Laser surface treatment of Inconel 718 alloy: Thermal stress analysis

Laser surface treatment of Inconel 718 alloy: Thermal stress analysis

ARTICLE IN PRESS Optics and Lasers in Engineering 48 (2010) 740–749 Contents lists available at ScienceDirect Optics and Lasers in Engineering journ...

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ARTICLE IN PRESS Optics and Lasers in Engineering 48 (2010) 740–749

Contents lists available at ScienceDirect

Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng

Laser surface treatment of Inconel 718 alloy: Thermal stress analysis B.S. Yilbas a,, S.S. Akhtar a, C. Karatas b a b

ME Department, KFUPM Box 1913, Dhahran, Saudi Arabia Engineering Faculty, Hacettepe University, Turkey

a r t i c l e in f o

a b s t r a c t

Article history: Received 4 December 2009 Received in revised form 12 March 2010 Accepted 15 March 2010 Available online 29 March 2010

Laser heating of Inconel 718 alloy is considered and the resulting temperature and stress fields are predicted using the finite element method (FEM). An experiment is carried out to treat the alloy surface by a laser beam at high pressure nitrogen environment. The metallurgical and morphological changes in the irradiated region are examined using the Scanning Electron Microscope (SEM), optical microscope, and X-ray Diffraction (XRD). It is found that the surface hardness of the alloy improves after the laser heating process, which is due to the microstructural changes and g-phase nitride formation in the surface region. The maximum value of the residual stress predicted in the irradiated region is close to the yielding limit of the alloy. & 2010 Elsevier Ltd. All rights reserved.

Keywords: Laser Heating Inconel 718 Thermal stress

Introduction Inconel 718 alloy is a nickel based superalloy and it is widely used in power industry due to its high resistance to harsh environments. However, niobium segregation resulted in large scatter in the mechanical properties. One way of avoiding the segregation is to form the fine structures through the laser controlled melting. Although the alloy has excellent resistance to oxidation, the use of the assisting gas in laser treatment process is necessary to prevent the formation of oxide species at elevated temperature in the laser irradiated region. The assisting gas is usually an inert gas and it prevents the high temperature exothermic oxidation reactions. In general, nitrogen is used as an assisting gas in laser surface treatment process. However, the formation of nitride species is possible during the heating process. In addition, high heating and cooling rates during the laser processing results in high thermal stress field in the treated region. The microhardness of the alloy can be improved through forming fine microstructures via non-equilibrium fast solidification during laser melting process. However, the large-scale Nb segregation should be avoided in the re-melted region, since it results in freckles and large-scale phases which causes microfissures in the irradiated region. Consequently, investigation into laser gas assisted heating of Inconel 718 and the formation of thermal stress field in the irradiated region becomes necessary. Considerable research studies are carried out to examine the treatment of Inconel alloys by a laser beam. Laser annealing of

 Corresponding author.

E-mail address: [email protected] (B.S. Yilbas). 0143-8166/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2010.03.012

Inconel 718 alloy was studied by Liufa et al. [1]. They showed that the matrix-strengthening phases (g00 and g0 ) in the surface layers were dissolved by the laser irradiation. The analysis of laser-induced grain boundary liquation in the heat-affected zone of Inconel 617 alloy was carried out by Luo et al. [2]. They indicated that intergranular and transgranular presence of massive NbC product phase occurred in the base metal. The effect of cooling rate on the solidification of Inconel 718 was investigated by Antonsson and Fredriksson [3]. They indicated that the effective cooling coefficient increased with increasing cooling rate. The microstructure and mechanical properties of Inconel 718 electron beam welds were investigated by Ram et al. [4]. They showed that a considerable amount of interdendritic niobium segregation and brittle intermetallic phase occurred in the laser re-melted regions. Laser assisted machining of Inconel 718 alloy was investigated by Shi et al. [5]. They introduced the heating and stress models to predict temperature and stress fields during the laser processing. The optimization study on welding of Inconel 718 alloy was carried out by Xiao et al. [6]. They indicated that mechanically sound welds with narrow fusion and heat-affected zones could be produced using the optimal processing scheme. The mechanical and microstructural characteristics of the laser-deposited Inconel alloy were studied by Blackwell [7]. He showed that increased grain size was associated with the reduced strength and the fine second phase particles (oxides, carbides) tend to inhibit the grain growth. The microstructure and mechanical properties of the laser formed shapes from Inconel 718 alloy were studied by Zhao et al. [8]. They indicated that the presence of the porosities in laser formed Inconel 718 samples resulted in the low ductility and stress rupture properties, which promoted the occurrence of the

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micro-porous coalescence failure in the tensile samples. The microstructures and mechanical properties of laser welded Inconel 718 were examined by Hong et al. [9]. They showed that the ductility of welds was considerably lower than that of the base material, which was attributed to the detrimental phases such as lanes on fusion zone. In the present study, laser gas assisted re-melting of Inconel 718 alloy is carried out. Temperature and stress fields in the irradiated region are predicted using the finite element method (FEM). The experiment is carried out to examine the metallurgical and morphological changes in the laser irradiated region. The nitrogen species formed in the irradiated region is examined using the X-ray Diffraction (XRD). The microhardness variation in the surface region is measured.

Experimental The CO2 laser (LC-ALPHAIII) delivering nominal output power of 2 kW was used to irradiate the workpiece surface. The nominal focal length of the focusing lens was 127 mm employed. The laser beam diameter focused at the workpiece surface will be 0.3 mm. Nitrogen assisting gas emerging from the conical nozzle and co-axially with the laser beam will be used. Laser treatment conditions are given in Table 1. Inconel 718 in sheet form with 2.5 mm thickness is used in the experiments. Material characterization of the laser nitrided surfaces was carried out using SEM and XRD. Jeol 6460 electron microscopy will be used for SEM examinations and Bruker D8 Advanced having MoKa radiation will be used for XRD analysis. A typical setting of XRD was 40 kV and 30 mA and scanning angle (2y) was ranged 20–801. Microphotonics digital microhardness tester (MP-100TC) was used to obtain microhardness across the depth of the nitride layer. The standard test method for Vickers indentation hardness of advanced ceramics (ASTM C1327-99) was adopted. Microhardness was measured at the workpiece surface after the laser treatment process. The measurements were repeated four times at each location for the consistency of the results. The experimental results appear to be all consistent.

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beam axis is the z-axis (Fig. 1). It should be noted that the laser beam intensity distribution is assumed to be Gaussian at the irradiated surface. In the case of a moving heat source along the x-axis with a constant velocity U, energy gain by the substrate material yields

r

DE @E @E ¼ r rU Dt @t @x

ð2Þ

or DE @ðCpTÞ @ðCpTÞ ¼r rU ð3Þ Dt @t @x Combining Eqs. (1)–(3) yields  @ðCpTÞ  @ðCpTÞ r ð4Þ ¼ rðkrTÞ þ rU þSo @t @x Eq. (4) is solved numerically with the appropriate boundary conditions to predict the temperature field in the substrate material. However, to analyze the phase change problem, the enthalpy method is used [10]. The thermal stress analysis using ABAQUS software was carried out earlier [12]. Therefore, the brief description of thermal stress analysis is given below. For a fully coupled temperature– displacement (or stress) analysis, ABAQUS solves a system of coupled equations [10] " #  ( ) Ru Kuu KUT Du ¼ ð5Þ KTU KTT RT DT

r

where DT and Du are the corrections to the incremental temperature and displacement, respectively, Kij are the submatrices of the fully coupled stiffness matrices, and RT and Ru are the thermal and mechanical residual vectors, respectively.

Laser Beam Focusing Lens Nitrogen Laser Heated Spot

x=0 y=0 z=0 x y

U

z

Laser Treated Region

Inconel 718 Mathematical analysis of temperature and stress fields Fig. 1(a) shows the schematic view of the laser heating situation. The Fourier heat transfer equation pertinent to the laser heating process can be written as  DE  r ð1Þ ¼ rðkrTÞ þ So Dt where E is the energy gain by the substrate material, k is the thermal conductivity, and So is the heat source term resembling the laser beam, i.e. 2

So ¼ Io ð1rf Þeððx

þ y2 =a2 ÞÞ

Io is laser power peak density, a is the Gaussian parameter, rf is the surface reflectivity, r is the density, and x and y are the axes while the laser beam scans the surface along the x-axis. The laser

Fig. 1. (a) Schematic view of laser heating situation and (b) Mesh used in the simulations.

Table 1 Laser heating conditions used in the experiment. Scanning Speed (cm/s) (mm/min)

Power (W)

Frequency (Hz)

Nozzle Gap (mm)

Nozzle Diameter (mm)

Focus setting (mm)

N2 Pressure (kPa)

10

140

500

1.5

1.5

127

600

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ABAQUS uses an implicit backward-difference scheme for time integration of both temperature and displacements at every material integration point [10]. The total strain vector, {De}, may be expressed as follows: fDeg ¼ fDeel g þ fDeth g þfDepl g el

ð6Þ th

where {De } is the elastic strain increment vector, {De } is the thermal strain increment vector, and {Depl} is the plastic strain increment vector. The elastic strain increment vector, {Deel} is related to the stress increment vector, {Dr} by Hooke’s law fDrg ¼ ½DfDeel g

ð7Þ

where [D] contains the elastic constants related to temperaturedependent elastic modulus, E and Poisson’s ratio, n. The incremental thermal strain vector, {Deth} arises from the volume changes that accompany the temperature increment, DT, which is calculated by the thermal analysis. It is normally accounted for in stress analyses through a temperature-dependent differential thermal expansion coefficient, a(T).

Numerical simulation Finite element discretization was carried out using the ABAQUS software [10]. The simulation is performed in Abaqus/ Standard and consists of sequential thermal-stress analysis. In the sequential thermal-stress analysis, 91 572 elements are used to create the model using two element types; for the heat transfer analysis, mesh used elements of type DC3D4 (4-node Linear heat transfer tetrahedron) and stress analysis used C3D4 (4-node Linear 3D stress tetrahedron). Fig. 1(b) shows the mesh used in the simulations. The fixed boundary conditions are applied on the both ends of the workpiece resembling the experimental laser heating situation. In the stress analysis, displacements are stored by ABAQUS at the nodal positions as a solution variable, and loads are defined as prescribed displacements and forces. Employing the interpolation functions, it is possible to calculate the strain and stress increments at any point within the element using the compatibility and constitutive equations. ABAQUS transforms the mechanical equilibrium equations into a set of simultaneous equations, such that the nodal displacements and forces are related to each other through the elemental stiffness matrix. However, ABAQUS uses a temperature-dependent total thermal strain coefficient, a0 (T). The differential and total thermal expansion coefficients are related to each other through Z T 1 a0 ðTÞ ¼ aðTÞdT TT o T o where To is a reference temperature designating the point at which the material exhibits no dilatational strain (set to the mechanical coherency temperature in the current problem). Laser heat flux with Gauss distribution and prescribed velocity of 10 cm/s along the x-axis through user subroutine DFLUX is applied to the thermal model. The Gauss parameter ‘‘a’’ is a ¼0.000333333 m, in accordance with the experimental power intensity distribution. The thermal model consisted of two steps. The first step, which lasts 0.05 s, simulates the response of plate under moving laser heat flux. The second step, which lasts for 1000 s, simulated the continued cooling in the model. Cooling was allowed to continue until all of the plate reaches initial temperature (room temperature). The temperature–time history resulted from the thermal analysis is used as input to the thermal stress analysis. The workpiece is considered as an elastic body, which is modeled as von-Mises elastic–plastic material with

Table 2 Properties of Inconel 718 used in the simulations [11]. Density (kg/m3) Latent Heat of Melting (J/kg) Solidus Temperature (K) Liquidus Temperature (K) Specific Heat Capacity (J/kg.K)

8200 250,000 1528 1610 600

Thermal Conductivity (W/m.K)

Temperature (K)

21 25 30 26 27 28

1000 1200 1500 1600 1700 1800

Temperature (K) 294 366 477 589 700 811 922 1033 1144 1227

Poisson’s ratio

Modulus of Elasticity (Pa)

0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3

2.08E +11 2.05E +11 2.02E +11 1.94E +11 1.86E +11 1.79E +11 1.72E +11 1.62E +11 1.27E +11 1.78E +10

Temperature (K) 366 477 589 700 811 922 1033 Temperature (K) 366 477 589 700 811 922 1033

Expansion Coefficient (1/K) 1.28E  005 1.35E  005 1.39E  005 1.42E  005 1.44E  005 1.51E  005 1.61E  005 Yield Stress (Pa) 11.72E + 008 11.24E + 008 10.96E +008 10.76E +008 10.69E +008 10.27E +008 7.58E + 008

isotropic hardening and with a yield stress that changes with temperature. Table 2 gives the properties of Inconel 718 used in the simulations.

Results and discussion Laser gas assisted re-melting of Inconel 718 alloy is studied. Temperature and stress fields are predicted using the finite element model (FEM) during and after the laser treatment process. The experiment is conducted and the microstructural and morphological changes in the laser irradiated region are examined. Fig. 2 shows temperature contours in the laser heated region while Fig. 3 shows temperature distribution along the x-axis, the laser scanning axis, at locations y¼0 and z ¼0. High temperature contours extend inside the workpiece and they extend further away from the center of the irradiated spot due to the movement of the laser beam along the x-axis. The maximum surface temperature predicted is less than the evaporation temperature of Inconel 718 alloy and the heating of liquid occurs at the surface at temperatures higher than the melting temperature of Inconel

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Fig. 2. Temperature contours in the heating periods.

2700

TEMPERATURE (K)

2400

y=0m&z=0m t = 0.01 sec t = 0.02 sec t = 0.03 sec t = 0.04 sec t = 0.05 sec

2100 1800 1500 1200 900 600 300 0

0.001

0.002

0.003

0.004

0.005

0.006

DISTANCE ALONG X-AXIS (m) Fig. 3. Temperature distribution along the x-axis at different heating periods.

718 alloy. Temperature rise to reach melting is gradual and the super heating of liquid phase takes place after the initiation of melting. Temperature decay is sharp from the maximum temperature. Consequently, temperature gradient along the x-axis is low up to the melting temperature and it becomes high in the cooling period after the melting initiation. High temperature gradient results in high stress levels during the cooling period. It should be noted that the times in the figure correspond to the position of the irradiated spot center along the x-axis, since the laser beam scans the surface at a constant speed. The location of the maximum temperature and the point of melting along the x-axis changes according to the laser scanning speed. Moreover, the rise of temperature up to the melting point changes at different laser beam locations along the axis. In this case, thermal conduction and convection from the irradiated surface are responsible for the slow rise of temperature at the locations far away from the

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Fig. 4. Temperature contours at different cooling periods.

TEMPERATURE (K)

1750

y = 0 m and z = 0 m

1500

t = 0.002 s t = 0.0044 s t = 0.0147 s t = 0.111 s t = 0.25 s t = 255 s

1250 1000 750 500 250 0.000

0.001

0.002

0.003

0.004

0.005

0.006

DISTANCE ALONG X-AXIS (m) Fig. 5. Temperature distribution along the x-axis at different cooling periods.

irradiated spot center. Consequently, initially heated regions cool as the laser beam moves along the x-axis. Fig. 4 shows temperature contours in the cooling periods while Fig. 5 shows temperature distribution at the surface (z¼0) along the x-axis for different cooling times. It should be noted that cooling periods starts after the completion of heating period (0.05 s). In this case cooling period 0.02 s corresponds to 0.07 s from the initiation of laser scanning. Temperature is high at location of the laser irradiated spot center and as the time progresses, the laser irradiated spot moves away from this location. The rate of cooling due to convection and conduction lowers the temperature at the location, which is initially heated by a laser beam spot. Temperature decay is sharp along the x-axis as the distance increases away from the location of the maximum temperature. However, temperature increase is gradual first and becomes sharp along the x-axis towards the location of the maximum temperature. Consequently, temperature distribution

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Fig. 6. Von Mises stress contours for different cooling periods.

vON MISES STRESS (Pa)

1.5E+09 t = 0.002 s t = 0.0044 s t = 0.0147 s t = 0.111 s t = 0.25 s t = 255 s

1.2E+09

9.0E+08

y = 0 m and z = 0 m

6.0E+08

3.0E+08

0.0E+00 0

0.001

0.002

0.003

0.004

0.005

0.006

DISTANCE ALONG X-AXIS (m) Fig. 7. Von Mises stress distribution along the x-axis for different cooling periods.

along the x-axis results in varying temperature gradient along the axis. This argument is true for all the cooling periods, except t¼ 255 s, in which case, workpiece cools down to almost the initial temperature. Fig. 6 shows von-Mises stress contours at different cooling periods while Fig. 7 shows von-Mises stress distribution along the x-axis for different cooling periods similar to those shown in (Fig. 5). It is evident that von-Mises stress attains low values in the region where temperature is high due to the softening of the substrate material at high temperatures. von-Mises stress becomes high in the region where the temperature gradient is high. However, after the sharp decay of von-Mises stress in the high temperature region, it increases sharply and, then, reduces as the distance along the x-axis increases further. This change in the von-Mises stress is because of the thermal strain developed due to the temperature gradient variation in this region. As the cooling progresses, the behavior of von-Mises stress along the x-axis changes due to the attainment of low temperature in the region where it was initially at high temperature due to the presence of laser heating spot. As the cooling progresses further (t ¼255 s),

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temperature reduces to almost room temperature and von-Mises stress becomes the residual stress inside the workpiece. The maximum residual stress is in the order of 1.2 GPa, which develops at the location where initial the maximum temperature occurs. The maximum value of the residual stress is close to yielding limit of the workpiece material. Fig. 8 shows temperature distribution along the y-axis for different heating periods. Temperature distribution along the y-axis follows almost the laser beam intensity distribution at the workpiece surface, which is Gaussian. As the irradiated spot moves along the x-axis, the resulting temperature distribution remains the same. It should be noted that the peak temperature is the maximum and remains the same for all heating periods, shown in the figure, since they correspond to the laser beam locations at the irradiated surface. Fig. 9 shows temperature distribution along the y-axis for different cooling periods. Temperature remains the highest when the laser irradiated spot passes in this region at the surface (t¼ 0.007 s). However, as the cooling period starts temperature decays at the surface. Temperature decay is sharp in the initially high temperature region while the decay is gradual in the initially low temperature region. The sharp decay of temperature is due to the attainment of high temperature gradient in this region. Consequently, high rate of thermal diffusion due to high temperature gradient in this region is responsible for the sharp decay of temperature. As the time progresses further, in the cooling period (t¼ 255 s), temperature reduces to almost room temperature and the cooling cycle ends. z = 0 m: t = 0.01 s: Laser beam at x = 0.00085 m z = 0 m: t = 0.03 s: Laser beam at x = 0.0028 m z = 0 m: t = 0.05 s: Laser beam at x = 0.0048 m

vON MISES STRESS (Pa)

2100 1500 900

x = 0.0048 m and z = 0 m

7.5E+08 5.0E+08 2.5E+08

10 00

Fig. 8. Temperature distribution along the y-axis for different heating periods.

20 0.

00

15 00 0.

10 00 0.

05 0.

00

00

05

00 0.

00

10

15

00

-0 .

-0

.0

02

0

DISTANCE ALONG Y-AXIS (m)

00

08

1.0E+09

2.0E+02

0.

00 0.

05 0.

00

03 00 0.

00 0.

00

3 00

5 .0 -0

-0

.0

00

8 00 .0 -0

-0

.0

01

0

300

t = 0.002 s t = 0.0044 s t = 0.0147 s t = 0.25 s t = 247.4 s

-0 .

2700 TEMPERATURE (K)

Fig. 10 shows von-Mises stress along the y-axis for different cooling periods similar to those shown in Fig. 9. von-Mises stress attains low values in the region of high temperature and it attains high values in the region of low temperatures. The rise and fall of von-Mises stress along the x-axis are associated with the variation of temperature gradient along the x-axis. As the cooling progresses, von-Mises stress rises in the initially high temperature region. von-Mises stress reaches its highest value once the cooling cycle is completed; in which case, the stress becomes the residual stress. Moreover, the similar arguments can be made for temperature distribution inside the substrate material. Temperature decay is sharp in the surface region (z r0.00015 m) and as the depth below the surface increases, temperature decay becomes gradual. The sharp decay of temperature in the surface region is attributed to the heat loss from the surface region due to conduction and convection heat transfer in this region. Moreover, high temperature gradient in the surface vicinity accelerates the heat diffusion towards the solid bulk. Consequently, heat conduction in the surface region becomes substantial due to the attainment of the high temperature gradient. It is evident that temperature distribution at different heating period remains the same due to the location of the laser irradiated spot at the workpiece surface i.e. the heating periods shown in the graph corresponds to the location of the laser irradiated spot. The gradual decrease of temperature is attributed to small temperature gradient at some distance away from the surface. In this case, small temperature gradient results in less energy diffusion in this region.

-0 .

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DISTANCE ALONG Y-AXIS (m) Fig. 10. Von Mises stress distribution along the y-axis for different cooling periods.

1200

1700 x = 0.0048 m and y = 0 m 1450 TEMPERATURE (K)

TEMPERATURE (K)

x = 0.0048 m and z = 0 m

t = 0.002 s t = 0.0044 s t = 0.0147 s t = 0.25 s t = 255 s

1450

950 700 450

950 700 450

0 00 1 0.

8 00 0 0.

5 00 0 0.

00 3 0. 0

0 00 0 0.

3 .0 00 -0

5 00 -0 .0

8 00 -0 .0

-0

.0 01

0

200

1200

t = 0.002 s t = 0.0044 s t = 0.033 s t = 0.25s t = 255 s

DISTANCE ALONG Y-AXIS (m) Fig. 9. Temperature distribution along the y-axis during the cooling periods.

200 0.0000

0.0002 0.0004 0.0006 0.0008 DISTANCE ALONG Z-AXIS (m)

0.0010

Fig. 11. Temperature distribution along the z-axis for different cooling periods.

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Fig. 11 shows temperature distribution inside the workpiece (along the z-axis) for different cooling periods at x-axis location is x ¼0.0048 m and y-axis location is y¼0 m. Temperature at the surface (z¼ 0 m) is the maximum and decays gradually in the surface vicinity and this decay becomes sharp in the region next to the surface vicinity. The behavior of temperature in the cooling cycle is different than that of the end of the heating cycle (t ¼0.05 s) at the same x- and y-axis locations. In this case, the gradual decay of temperature in the surface vicinity suppresses temperature gradient, which in turn, lowers the heat diffusion from the surface vicinity to the solid bulk. As a consequence, temperature gradient attains high values in the region next to the surface vicinity. Moreover, as the heating period progresses further (t¼255 s), temperature inside the workpiece becomes almost the same as the initial temperature. Fig. 12 shows von-Mises stress inside the workpiece (along the z-axis) for different cooling periods as similar to those shown in Fig. 11. Von-Mises stress attains low values in the early heating period (tr0.0094 s), provided that the stress is low in the surface vicinity, where temperature is initially high. As the cooling progresses (t ¼255 s), von-Mises stress reaches its maximum and decays gradual in the surface vicinity. As the distance from the surface vicinity increases, it decays sharply. The high von-Mises stress (  900 MPa) region extends almost 225 mm below the surface. Since the cooling rate is high, the stress field is considered as the residual stress. Although the stress level is close to the yielding limit of the workpiece material, no cracks or fissures are observed across the cross-section of the irradiated region. Moreover, high residual stress contributes to the hardness of the workpiece in the surface region. Fig. 13 shows SEM micrographs of the top surface of the laser treated Inconel 718 alloy. It is evident that the surface is free from microcracks as well as cavities due to attainment of high stress levels and rapid evaporation of the surface. Moreover, the laser scanning resulted in the melting tracks at the surface. The overlapping ratio of the melted spots is estimated as 60%. The presence of the surface melt tracks reveals that some small amount of molten material flow took place between the tracks. Fig. 14 shows SEM micrographs of the cross-section of the laser treated Inconel alloy. The depth of laser melt layer extends about 50 mm below the surface. The close examination of the micrograph reveals that uniform melt depth occurs in the surface region which is attributed to the constant scanning speed of the laser beam. The presence of columnar dendrites, which grew almost normal to the workpiece surface is evident. The formation of multi-directional columnar structure is attributed to the high cooling rates, which takes place in non-uniform manner. The columnar spacing is, in average, below 1 mm due to the extremely

1.20E+09 vON MISES STRESS(Pa)

x = 0.0048 m and y = 0 m 1.00E+09

t = 0.002 s t = 0.0044 s t = 0.25 s t = 255 s

8.00E+08 6.00E+08 4.00E+08 2.00E+08 2.00E+02 0.0000

0.0002

0.0004

0.0006

0.0008

0.0010

DISTANCE ALONG Z-AXIS (m) Fig. 12. Von Mises stress distribution along the z-axis for different cooling periods.

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Laser Tracks

Fig. 13. SEM micrograph of top surface of laser treated Inconel 718 alloy.

high cooling rates. This provides improved mechanical properties with structures free from defects. The precipitation of particles in the grain boundary is also observed. It should be noted that the primary carbides (NbC) and d-phase precipitates at grain boundaries are expected to take place [13]. Although the precipitation of niobium-rich phases [d-phase (Ni3Nb)] at the grain boundaries expected to cause grain boundary cracking [14], this situation is not observed from the micrographs. Consequently, the presence of d-phase at the grain boundary is less in amount. The close examination of SEM micrographs corresponding to the near surface region reveals that the fine discrete particles showing the Laves morphology are evident. The presence of discrete Laves phase indicates that the precipitation of strengthening phases g00 (Ni3Nb) and g0 (Ni3 (Al, Ti)) are presented in this region. However, it is expected that the highly interconnected coarse Laves phase triggers the early initiation of cracks while resulting in low energy fracture path [15]. This structure is not observed in the surface region of the workpiece. Table 3 gives the microhardness measurement results at the workpiece surface. The microhardness increases at the surface after the laser treatment process. This may be attributed to the nitride species (g00 N and g0 N) formation in the surface region, which is evident from the XRD diffractogram (Fig. 15), and dissolution of Laves phase in the region close to the surface. In this case, the amount of Nb available for g00 precipitation increases and it contributes to the enhancement of hardness.

Conclusion Laser gas assisted melting of Inconel 718 alloy is carried out. Temperature and stress fields in the laser irradiated region are predicted using the FEM. The metallurgical and morphological changes in the laser irradiated region are examined using SEM, optical microscope, and XRD. The microhardness prior and after the laser treatment is measured. It is found that temperature decays sharply around the irradiated spot particularly inside the workpiece. Initial heating of the workpiece along the x-axis, due to laser movement, reduces the temperature gradient along the xaxis. Temperature variation along the y-axis follows almost the laser beam intensity distribution, which is Gaussian, during heating period. However, as the cooling starts after 0.05 s, temperature decay becomes sharp in the region initially irradiated by the laser beam while it is gradual in the region next to this region. This is because of the attainment of high temperature gradient in the initially irradiated region while increasing heat diffusion to the solid bulk. Von-Mises stress attains low values in the region of high temperature due to the softening of the

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Laser Treated Region

Cross-sectional view of laser treated Inconle 718 alloy.

Grain Boundary Precipitations

Cross-sectional view of laser treated Inconle 718 alloy.

Fine Dendrites Laves Phase

Cross-sectional view of laser treated Inconle 718 alloy in the surface region. Fig. 14. Cross-sectional view of laser treated Inconle 718 alloy.

Table 3 Microhardness results prior and after the laser treatment process. Micro Hardness (HV) 450 720 300 715

Laser Treated Untreated

350 INTENSITY (cps)

Acknowledgements

γ'

300

agreement with the predictions. The precipitation of primary carbides is observed at grain boundaries. Moreover, finer discrete particles showing Laves phase are seen in the region close to the surface. The microhardness improves at the surface due to formation of gN species and the grain refinement in the surface region because of the high cooling rates.

The authors acknowledge the support of King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia.

250 200 150 100

γ'

γ' γ 'N

50

γ 'N

0 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 2θ Fig. 15. XRD diffractogram for laser treated Inconel 718 alloy. g0 and g0 N are shown.

workpiece material. However, von-Mises stress attains considerably high values in the region where temperature decay is sharp. Although the magnitude of residual stress predicted is within the limit of yielding limit of the alloy, SEM micrographs reveal that the surface is free from microcracks. The depth of laser treated region extends almost 50 mm below the surface, which is in good

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