Thermal cycle and its influence on the microstructure of laser welded butt joint of 8 mm thick Ti-6Al-4V alloy

Tian Deyong,Yan Tingyan,Gao Qiyu,Wang Feiyun,Zhan Xiaohong

1.National Key Laboratory of Science and Technology on Helicopter Transmission,Nanjing University of Aeronautics and Astronautics,Nanjing 211106,China;

2.College of Science,Nanjing University of Aeronautics and Astronautics,Nanjing 211106,China

Abstract Ti-6Al-4V alloy is extensively used in the manufacture of components in aviation.In the current study,the laser welding process is adopted to joint the Ti-6Al-4V alloy plate which has the thick of 8 mm.A three-dimensional finite element model is established to simulate the temperature distribution of laser welding process.The thermal cycle curves are produced on the strength of the simulation results.Meanwhile,the microstructure characteristics of the welded joint are investigated combined with simulation results.The results show that weld zone,heat affected zone and based metal experience similar thermal cycles process and the cooling rate has an important influence on the formation of microstructure.Moreover,the simulation results are well matched with experiment results.

Key words Ti-6Al-4V alloy,laser welding,temperature distribution,thermal cycle,microstructure

0 Introduction

Titanium and its alloys have been widely used in many industries fields,to fabricate a number of complicated components,on account of their excellent properties such as low density,good corrosion resistance and high operating temperature,etc.Among these materials,the development of Ti-6Al-4V alloy has been driven by the aviation and aerospace industry for structural applications.Such complex components often require high-quality welded joint to meet their service demand.Thus,it is necessary to employ an appropriate advanced welding process.Compared to traditional welding process,laser welding with its distinctive advantage provides extra benefits.For instance,a comparative test with laser and TIG welding was conducted by Gao et al.[1]showing that laser welding has less residual distortion.As proposed by Lacki[2],laser welding is capable to operate in antivacuum environment,which makes the test cost lower than electron beam welding.

Considerable researches on the laser welding of titanium alloys were carried out, where different process parameters[3-5], microstructure and mechanical properties[6-9]were discussed. Cao et al.[10]proved that Nd:YAG laser is an applicable source of laser power for welding titanium alloy. Chen et al.[11]revealed the microstructure of weld zone is coarse and original β-columnar crystal. As the laser power increases, the martensite distribution is more dense.Wang et al.[12]showed that laser beam welded Ti-6Al-4V sheet achieved good mechanical properties even if tests temperature rises up to 450 ℃.

In order to obtain a good understand of laser welding process for Ti-6Al-4V alloy,some comprehensive studies using the simulation method.Akbari et al.[13]set up a transion the progress of the laser welding process were finished ent three-dimensional model to predicted the heat affected if tests temperature rises up to 450 ℃.zone (HAZ),depth and width of the molten pool.Ahn et al.[14]predicted residual stresses and distortion in the fibre laser weld sample.Panwisawas et al.[15]obtained the morphology of the molten pool during laser welding process under different laser powers.

A lot of work about Ti-6Al-4V thin plates (below 3 mm)have been finished,while the investigations with the laser welding for Ti-6Al-4V alloy thick plates are rarely mentioned.Therefore,it is necessary to investigate the laser weldng process of thick plate.In the current study,a threedimensional finite element model is established to simulate the temperature distribution of laser welding process.Meanwhile,the thermal cycle curves during laser welding process are produced on the strength of the simulation results.Moreover,the microstructure characteristics of the welded joint are observed through optical microscope.

1 Welding procedure and material

Laser welding experiments are performed on Ti-6Al-4V alloy plates with dimensions of 100 mm×50 mm×8 mm.The chemical composition of Ti-6Al-4V is shown in Table 1.The equipment for the laser welding is shown in Fig.1,which consists of a laser power,KUKA robot and watercooled machine.The laser power produces no more than 12 kW continuous wave solid state Nd-YAG laser.The principle of the laser welding is simply shown in Fig.2.During the laser welding process,the KUKA robot moves according to the predetermined welding path and the laser beam focuses on the surface of plates by a convey lens with a focal length of 30 mm.

Table 1 The chemical compositions of Ti-6Al-4V(wt%)

Fig.1 Laser welding experimental equipment (a)KUKA Robot (b)Laser device (c)Water-cooled machine

Fig.2 Experimental principle of laser welding process(a)The three-dimensional schematic diagram(b)The three-dimensional schematic diagram in the welded joint

The satisfied samples are selected as the research objects of this paper,whose experimental parameters are given in Table 2.The microstructure characteristics of crosssection is revealed through optional microscope.

Table 2 Parameters of laser welding process

2 Finite element model

Before the simulation of the laser welding process,three-dimensional geometric model is established and meshed,which is shown in Fig.3.As illustrated,fine mesh is employed near the weld bead where the temperature is high.While the area far away from the weld bead is divided with a relatively coarse mesh to reduce the number of grids.

Fig.3 Meshing of the laser welding process(a)Overall grid division(mm)(b)Weld grid division

In order to simulate the welding process more realistically,the Rotary-Gauss body heat source is adopted in this study,which is shown in Fig.4.The heat density expression is given by Eq.(1).

whereQis the effective laser power,R0is the radius of heat source,H is the depth of heat source.From the actual weld profile,these distribution parameters are set to be 7 500 W,2 mm and 7 mm respectively.

Fig.4 The energy density distribution of the Rotary-Gauss body heat source model

The essence of the heat transfer law is the first law of thermodynamics,which is described by the following Eq.(2).whereρis the density of material,cpis the specific heat,λis the heat transfer coefficient andQis the internal heat source.

The initial condition involves in the simulation of laser welding process consists of the initial temperature of Ti-6Al-4V alloy.It is necessary to consider that the initial temperature of the based metal equals to the room temperature which is set to be 25 ℃.The specific initial condition of based metal can be given by Eq.(3).

whereTis the temperature distribution function of the based metal.Tbis the initial temperature of based metal.In this simulation,the boundary condition considering the heat convection among molten pool,based metal and environment is applied during laser welding process can be described in Eq.(4).

whereqis the heat density located in the boundary,his the convective coefficient which is set to be 40 J/m2K,T1is the transient temperature of based metal,T0is the room temperature.As shown in Fig.5,the simulated result using the Rotary-Gauss body heat source model is compared with the experimental results.It is explicit that the shape and size of the molten pool is in good agreement with the experimental result.

Fig.5 Comparison between simulated result and experimental result

3 Results and discussion

The distribution of temperature field in laser welding process at 35th time-step is shown in Fig.6a.It is evident that the peak temperature appears in the center of the molten pool and the temperature gradually reduces from the center outward.The distribution of temperature gradient shows some difference at diverse locations in the molten pool.The spacing of isotherm in the front of molten pool is far less than that of the rear one,which means the temperature gradient in the front of pool is larger than that in the back.The thermal cycles of three nodes,located at the cross section of the molten pool,are shown in Fig.6b-d.It is signified that three node experience similar thermal cycles.On account of the high density of laser energy,the temperature rises in no time with heat source coming closer and decreases gradually as the laser beam goes away.It is explicit that the slope of the decline at node 1 (located at weld zone)is much larger than node 2 (located at heat affected zone)and node 3 (located at based metal).

Fig.6 Temperature distribution during laser welding process (a)Temperature field at 35th time-step(b)Thermal history at node 1(c)Thermal history at node 2(d)Thermal history at node 3

Fig.7 gives the microstructure of welded joint.As indicated in Fig.7a,the profile of weld bead presents Y-shaped.Fig.7b reveals the different orientation acicular α’ solidification phases tangle in the β grains in the weld zone after the laser welding process.Fig.7c shows that the microstructure of heat affected zone (HAZ)is composed of the acicular α′phase and the original α phase.In heat affected zone,the part near weld bead is mainly distributed with acicular α′phase,while the other side away from weld bead is distributed with original α phase.In addition,the microstructure of based metal is distributed with original α phase,shown in Fig.7e.

Fig.7 Microstructure of welded joint(a)The overall structure of welded joint(b)The microstructure of weld zone(c)The microstructure of heat affected zone(d)The Microstructure of around the inner wall of porosity(e)The microstructure of based metal

The region where the cooling rate is between 410 ℃/s and 525 ℃/s normally results in transformation to the acicular α’ phase[16].Calculated by Fig.7a,the cooling rate in weld zone is 479 ℃/s,thus acicular α’ phase tangles in the β grains in the weld zone.Moreover,as shown in Fig.7c and Fig.7d,the cooling rates in heat affected zone and based metal are both below 410 ℃/s,since the original α phase appears.Acicular α’ phase is only distributed in the heat affected zone near the weld bead.As shown in Fig.7d,the microstructure around the inner wall of porosity is acicular α’ phase,which indicates that the solidification of the metal liquid around the bubble is extremely fast.

4 Conclusions

(1)The simulations on thermal distribution in the Ti-6Al-4V alloy during the laser welding process are achieved.The experiment results and the simulation results are in good agreement.Hence the simulations can provide a powerful guidance for further optimization process.

(2)Weld zone,heat affected zone and based metal experience similar thermal cycles.The slope of the decline at node 1 (located at weld zone)is much larger than node 2(located at heat affected zone)and node 3 (located at based metal).

(3)The cooling rate in weld zone is 479 ℃/s,thus acicular α′ phase tangles in the β grains in the weld zone.While the cooling rates in heat affected zone and based metal are both below 410 ℃/s,since the original α phase appears.Acicular α′ phase is only distributed in the heat affected zone near the weld bead..