different heat input

Welding Power (kw)

Welding speed (m/min)

Heat input (kJm-1)

Hardness, (HV,0.5kg)

1 2 30 417.5 1.5 3 30 440.9 2 4 30 443 2 3 40 484.1

1.5 2 45 516.5 2 2 60 532

3.4. WELDED STRUCTURE

The cross section of this welded part is shown in Fig. 5. The top and bottom part of the weld for both titanium alloys shows columnar grain structure. Which suggest the solidification rate is higher in top and bottom part of the weld. The true path of the grains was from the side walls extending toward the center of the top bead. In the middle of the plate, the grains are "swept" in the weld direction.In the range of investigated parameters of bead-on-plate laser welding of the titanium Ti-6Al-4Fe-0.25Si alloys sheets the mechanism of melting and welding is a key-hole welding (Fig. 5).

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0300

350

400

450

500

550 1.5kw-2m/min 2kw-2m/min 2kw-3m/min

Har

dnes

s, H

V (0

.5 k

g)

From centre of weld metal to base metal, distance (mm)-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0

300

350

400

450

500

550 1kw-2m/min 1.5kw-3m/min 2kw-4m/min

Har

dnes

s, H

V (0

.5 k

g)

From centre of weld metal to base metal, distance (mm)

Proceedings of the 1st ICEMME, 22-24, Dec, 2016, Dhaka, Bangladesh

FIG. 5: CROSS SECTION OF THE WELD

The noticeable feature is the highly directional nature of the microstructure around the axis of the laser beam. This is due to solidification of the weld metal at high cooling rate compared to that of conventional GTA welding [7]. It can be noticed that the higher the welding speed, the finer the columnar structure. This is attributed to an increase in both solidification and cooling rates due to low heat input resulted from high welding speed. Concerning the effect of laser power, the higher the laser power, the coarser is the columnar structure due to decreasing cooling rate. Hence lower laser power and higher welding speed gives the higher cooling rate and weld as increases the hardness.

(a) 1 kw , 2m/min (b) 1.5 kw , 2m/min

(c) 1.5 kw , 3m/min (d) 2 kw , 2m/min

(e) 2 kw , 3m/min (f) 2 kw , 4m/min

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According to the hardness profile and the welded structure analysis, optimum welding condition would be (a) 1.5kW power with 2m/min welding speed or (b) 2kW power with 3m/min welding speed. For further analysis Tensile tests were carried out to find the optimum welding condition.

3.5. TENSILE PROPERTIES

It is reported earlier that the tensile strength of base metal 1107 MPa. Now the experiment was carried out for the welded metal. The specimen standard size was followed according ASTME E-8 sub size. The average tensile strength and elongation are given in the Table 5. The tensile strength of the weld metal is similar with the as received base metal. The elongation decreases due to the higher hardness at the weld zone.

Table 5: Tensile strength and elongation of the welded metal

Properties 1.5kw-2m/min 2kw-3m/min As received

Tensile strength [MPa] 1098 1087 1100

Elongation (%) 4.8 4.9 14.5

Tensile fracture occurred at weld zone due to higher hardness at the weld zone. From the above discussions, 2kW power with 3m/min welding speed is the optimum welding condition due higher welding speed with maximum hardness and acceptable tensile strength.

4. CONCLUSIONS 1. Ti-6Al-4Fe-0.25Si is an α-β alloy of Titanium. 2. Laser welding gives narrow weld zone and HAZ. 3. In laser welding, lower welding power and higher welding speed give the higher cooling rate, hence the finer

structure as well as increased hardness. 4. 2kW power with 3m/min welding speed is the optimum welding condition for Ti-6Al-4Fe-0.25Si alloys in

LASER welding processdue higher welding speed with maximum hardness and acceptable tensile strength.

5. REFERENCES [1] Yong Hwan Song, Joo-Hee Kang, Chan Hee Park, Seong-WoongKim,Yong-Taek Hyun, Nam Hyun Kang,

and Jong-TaekYeom. Microstructure Evolution of Ti-6Al-4Fe-0.25Si Through Aging Heat Treatment. [2] E.H. Kim, H.W. Jeong, S.E. Kim, Y.T. Hyun, Y.T. Lee and J.W. Yoon. Tensile Properties Of Cast And

Hot Isostatic Pressurized Ti-6Al-4Fe-Xsi Alloys. [3] A. G. Olabi, K. Y. Benyounis and M. S. J. Hashmi, “Applicationof Response Surface Methodology in

Describingthe Residual Stress Distribution in CO2 Laser Weldingof AISI304,” Strain: An International Journal ExperimentalMechanics, Vol. 43, No. 1, 2007, pp. 37-46.

[4] G. Casalino, F. Curcio and F. M. C. Minutolo, “Investigationon Ti6Al4V Laser Welding Using Statistical andTaguchi Approaches,” Journal of Materials ProcessingTechnology, Vol. 167, No. 2-3, 2005, pp. 422-428.doi:10.1016/j.jmatprotec.2005.05.031

[5] A. G. Olabi, G. Casalino, K. Y. Benyounis and M. S. J.Hashmi, “An ANN and Taguchi Algorithms IntegratedApproach to the Optimization of CO2 Laser Welding,”Advances in Engineering Software, Vol. 37, No. 10, 2006,pp. 643-648. doi:10.1016/j.advengsoft.2006.02.002

[6] Abdel-Monem El-Batahgy. Effect of laser welding parameters on fusion zone shape and solidification structure of austenitic stainless steels.Materials letters 32(1997) 155-163.

[7] S.S. David, J.M. Vitek and T.L. Hebble, Welding J. 66(1987)289.