Deposition of Ti–6Al–4V using a high power diode laser and wire Part I

Deposition of Ti –6Al –4V using a high power diode laser and wire,Part I:

Investigation on the process characteristics

Sui Him Mok,Guijun Bi ?,Janet Folkes,Ian Pashby

Innovative Manufacturing Processes Group,School of Mechanical,Materials and Manufacturing Engineering,

University of Nottingham,University Park,NG72RD,UK

Received 24September 2007;accepted in revised form 4February 2008

Available online 20February 2008

Abstract

In this paper the deposition of Ti –6Al –4V wire with High Power Diode Laser was investigated by producing single tracks.The effect of the wire feeding direction and angle was firstly studied.The influence of laser power,traverse speed and wire feed rate on the weight and dimension of the deposited single tracks was then investigated.The microstructure and hardness of the single tracks were examined.Deposition with diode laser and wire was proved to provide a high deposition rate with good quality.Columnar grains were found in the deposits.Wire feeding orientation,laser power,traverse speed and wire feed rate were verified as factors which influenced the quality of the deposit.With similar energy level,different power/traverse speed produced deposits with different hardness value.Hardness values tended to increase from the deposit,via the re-melted zone till to the heat affected zone,and then decrease again when the measurements were taken in the unaffected base material.?2008Elsevier B.V .All rights reserved.

P ACS:81.15.Fg

Keywords:Deposition;Diode laser;Ti6Al4V

1.Introduction

Direct metal deposition with laser has been verified as a promising process for surface modification,repair and compo-nent building [1,2].Recently,further studies have been carried out to explore the feasibility of applying this process in the aero-space industry [3–6].The surface cladding can be used to generate a thin functional layer on the surface of the component made by cost-effective material.The material of the clad layer is different from the base material,which can provide high tem-perature corrosion and wear resistance.For the application of the metal deposition process,a high deposition rate or volume is required.The components produced should also have the same properties and functions as ones made by conventional methods.In the development stage,traditionally,the components are made by stock removal method,forging or casting,such as,precision sand casting and investment casting.However,when

there is any modification on the design,the production time for the new components would be very long.The concept of the proposed application methodology is by using the traditional methods to produce a “basic ”structure and applying the direct metal deposition to add the feature onto it.So,it could help to save time for manufacturing the whole structure and have higher flexibility in modifying the design in a shorter period of time.Moreover,for the repair process,it can save time and costs compared with the replacement by a new component [7].

Due to its outstanding strength-to-weight ratio,Ti –6Al –4V is one of the main materials used in the aerospace industry [8].In the U.S.this alloy accounts for about half of all titanium alloy usage [9,10].Ti –6Al –4V plays a major role in the blades,disc and the cooler parts of the compressor in the jet engines because high strength and creep resistance are required in these applications [11,12].Nowadays,there is increasing use of Ti –6Al –4V for quite critical applications for the forged structural members in aircraft,including undercarriage components,flap and slat tracks in wings and for engine mountings [13].

Most of the researches have focused on the deposition with powder,which is difficult to achieve a comparable deposition

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Surface &Coatings Technology 202(2008)3933–

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?Corresponding author.

E-mail address:biguijun@https://www.360docs.net/doc/f39999675.html, (G.Bi).

0257-8972/$-see front matter ?2008Elsevier B.V .All rights reserved.doi:10.1016/j.surfcoat.2008.02.008

rate to the traditional processes,such as PTA and TIG processes.Therefore,the proposed methodology,direct diode laser depo-sition with wire would be a possible way to overcome limi-tations in surface cladding,manufacturing and repair.For this technology metal wires are used as the additive material.During the powder deposition process a certain amount of powders can not be caught by the melt pool.The powders are blown to the surrounding environment,which causes potential hazard to the operators and the https://www.360docs.net/doc/f39999675.html,pared with powder feeding,wire feeding method has higher material usage efficiency.Al-most all the materials fed into the melt pool during the process are used to form the deposit.It lowers the risks to the operators and makes the process environment friendly.Moreover,the powder feeding nozzle plays an important role in the powder deposition process.It makes the deposition with powder more complicated than with wire.Another merit is that metal wires are easily available and cheaper than powders,which makes the wire deposition very cost-competitive.

In the literature [14]the study on the deposition of stainless steel wire with diode laser was reported.Wire deposition with Nd:YAG laser was also investigated in the literatures [15,16].However,no detailed research has been carried out to study the influence of the wire orientation,laser power,traverse speed and wire feed rate on the deposit quality.Also the deposition of Ti-

alloy wire with diode laser is a promising research area.How-ever,most of the researches focused on the deposition with Ti-alloy powders [3–6,17,18].

The aim of this project is to get an insight into the process characteristics in an attempt to understand the property,struc-ture and process interaction associated with the direct diode laser deposition with wire feeding of Ti –6Al –4V.Deposit quality and deposition rate will also be investigated.Parameters will be optimised.The main properties of the deposit including the microstructure,micro-hardness will be investigated.2.Experimental procedure 2.1.Experimental setup

The experimental system comprises a 2.5kW Rofin DL025diode laser with beam delivery system,wire feeding apparatus (wire feeder and feeding head Planetics 501)and computer nu-merically controlled (CNC)table for 4motions:horizontal motion (x axis and y axis),vertical motion (z axis)and rotational motion (A axis).In addition,the system also includes a cooling system (top and base cooling plates),a camera system for process obser-vation and video capture,a chamber filled with Argon and a Dansensor (oxygen sensor).The laser beam was set in the focal length of 85mm along the z axis and with the beam size of 2mm×7mm in focus.Fig.1shows the schematics of the experimental set-up and the wire feeding orientations.2.2.Materials

Ti –6Al –4V is the most common titanium alloy used in the aerospace industry.Such an alloy presents a relatively high strength up to 300°C and is ideal for welding.In this work 1.2mm Ti –6Al –4V wire was deposited onto 10mm Ti –6Al –4V plates.The chemical composition of the material is shown in Table 1.2.3.Arrangement of the experiments

The direct diode laser possesses a top-hat power density distribution in the slow axis direction,which is preferred for the cladding process with wire,because the top-hat and wide beam can give more tolerance for the wire feeding.A stable process can be guaranteed as long as the wire is fed into the beam.In comparison,no stable deposition process can be achieved in the fast axis direction,while the beam is too narrow and the wire vibrates due to the tension of rolled wire,it is very difficult to keep the wire in the laser beam.Thus,all the experiments were carried out in the slow axis direction of the diode laser,as shown in Fig.1.70mm-long single tracks were deposited.The experiments were scheduled in three stages.In the first stage,the correlation between deposit weight and surface quality

with

Fig.1.Schematics of the experimental setup and the wire feeding orientations.

Table 1

Composition of the Ti –6Al –4V Element Ti Al V Fe Si C O N Weight %

Bal.

6.01

3.84

0.3

0.15

0.10

0.15

0.15

3934S.H.Mok et al./Surface &Coatings Technology 202(2008)3933–3939

various feeding orientations were presented.Conclusions were drawn and preferred wire feed angle and direction were defined for further experiments.In the second stage,the correlations of deposit weight with traverse speed and wire feed rate were presented and preliminary observations of the deposition pro-cess were made.Preferred wire feeding rates were defined for further experiments.In the third stage,further works were car-ried out on exploring the characteristics of the direct laser depo-sition process in terms of deposit dimension,microstructure and micro-hardness.

2.4.Characterization of the samples

The samples were cross-sectioned and polished to examine deposit quality with respect to dimension,porosity,and micro-hardness.The samples were then etched using 2%HF +5%HNO3in water for the purpose of micro-structure analyses.Optical Microscopes were used to study and analyse the micro-structure.Micro-hardness test machine M-400developed by LECO was used to examine the hardness values of the single track deposition.For the testing,a 500g load and 15second loading time were applied.3.Results and discussion

3.1.Determination of the wire feed orientation

Experiments were carried out by changing the wire feed angles (15°,45°,and 60°)and directions (front,side and back

feeding),as shown in Fig.1,under a fixed laser power (2.06kW)and wire feed rate (2m/min)at various traverse speeds (100,200,300and 400mm/min).The graphs in Fig.2show the weight of the deposit at different angles with the same feeding direction.The data indicated that under the front feeding condition,feeding at 45°gave the highest deposition weight (6g at 100mm/min).Furthermore,on back feeding and side feeding,results showed that feeding at 60°and 15°were preferred,respectively.They had the weights of deposition of 5.7g and 6.6g at 100mm/min.No positive results were found when back feeding at 15°because there was very small clearance between the fed wire and the deposit during the process.Therefore,the wire easily hit the solidified deposit and bent onto the protective glass of the laser head,as shown in Fig.3.

Generally,front feeding and side feeding provided smoother surface compared with the back feeding.During back feeding,wire was fed on top of deposit.Thus,the materials seemed to be pushed away from the deposit.Therefore,a wavy surface with bulbs on the side of the deposit was formed in back feeding specimens.This is concordant to the results showed in [14]for deposition with stainless steel wire.For front and side feeding,the wires were fed from the direction towards the deposits.Therefore,the wires were melted,formed the deposits and pushed towards the previous still-red-hot deposits.The flow characteristics of melt in the melt pool are better in these con-ditions.Due to the surface tension effect,a better surface could be formed.Side feeding can provide a smooth surface but with uneven edges due to the feeding direction,as shown in Fig.4.The uneven edges were formed by the wire which firstly reacted with the melt pool on one side and more material was deposited on that side.

For selecting a suitable feeding orientation,high deposi-tion rate was a crucial factor.However,consistent feeding was also significant,especially in net-shape component deposition when making real parts.Multiple feeding directions could occur during the same process.As shown from the results,feeding at 15°is not suitable because deposition did not occur with back feeding at this angle.Therefore,feeding at 45°or 60°is preferred.Directional-wise,front feeding was more preferred because a more consistent feeding was achieved at the three different angles,within about 4.5g to 6g in total weight.As a result,front feeding at 45°was chosen for the further

experiments.

Fig.3.Trial in back feed at 15°with 100mm/min traverse

speed.

Fig.2.Weight of the deposit at different angles grouped by the same feeding direction.

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3.2.Determination of the wire feed rate

Laser power was set at 2.06kW,the wire feed rate was varied from 0.83mm/min to 3.33m/min and the traverse speed was varied from 50mm/min to 300mm/min.The weight of deposit against traverse speed was plotted,as shown in Fig.5.It decreased while increasing the traverse speed in all the wire feed settings.The weight difference between the different wire feed rate reduced from about 4g to less then 1g,while the traverse speed increased.Especially,while the traverse speed reached at 200mm/min,the weight of deposits was almost the same above wire feed rate of 2.22m/min.The humps observed for 2.22and 3.33m/min wire feed rates resulted from the unstable process caused by the too high wire feed rates.Some un-melted wire can be seen on the surface of the deposit.Furthermore,the relations between the weight of deposits and the wire feed rate at different traverse speed were plotted in Fig.6.The weight of the deposits increased while increasing the wire feed rates but trended to level off or decrease when it reached 2m/min.When the wire feed rate was set at high levels,wire could not be fully melted.Since there was no way for the wire to escape,it trended to push backwards into the wire feeder and the “cracking ”sounds were heard.There-fore,a preferred wire feed rate for the laser power 2.06kW and 1.65kW was chosen as 2m/min.Similarly,the preferred wire feed rate for the laser power 1.2kW was chosen as 1m/min.3.3.Effect of the laser power and traverse speed on the dimension of the deposit

Further experiments were carried out to investigate the in-fluence of the laser power and traverse speed on the dimen-sion of the deposit.A set of experiments with three different laser powers (2.06kW,1.65kW and 1.2kW),various traverse speeds (50mm/min to 250mm/min),fixed wire feed rates (1m/min and 2m/min)and front feeding at 45°were adopted.Deposits were generally clean and free of cracks and porosity as shown in Fig.7.Deposit materials were fully melted and metallurgically bound to the base plates.Dimensional analyses were carried out with the cross sectioned samples.Deposit height,width and deposit angle were measured according to the schematic in Fig.8.As can be observed in Fig.9,at the same setting of laser power and wire feed rate,both deposit height and deposit width decreased with increasing traverse speed,especially the deposit height.For the deposition with 2.06kW laser power and 2m/min wire feed rate,the deposit height decreased from 4.96mm to 1.40mm (decrease up to 72%)with increasing traverse speed.When the traverse speed was kept constant,such as at 50mm/min,the deposit height changed from 4.96mm to 3.13mm (a reduction of about 39%).These showed that the changes of traverse speed have a more sig-nificant effect on the deposit height than the laser power.The deposit width changed only from 7.92mm to 7.16mm (decrease less 10%)with increasing traverse speed.With the increasing laser power from 2060W to 1200W and a constant traverse speed,such as at 50mm/min,the deposit width de-creased from 7.92to 6.84mm (declines 13.6%).In the case of 150mm/min traverse speed,the deposit width varied from 7.27to 6.42mm (varies about 11.7%).Such results indicated that laser power affects deposit width more significantly than the traverse speed.The very low changes of deposit width resulted from the constant melt pool size created by the laser beam at the same power

level.

Fig.4.Photos of the surface and cross-section of the single track deposited by side

feeding.

Fig.5.Weight of the deposit against traverse

speed.Fig.6.Weight of the deposit against the wire feed rate.

3936S.H.Mok et al./Surface &Coatings Technology 202(2008)3933–3939

Deposit angle,also known as the contact angle (θc ),is one of the factors affecting the deposition quality,especially in surface deposition and structure build.In the laser deposition process,the contact angle is determined not only by the surface tension effect in the molten material,but also predominately by the laser beam size and spatial power density distribution.Acute angles are preferred in the direct diode laser deposition process.Therefore,good dense deposition could be achieved and no inter-run porosity would form.If the deposit angle is greater than 90o ,gaps in between overlapping tracks during the deposition would form [19].

Five cross-sections were prepared for each single track and the deposited angles were measured at both left and right sides.The average values were adopted for the discussion.As shown in Table 2,most of the deposit angles are acute angles,ranging from 31o to 68o except the two highlighted settings (laser power =2.06kW and 1.65kW with traverse speed =50mm/min).It suggested that most of the settings are suitable for the surface deposition and structure building.Furthermore,at the same wire feeding rate,the deposit angle shows significant

dependence on the ratio of laser power and traverse speed.The angle increases with increasing ratio.3.4.Micro-structure

Fig.10shows that all the deposits possess a columnar structure.The grains in the deposit grew directionally from the outer surface towards the centre of the melt pool.The cooling direction dictated the grain structure.Heat conduction through the base plate domi-nated the cooling effect compared with cooling via the deposit surface and the base plate through radiation and convection.The heat conduction in the deposit was slow,compared to the re-melted and Heat Affected Zone (HAZ).Therefore,large size columnar grains grew parallel to the cooling direction,as shown in Fig.10a,c and e.The grain size ranges from a few hundred microns to several millimeters depending on different laser power and traverse speed.On the other hand,the materials in the re-melted zone and HAZ were cooled down quickly by conduction.Fig.10b,d and f shows the microstructures of these two regions.The

grains

Fig.7.Cross-section of the single tracks deposited with different set of

parameters.

Fig.8.Schema of the cross-section for the dimension

measurement.Fig.9.Dimension of the single tracks deposited with different set of parameters.

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in the re-melted area were larger in size(100to200μm)compared with the grains next to the bottom of HAZ(less than100μm).This indicates that the cooling in the bottom of HAZ is faster than that in the middle.

Comparing the grain sizes of the samples with different tra-verse speed and laser power–traverse speed:a and b versus c and d;laser power:a and b versus e and f,results indicate that these two process parameters affected the grains sizes of the deposits.The grain size of both deposits and HAZs decreased while increasing the traverse speed.With reduced laser power,grain size became smaller,especially the size in the deposit.However,the impact was not as significant as the influence of traverse speed.

3.5.Micro-hardness

By considering the influence of different laser power and traverse speed in terms of micro-hardness,Fig.11shows the micro-hardness tested with selected samples.As can be seen,the hardness declined gradually with the increase of the distance away from the interface.The hardness value decreased rapidly between310HV and350HV after entering a particular region. This region was in the base plate,which was not affected by the laser energy.

The fluctuation of the hardness in one track resulted from the different cooling rate in different regions.After the laser passed through,the re-melted area and the deposit layer started cooling down from a high temperature(over1000°C)to room tem-perature(around25°C)in a short period of time.The materials were subjected to a condition like quenching in heat treatment and small grains in size were formed.The heat generated by the laser in the re-melted area and HAZ region was dominantly transferred in all directions to the base plate by conduction.And the

heat

Fig.10.Microstructure of the selected

deposits.

Fig.11.Hardness of the selected single tracks.

Table2

Dependence of the deposit angle on the traverse speed and laser power

Laser power Traverse speed(mm/min)

Angle50100150200250

Power=2.06kW Left98°68°47°35°33°

Right92°62°46°36°31°

Power=1.65kW Left96°61°42°34°

Right88°60°48°38°

Power=1.2kW Left56°49°33°

Right60°51°36°

3938S.H.Mok et al./Surface&Coatings Technology202(2008)3933–3939

conduction is more effective in these two regions than in the deposited layer,which resulted in the finer grain size in the re-melted area and HAZ,as discussed in Section3.4.As a result a higher hardness in these two regions than in the deposited layer was observed.The difference of the three hardness curves can be explained by the different energy input and the resulted different size of the microstructure.Results show that the highest values of energy input(2.06kW and50mm/min)which mean the deposits and base materials absorbed the largest quantity of energy to heat up the materials.Therefore,obviously,the longest time is needed to cool down back to the room temperature and longer time for the grains to grow.As a result,the deposit possessed a larger grain size.The larger the grains size,the less the grain boundaries in the structure and the lower values of hardness resulted.

4.Conclusions

The characteristics of the deposition of Ti6Al4V wire with high power diode laser for the surface modification and component building was investigated.Based on the results showed above,the following conclusions can be drawn:

?Direct Diode Laser Deposition with wire provides a high deposition rate process:1kg/h of titanium can be achieved;

the deposition showed good dense,free of porosity,clean and smooth surfaces

?Front wire feeding with45°angle was verified as the best feeding orientation and a maximum wire feeding rate of2m/ min was achieved for the set-up in this research

?The results manifested that the deposition width is mainly determined by laser power;however,traverse speed influ-ences the height more significantly than laser power;the deposit angle achieved in this study is mainly acute,which is suitable for the modification of large surface and component building with overlapping deposition

?Epitaxial columnar grains in the deposit were found parallel to the building direction;The higher the energy acting on the process,the larger the grain size obtained ?For the same laser power,higher traverse speed setting resulted in a higher hardness value;for the same traverse speed,the lower laser power brought a higher hardness References

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