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Rebuilding of metal components with laser cladding forming
Applied Surface Science 252 (2006) 7934–7940 www.elsevier.com/locate/apsusc
Rebuilding of metal components with laser cladding forming Jianli Song a,b,*, Qilin Deng a, Changyuan Chen a, Dejin Hu a, Yongtang Li b a
School of Mechanical and Power Engineering, Shanghai Jiaotong University, Shanghai 200030, PR China b Taiyuan University of Science and Technology, Taiyuan 030024, PR China Received 29 June 2005; received in revised form 2 October 2005; accepted 2 October 2005 Available online 11 July 2006
Abstract Laser cladding forming (LCF) is a novel powerful tool for the repairing of metal components. Rebuilding of V-grooves on medium carbon steel substrates has been carried out with laser cladding forming technique using stainless steel powder as the cladding material. Microstructure of the deposited layers has been characterized using optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray microanalysis (EDAX), electron probe microanalysis (EPMA) and X-ray diffraction (XRD). Mechanical properties of the rebuilt V-groove samples have been evaluated by tensile and impacting tests and microhardness measurement. Experimental results show that good fusion bonding between the rebuilt layers and the substrate has been formed, and the microstructure of the cladding layers is mainly composed of fine, dense and defect-free epitaxial columnar dendrites. Due to the effect of grain size refinement, the tensile strength, impacting toughness, elongation and microhardness of the rebuilt samples have been greatly enhanced compared to those of the substrate. Microhardness is also very uniform throughout the rebuilt regions. With the growth of the deposited layers, the microhardness increases gradually. The good ductility of the deposited regions is verified by the SEM fracture analysis. # 2005 Elsevier B.V. All rights reserved. Keywords: Laser cladding forming (LCF); Rebuilding; V-groove; Stainless steel powder
1. Introduction In the fields of mechanical engineering, metallurgical industry and petrochemical and electric power, a large number of components work in formidable conditions involving impact, abrasion, high-temperature and pressure, and are liable to breakdown. If successful repairing cannot be carried out, the damaged components will have to be discarded and a significant loss will be caused. Conventional repairing methods presently adopted mainly include mechanical machining, brushelectroplate, deposited welding, TIG welding and thermal spraying (flame spraying and plasma spraying). Although, these methods have different advantages, there are still many drawbacks, such as being time-consuming and labor intensive, having limited thickness of deposition layers and machinable times, poor bonding strength, large amount of porosities and cracks, or significant heat injection and distortion of the substrates [1–3]. Therefore, it is of great interest to develop high-efficiency and precision repairing technologies that will extend the lifetime of components. * Corresponding author. Tel.: +86 21 62932360; fax: +86 21 62932611. E-mail address: [email protected] (J. Song). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.10.025
The novel versatile laser cladding forming technology is such a repairing technology, which integrates laser technology, CAD/CAM technology, advanced materials processing technology and photoelectric measuring and control technology. Different similar techniques, for example direct light fabrication (DLF) [4], laser engineered net shaping (LENSTM) and direct metal deposition [5] have been developed and studied by many research groups in recent years. Laser cladding forming combines the two technologies of rapid prototyping manufacturing (RPM) and laser cladding surface modification, and can be used to fabricate three-dimensional fully dense metal components directly from the CAD model. In the laser cladding forming process, a high-energy laser beam is focused onto the substrate to create a molten pool; metal powders are simultaneously delivered into the focal zone by the powder delivering nozzles and then melted and rapidly solidified. A single layer can be built with the motion of the worktable in the X–Y plane. Then the laser beam is elevated for a predefined distance in the Z-direction to deposit a new layer, thus a three-dimensional component can be stacked consecutively. The principle of this technology is shown in Fig. 1 [6,7]. The process overcomes the inevitable difficulties of deformation and thermal fatigue occurred in the conventional arc and TIG
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Fig. 2. Sketch of the LCF substrate. Fig. 1. Principle of laser cladding forming technology.
welding process and the poor bonding quality in electroplating process. Compared with those conventional repairing technologies, laser cladding forming has many unique characteristics [8,9]: higher flexibility, lower cost and shorter lead time, stronger bonding interface between laser cladding forming layers and the substrate, smaller dilution, segregation and distortion, finer microstructure and excellent mechanical properties, and tailored materials in required repairing positions. In recent years, laser cladding forming of various materials has been studied by many scholars. D’Oliveira et al. deposited five-layers of Stellite 6 alloy onto a 304 stainless steel plate, and investigated the residual stress and microstructures [3]. Li et al. studied the microstructure and wear resistance of multi-layer laser cladding HMSP2573 alloy [10]. Andrew and Lin reported the multi-layer cladding of stainless steel using a high-power diode laser and investigated the effects of process parameters on the surface properties and dimensions of the samples [11]. Most of the multi-layer cladding experiments have been carried out on a planar steel plate and only three to five-layers have been deposited. However, damaged surface regions are not always regular; sometimes deep grooved regions need to be deposited to recover the shapes and properties of the worn components. In this paper, laser cladding forming rebuilding of V-grooves up to 20 mm in depth on common carbon AISI 1045 steel substrates have been systematically investigated, microstructure and mechanical properties of the repaired regions have been characterized and assessed with different testing methods.
greasy and rust. Parameters used in the experiments are: laser power 2.25–3.75 kW, overlapping 27.5–37.5%, scanning speed 100–150 mm/min, beam diameter 4 mm, and powder delivery velocity 1.5–3.5 g/min, respectively. The grooves were then rebuilt with the method described in Fig. 1, and the molten pool was shrouded by Argon gas. In order to optimize the parameters of the rebuilding process, four-layer laser cladding forming experiments were firstly carried out on the surface of a planar metal plate with different combinations of process parameters, and the samples of the experiments have been characterized, respectively. Then the V-grooves were deposited with the optimized cladding parameters, which were determined from the previous testing results. After the rebuilding, standard specimens were cut from the samples with wire electrode discharge machining (WEDM), polished with sand paper and glazing machine, and eroded with aqua regia. The microstructure of the as-deposited samples was observed under KEYENCE three-dimensional digital microscope. Tensile strength and impacting toughness tests were, respectively, carried out on Zwick/Roell and SATEC SYSTEMS SI-lC3 material testing machines, morphology of the fractures was observed with Philips Scanning Electron Microscope and compositions of the laser cladding forming samples were analyzed with Shimadzu EPMA-8750 electronic probe microanalyzer and EDAX fitted on Philips Scanning Electron Microscope. Phases of the cladding layers were analyzed with D/max 2550V X-ray diffraction analysis and the microhardness was tested with a HX-1000 microhardness machine.
2. Experimental procedure
3. Results and discussion
The experimental system consists of a 6 kW CO2 laser source, an automatic wide-band powder feeding device, a SIEMENS numerical control system and a four-axis working plate. V-grooves with dimensions shown in Fig. 2 were premachined to a roughness of Ra12.5 mm on AISI 1045 block substrates with a size of 75 mm 150 mm 200 mm. 316L stainless steel powder (<0.03 C, 16–18 Cr, 12–14 Ni, 2–3 Mo, 0.68 Si and balance Fe, wt.%) was used as the rebuilding material. Before laser deposition, the 316L stainless steel powder was dried in a vacuum stove; the substrate was polished with sand paper and cleaned with acetone to remove the
3.1. Microstructure analysis The morphology and microstructure of the rebuilt V-groove regions are shown in Fig. 3. Fig. 3(a) and (b) illustrates the low magnification morphology of the deposited layers at the bottom part and the side part of the grooves, while Fig. 3(c)–(h) are the specifics of the microstructure. Fig. 3(c) and (d) shows the microstructure near the bonding interface, which are the side part and the bottom part of the deposited V-grooves, respectively. It can be seen that the bonding line is smooth or distinct fusion bonding interface has been obtained between the
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Fig. 3. Morphology and microstructure of the LCF rebuilt V-groove zone: (a) and (b) low magnification morphology of the V-groove zone; (c) and (d) microstructure near the bonding interface; (e) non-remelted zone; (f) longitudinal cross-section; (g) overlapping and epitaxial growth region; (h) cross-section of the columnar dendrites.
rebuilding layers and the layer-substrate boundary. The microstructures allover the rebuilt V-grooves are fine, free of porosities and cracks. The solidified microstructure is determined by the composition of the alloy, the temperature gradient G, the energy of the interface and the solidification velocity V, etc. At the beginning
of the cladding process, the ratio G/V on the solidification interface is very large, and there is hardly any ingredient super cooling, so the interface grows with a planar pattern, as shown in Fig. 3(c) and (d). The growth of the crystal is also related to the orientation of crystal cells. At the orientation that is most favorable for the growth of the crystal, the growth velocity is the
J. Song et al. / Applied Surface Science 252 (2006) 7934–7940
fastest. In laser cladding forming the preferential orientation is in accordance with the direction of the maximum heat flux [12,13]. During the laser cladding forming process, because the heat of the cladding layers dissipates through the substrate, the temperature gradient and the heat flux density perpendicular to the interface is the largest. At the same time, the liquid metal always keeps a tight contact with the solid metal matrix, and typical rapid solidified epitaxial columnar dendrites of different lengths are formed preferentially along the maximum temperature gradient in most deposited areas. With the propelling of the solid/liquid interface and heat accumulation, the temperature gradient G in the cladding layers decreases gradually. Heat flow via the substrates is no longer predominating, and heat of the cladding layers starts to radiate through the surrounding atmosphere. In the process of laser cladding forming, solidification velocity V at the front side isothermal line of the molten pool can be expressed as: V = Vb cos u (where Vb is the scanning speed and u is the angle between the solidification speed and the scanning speed). With the increasing of the scanning speed, the dwell time of the heat source at each point of the track decreases, and therefore, the solidification rate increases. At the bottom of the molten pool, solidification velocity V is very small, so that a planar interface is formed, while near the top of the molten pool, the value of cos u and the growth velocity vector along the horizontal direction is the maximum. Accordingly, the preferential growth orientation of the dendrites changes to the laser beam scanning direction, and dendrites growing along this orientation solidify rapidly. This competition leads to the microstructure as shown in zone A of Fig. 3(e) at the boundary of some layers, and also the microstructure in zone B of Fig. 3(f), the longitudinal crosssection of the cladding layers [13]. If the tilted dendrites shown in zones A and B can be completely remelted during the cladding of the succeeding layers, continuous epitaxial dendrites throughout all the layers can be obtained as shown in Fig. 3(g). Therefore, if the laser cladding forming process parameters and the remelted depth can be reasonably controlled, the orientation of the dendrites will be controlled
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Fig. 4. X-Ray Diffraction of the cladding layers.
correspondingly. Fig. 3(h) shows the cross-section of the columnar dendrites, which is very homogeneous and presents typical honeycomb structures. According to the rapid solidification theoretical models [14–16], there is a relationship between the primary dendrite arm distance l1, the solidification velocity V, and the temperature gradient G: l1 / VxGy (where x and y are the material correlated coefficients, which have been verified to have a value of about 0.25 and 0.5, respectively). The formula indicates that the dendrite arm distance is in an inverse ratio with V and G. Based on this formula, because the temperature gradient G in laser cladding forming process is very large (up to 106 K/mm [10,17]), l1 will be rather small, or in other words, a very fine microstructure of the rebuilt layers will be resulted. The dendrite arm distance obtained by this experiment is between 8 and 20 mm, well conforming to the formula. Fig. 4 is the X-ray diffraction of the rebuilt layers with laser cladding forming. Due to the super-rapid cooling rate, the microstructures are all g phase [17]. Fig. 5 is the EPMA composition line scanning results of the laser cladding forming rebuilding regions. The scanning direction is indicated in Fig. 5(a). Fig. 5(b) is the result of
Fig. 5. EPMA image of the laser cladding rebuilt regions: (a) scanning directions; (b) perpendicular to the interface; (c) perpendicular to the dendrite axis.
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Fig. 6. EDAX micro-chemical analysis: (a) bonding interface; (b) interdendritic regions; (c) columnar dendrites; (d) average value at different locations of the cladding layers.
scanning perpendicular to the cladding layer-substrate interface, i.e. along the direction (A). It can be seen that the distribution of compositions in the cladding layers is rather uniform. Fig. 5(c) is the image obtained perpendicular to the columnar dendrite axis, i.e. along the direction (B). It shows a very small interdendritic segregation of the four main elements. To further investigate the quantitative uniformity of the element distribution, EDAX micro chemical analysis of the cladding
layers has been carried out, shown in Fig. 6. Three points on the interface, the stem of the columnar dendrites and the interdendritic regions far from the bonding zone were, respectively, tested and the average values of the compositions have been calculated. It indicates that at the interface of the cladding layers and the substrate, slight element diffusion happens, with contents of Mo and Si decreasing slightly. With the increasing of the distance from the interface, there is a
Fig. 7. Tensile strength and impacting test samples of the V-grooves: (a) tensile strength sample parallel to the scanning speed; (b) tensile strength sample billets perpendicular to the scanning speed; (c) impacting sample.
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Table 1 Comparison of the mechanical properties between LCF rebuilt layers and the substrate Materials LCF rebuilt layers Parallel Perpendicular Substrate
Tensile strength, sb (MPa)
Elongation, d (%)
Impacting energy, AK (J)
664–752 647–738 600
21–32 19–27 16
51 39
content reduction of Cr and Ni. Compared with the original powder compositions, there is a slight element burning loss. Interdendritic segregation of Cr is about 1.72% and the segregation of Ni is 2.41%. The difference of compositions between the bonding interface and the cladding layers is less than 5%. The insignificant element fluctuation and segregation can be attributed to the inherent characters of the process. Since, laser cladding forming is a process of rapid solidification, macroscopic diffusion layers are very small and segregation of the interdendritic grains is also insignificant. 3.2. Mechanical properties To study the mechanical properties of the rebuilt V-groove regions with laser cladding forming, four pieces of samples with pre-machined V-grooves were rebuilt. Within the deposited regions and along the direction parallel to the scanning direction, several pieces of cylindrical tensile strength samples and rectangular block impacting testing samples were prepared, respectively. The dimensions of the samples are shown in Fig. 7(a) and (c). In addition, tensile strength testing samples perpendicular to the direction of the scanning speed were also prepared, in which, part of the substrate is also included to assess the bonding strength of the interface. The billets of the samples are shown in Fig. 7(b), areas circled in the black lines are deposited with laser cladding forming method and materials in the other regions are the substrate. The dimensions of the final samples are the same as indicated in Fig. 7(a). Microhardness measurements are also carried out on the transverse cross-section perpendicular to the scanning speed, beginning from the surface of the cladding layers.
Fig. 9. SEM morphology of the tensile fracture.
Table 1 lists the mechanical properties of the rebuilt Vgroove samples and the substrate. The tensile strength of the deposited layers parallel to the scanning speed is between 664 and 752 MPa, the elongation is 22–32%, while the tensile strength perpendicular to the scanning speed varies from 647 to 738 MPa and the elongation is 19–27%. Due to the overlapping effect of the cladding tracks and the influence of the substrate, tensile properties perpendicular to the scanning speed is somewhat inferior to those in the parallel direction. Both the tensile strength and the elongation are better than those of the substrate. The average impacting energy of the laser-rebuilt samples is 51 J, 130% of the impacting value of the substrate. The microhardness distributed along the cladding layers varies from 276 to 312 HV, as shown in Fig. 8. It can be seen that the microhardness is rather homogeneously distributed throughout the rebuilt layers and much higher than that of the substrate. With the increasing of the distance from the surface, the hardness decreases gradually and there is an abrupt increase around the bonding interface. This result can be attributed to the coarse microstructure at the bottom and finer dendrites at the upper parts of the cladding layers that are caused by the laser beam reheating of the underlying layers and the super-rapid cooling rate at the interface. Fig. 9 shows the scanning electronic microscope morphology (SEM) of the tensile fracture. It is composed of reticularly connected white tearing edges and plastic dimples, which indicate good ductility of the deposited materials. The general better mechanical property is a result of grain size refinement. 4. Conclusions
Fig. 8. Microhardness distribution along the rebuilt cladding layers.
In this study, pre-machined V-grooves on common carbon steel substrates were successfully rebuilt with laser cladding forming method. Microstructure and mechanical properties of the rebuilt V-groove samples were characterized and analyzed. From the experimental results, the following conclusions can be drawn.
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The rebuilt V-groove regions present a strong metallurgical bonding with the substrate. Microstructure of the deposited regions mainly consists of fine and defect-free epitaxial columnar dendrites. Compositions distributed in the cladding layers are rather uniform and interdendritic segregation of the main elements is relatively small, which is attributed to the inherent rapid heating and cooling characters of the laser cladding forming process. Due to the effects of grain size refinement, the comprehensive mechanical properties of the rebuilt layers have greatly exceeded that of the substrate. Tensile strength of the deposited V-groove layers varies from 647 to 752 MPa and the elongation is within the range of 19–32%. The average impacting energy of the laser-rebuilt samples is 130% of the substrate’s, and the laser cladding forming rebuilt regions exhibit good ductility. The microhardness is quite homogeneous throughout the rebuilt layers. With the increase of the distance from the surface, the hardness gradually decreases from 312 to 276 HV. At the location around the bonding interface, there is a sharp decrease. The results are attributed to the reheating of the underlying layers and the grain size variation of the microstructure. Acknowledgements The authors would like to acknowledge the financial support of the National Natural Science Foundation of China (50375096).
References [1] J. Mei-li, Mod. Mach. 1 (2004) 52–53 (in Chinese). [2] J. Grum, J.M. Slabe, Surf. Coat. Technol. 180/181 (2004) 596–602. [3] A.S.C.M. D’Oliveira, P.S.C.P. da Silva, R.M.C. Vilar, Surf. Coat. Technol. 153 (2002) 203–209. [4] J.O. Milewski, G.K. Lewis, D.J. Thoma, G.I. Keel, R.B. Nemec, R.A. Reinert, J. Mater. Process Technol. 75 (1998) 165–172. [5] J. Mazumder, D. Dutta, N. Kikuchi, A. Ghosh, Opt. Laser Eng. 34 (2000) 397–414. [6] K.I. Schwendner, R. Banerjee, P.C. Collins, C.A. Brice, H.L. Fraser, Scripta Mater. 45 (2001) 1123–1129. [7] M.L. Grifth, M.E. Schlienger, L.D. Harwell, M.S. Oliver, M.D. Baldwin, M.T. Ensz, M. Essien, J. Brooks, C.V. Robino, J.E. Smugeresky, W.H. Hofmeister, M.J. Wert, D.V. Nelson, Mater. Des. 20 (1999) 107–113. [8] G.K. Lewis, E. Schlienger, Mater. Des. 21 (2000) 417–423. [9] W. Hofmeister, M. Wert, J. Smugeresky, J.A. Philliber, M. Griffith, M. Ensz, JOM 53 (9) (2001) 30–34. [10] L. Mingxi, H. Yizhu, S. Guoxiong, Appl. Surf. Sci. 230 (2004) 201– 206. [11] A.J. Pinkerton, L. Li, Thin solid Films 453/454 (2004) 471–476. [12] Z. Molian, Foundation of Welding Theories and Processes, 1st ed., Beijing University of Aeronautics and Astronautics Press, Beijing, 1994, pp. 132– 138 (in Chinese). [13] M. Ga¨umann, S. Henry, F. Cle´ton, J.-D. Wagnie`re, W. Kurz, Mater. Sci. Eng. A A271 (1999) 232–241. [14] Z. Yaohe, H. Zhuangqi, J. Wanqi, Solidification Technology, 1st ed., China Machine Press, Beijing, 1998, pp. 235–236 (in Chinese). [15] W. Kurz, D.J. Fisher, Acta Metall. 29 (1981) 11–20. [16] R. Trivedi, Metall. Trans. 15A (1984) 977–982. [17] L. Yanmin, Research on technical characters and microstructure of laser solid forming, Ph.D. Thesis, Xi’an, China, 2001 (in Chinese).
Rebuilding of metal components with laser cladding forming Jianli Song a,b,*, Qilin Deng a, Changyuan Chen a, Dejin Hu a, Yongtang Li b a
School of Mechanical and Power Engineering, Shanghai Jiaotong University, Shanghai 200030, PR China b Taiyuan University of Science and Technology, Taiyuan 030024, PR China Received 29 June 2005; received in revised form 2 October 2005; accepted 2 October 2005 Available online 11 July 2006
Abstract Laser cladding forming (LCF) is a novel powerful tool for the repairing of metal components. Rebuilding of V-grooves on medium carbon steel substrates has been carried out with laser cladding forming technique using stainless steel powder as the cladding material. Microstructure of the deposited layers has been characterized using optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray microanalysis (EDAX), electron probe microanalysis (EPMA) and X-ray diffraction (XRD). Mechanical properties of the rebuilt V-groove samples have been evaluated by tensile and impacting tests and microhardness measurement. Experimental results show that good fusion bonding between the rebuilt layers and the substrate has been formed, and the microstructure of the cladding layers is mainly composed of fine, dense and defect-free epitaxial columnar dendrites. Due to the effect of grain size refinement, the tensile strength, impacting toughness, elongation and microhardness of the rebuilt samples have been greatly enhanced compared to those of the substrate. Microhardness is also very uniform throughout the rebuilt regions. With the growth of the deposited layers, the microhardness increases gradually. The good ductility of the deposited regions is verified by the SEM fracture analysis. # 2005 Elsevier B.V. All rights reserved. Keywords: Laser cladding forming (LCF); Rebuilding; V-groove; Stainless steel powder
1. Introduction In the fields of mechanical engineering, metallurgical industry and petrochemical and electric power, a large number of components work in formidable conditions involving impact, abrasion, high-temperature and pressure, and are liable to breakdown. If successful repairing cannot be carried out, the damaged components will have to be discarded and a significant loss will be caused. Conventional repairing methods presently adopted mainly include mechanical machining, brushelectroplate, deposited welding, TIG welding and thermal spraying (flame spraying and plasma spraying). Although, these methods have different advantages, there are still many drawbacks, such as being time-consuming and labor intensive, having limited thickness of deposition layers and machinable times, poor bonding strength, large amount of porosities and cracks, or significant heat injection and distortion of the substrates [1–3]. Therefore, it is of great interest to develop high-efficiency and precision repairing technologies that will extend the lifetime of components. * Corresponding author. Tel.: +86 21 62932360; fax: +86 21 62932611. E-mail address: [email protected] (J. Song). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.10.025
The novel versatile laser cladding forming technology is such a repairing technology, which integrates laser technology, CAD/CAM technology, advanced materials processing technology and photoelectric measuring and control technology. Different similar techniques, for example direct light fabrication (DLF) [4], laser engineered net shaping (LENSTM) and direct metal deposition [5] have been developed and studied by many research groups in recent years. Laser cladding forming combines the two technologies of rapid prototyping manufacturing (RPM) and laser cladding surface modification, and can be used to fabricate three-dimensional fully dense metal components directly from the CAD model. In the laser cladding forming process, a high-energy laser beam is focused onto the substrate to create a molten pool; metal powders are simultaneously delivered into the focal zone by the powder delivering nozzles and then melted and rapidly solidified. A single layer can be built with the motion of the worktable in the X–Y plane. Then the laser beam is elevated for a predefined distance in the Z-direction to deposit a new layer, thus a three-dimensional component can be stacked consecutively. The principle of this technology is shown in Fig. 1 [6,7]. The process overcomes the inevitable difficulties of deformation and thermal fatigue occurred in the conventional arc and TIG
J. Song et al. / Applied Surface Science 252 (2006) 7934–7940
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Fig. 2. Sketch of the LCF substrate. Fig. 1. Principle of laser cladding forming technology.
welding process and the poor bonding quality in electroplating process. Compared with those conventional repairing technologies, laser cladding forming has many unique characteristics [8,9]: higher flexibility, lower cost and shorter lead time, stronger bonding interface between laser cladding forming layers and the substrate, smaller dilution, segregation and distortion, finer microstructure and excellent mechanical properties, and tailored materials in required repairing positions. In recent years, laser cladding forming of various materials has been studied by many scholars. D’Oliveira et al. deposited five-layers of Stellite 6 alloy onto a 304 stainless steel plate, and investigated the residual stress and microstructures [3]. Li et al. studied the microstructure and wear resistance of multi-layer laser cladding HMSP2573 alloy [10]. Andrew and Lin reported the multi-layer cladding of stainless steel using a high-power diode laser and investigated the effects of process parameters on the surface properties and dimensions of the samples [11]. Most of the multi-layer cladding experiments have been carried out on a planar steel plate and only three to five-layers have been deposited. However, damaged surface regions are not always regular; sometimes deep grooved regions need to be deposited to recover the shapes and properties of the worn components. In this paper, laser cladding forming rebuilding of V-grooves up to 20 mm in depth on common carbon AISI 1045 steel substrates have been systematically investigated, microstructure and mechanical properties of the repaired regions have been characterized and assessed with different testing methods.
greasy and rust. Parameters used in the experiments are: laser power 2.25–3.75 kW, overlapping 27.5–37.5%, scanning speed 100–150 mm/min, beam diameter 4 mm, and powder delivery velocity 1.5–3.5 g/min, respectively. The grooves were then rebuilt with the method described in Fig. 1, and the molten pool was shrouded by Argon gas. In order to optimize the parameters of the rebuilding process, four-layer laser cladding forming experiments were firstly carried out on the surface of a planar metal plate with different combinations of process parameters, and the samples of the experiments have been characterized, respectively. Then the V-grooves were deposited with the optimized cladding parameters, which were determined from the previous testing results. After the rebuilding, standard specimens were cut from the samples with wire electrode discharge machining (WEDM), polished with sand paper and glazing machine, and eroded with aqua regia. The microstructure of the as-deposited samples was observed under KEYENCE three-dimensional digital microscope. Tensile strength and impacting toughness tests were, respectively, carried out on Zwick/Roell and SATEC SYSTEMS SI-lC3 material testing machines, morphology of the fractures was observed with Philips Scanning Electron Microscope and compositions of the laser cladding forming samples were analyzed with Shimadzu EPMA-8750 electronic probe microanalyzer and EDAX fitted on Philips Scanning Electron Microscope. Phases of the cladding layers were analyzed with D/max 2550V X-ray diffraction analysis and the microhardness was tested with a HX-1000 microhardness machine.
2. Experimental procedure
3. Results and discussion
The experimental system consists of a 6 kW CO2 laser source, an automatic wide-band powder feeding device, a SIEMENS numerical control system and a four-axis working plate. V-grooves with dimensions shown in Fig. 2 were premachined to a roughness of Ra12.5 mm on AISI 1045 block substrates with a size of 75 mm 150 mm 200 mm. 316L stainless steel powder (<0.03 C, 16–18 Cr, 12–14 Ni, 2–3 Mo, 0.68 Si and balance Fe, wt.%) was used as the rebuilding material. Before laser deposition, the 316L stainless steel powder was dried in a vacuum stove; the substrate was polished with sand paper and cleaned with acetone to remove the
3.1. Microstructure analysis The morphology and microstructure of the rebuilt V-groove regions are shown in Fig. 3. Fig. 3(a) and (b) illustrates the low magnification morphology of the deposited layers at the bottom part and the side part of the grooves, while Fig. 3(c)–(h) are the specifics of the microstructure. Fig. 3(c) and (d) shows the microstructure near the bonding interface, which are the side part and the bottom part of the deposited V-grooves, respectively. It can be seen that the bonding line is smooth or distinct fusion bonding interface has been obtained between the
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Fig. 3. Morphology and microstructure of the LCF rebuilt V-groove zone: (a) and (b) low magnification morphology of the V-groove zone; (c) and (d) microstructure near the bonding interface; (e) non-remelted zone; (f) longitudinal cross-section; (g) overlapping and epitaxial growth region; (h) cross-section of the columnar dendrites.
rebuilding layers and the layer-substrate boundary. The microstructures allover the rebuilt V-grooves are fine, free of porosities and cracks. The solidified microstructure is determined by the composition of the alloy, the temperature gradient G, the energy of the interface and the solidification velocity V, etc. At the beginning
of the cladding process, the ratio G/V on the solidification interface is very large, and there is hardly any ingredient super cooling, so the interface grows with a planar pattern, as shown in Fig. 3(c) and (d). The growth of the crystal is also related to the orientation of crystal cells. At the orientation that is most favorable for the growth of the crystal, the growth velocity is the
J. Song et al. / Applied Surface Science 252 (2006) 7934–7940
fastest. In laser cladding forming the preferential orientation is in accordance with the direction of the maximum heat flux [12,13]. During the laser cladding forming process, because the heat of the cladding layers dissipates through the substrate, the temperature gradient and the heat flux density perpendicular to the interface is the largest. At the same time, the liquid metal always keeps a tight contact with the solid metal matrix, and typical rapid solidified epitaxial columnar dendrites of different lengths are formed preferentially along the maximum temperature gradient in most deposited areas. With the propelling of the solid/liquid interface and heat accumulation, the temperature gradient G in the cladding layers decreases gradually. Heat flow via the substrates is no longer predominating, and heat of the cladding layers starts to radiate through the surrounding atmosphere. In the process of laser cladding forming, solidification velocity V at the front side isothermal line of the molten pool can be expressed as: V = Vb cos u (where Vb is the scanning speed and u is the angle between the solidification speed and the scanning speed). With the increasing of the scanning speed, the dwell time of the heat source at each point of the track decreases, and therefore, the solidification rate increases. At the bottom of the molten pool, solidification velocity V is very small, so that a planar interface is formed, while near the top of the molten pool, the value of cos u and the growth velocity vector along the horizontal direction is the maximum. Accordingly, the preferential growth orientation of the dendrites changes to the laser beam scanning direction, and dendrites growing along this orientation solidify rapidly. This competition leads to the microstructure as shown in zone A of Fig. 3(e) at the boundary of some layers, and also the microstructure in zone B of Fig. 3(f), the longitudinal crosssection of the cladding layers [13]. If the tilted dendrites shown in zones A and B can be completely remelted during the cladding of the succeeding layers, continuous epitaxial dendrites throughout all the layers can be obtained as shown in Fig. 3(g). Therefore, if the laser cladding forming process parameters and the remelted depth can be reasonably controlled, the orientation of the dendrites will be controlled
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Fig. 4. X-Ray Diffraction of the cladding layers.
correspondingly. Fig. 3(h) shows the cross-section of the columnar dendrites, which is very homogeneous and presents typical honeycomb structures. According to the rapid solidification theoretical models [14–16], there is a relationship between the primary dendrite arm distance l1, the solidification velocity V, and the temperature gradient G: l1 / VxGy (where x and y are the material correlated coefficients, which have been verified to have a value of about 0.25 and 0.5, respectively). The formula indicates that the dendrite arm distance is in an inverse ratio with V and G. Based on this formula, because the temperature gradient G in laser cladding forming process is very large (up to 106 K/mm [10,17]), l1 will be rather small, or in other words, a very fine microstructure of the rebuilt layers will be resulted. The dendrite arm distance obtained by this experiment is between 8 and 20 mm, well conforming to the formula. Fig. 4 is the X-ray diffraction of the rebuilt layers with laser cladding forming. Due to the super-rapid cooling rate, the microstructures are all g phase [17]. Fig. 5 is the EPMA composition line scanning results of the laser cladding forming rebuilding regions. The scanning direction is indicated in Fig. 5(a). Fig. 5(b) is the result of
Fig. 5. EPMA image of the laser cladding rebuilt regions: (a) scanning directions; (b) perpendicular to the interface; (c) perpendicular to the dendrite axis.
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Fig. 6. EDAX micro-chemical analysis: (a) bonding interface; (b) interdendritic regions; (c) columnar dendrites; (d) average value at different locations of the cladding layers.
scanning perpendicular to the cladding layer-substrate interface, i.e. along the direction (A). It can be seen that the distribution of compositions in the cladding layers is rather uniform. Fig. 5(c) is the image obtained perpendicular to the columnar dendrite axis, i.e. along the direction (B). It shows a very small interdendritic segregation of the four main elements. To further investigate the quantitative uniformity of the element distribution, EDAX micro chemical analysis of the cladding
layers has been carried out, shown in Fig. 6. Three points on the interface, the stem of the columnar dendrites and the interdendritic regions far from the bonding zone were, respectively, tested and the average values of the compositions have been calculated. It indicates that at the interface of the cladding layers and the substrate, slight element diffusion happens, with contents of Mo and Si decreasing slightly. With the increasing of the distance from the interface, there is a
Fig. 7. Tensile strength and impacting test samples of the V-grooves: (a) tensile strength sample parallel to the scanning speed; (b) tensile strength sample billets perpendicular to the scanning speed; (c) impacting sample.
J. Song et al. / Applied Surface Science 252 (2006) 7934–7940
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Table 1 Comparison of the mechanical properties between LCF rebuilt layers and the substrate Materials LCF rebuilt layers Parallel Perpendicular Substrate
Tensile strength, sb (MPa)
Elongation, d (%)
Impacting energy, AK (J)
664–752 647–738 600
21–32 19–27 16
51 39
content reduction of Cr and Ni. Compared with the original powder compositions, there is a slight element burning loss. Interdendritic segregation of Cr is about 1.72% and the segregation of Ni is 2.41%. The difference of compositions between the bonding interface and the cladding layers is less than 5%. The insignificant element fluctuation and segregation can be attributed to the inherent characters of the process. Since, laser cladding forming is a process of rapid solidification, macroscopic diffusion layers are very small and segregation of the interdendritic grains is also insignificant. 3.2. Mechanical properties To study the mechanical properties of the rebuilt V-groove regions with laser cladding forming, four pieces of samples with pre-machined V-grooves were rebuilt. Within the deposited regions and along the direction parallel to the scanning direction, several pieces of cylindrical tensile strength samples and rectangular block impacting testing samples were prepared, respectively. The dimensions of the samples are shown in Fig. 7(a) and (c). In addition, tensile strength testing samples perpendicular to the direction of the scanning speed were also prepared, in which, part of the substrate is also included to assess the bonding strength of the interface. The billets of the samples are shown in Fig. 7(b), areas circled in the black lines are deposited with laser cladding forming method and materials in the other regions are the substrate. The dimensions of the final samples are the same as indicated in Fig. 7(a). Microhardness measurements are also carried out on the transverse cross-section perpendicular to the scanning speed, beginning from the surface of the cladding layers.
Fig. 9. SEM morphology of the tensile fracture.
Table 1 lists the mechanical properties of the rebuilt Vgroove samples and the substrate. The tensile strength of the deposited layers parallel to the scanning speed is between 664 and 752 MPa, the elongation is 22–32%, while the tensile strength perpendicular to the scanning speed varies from 647 to 738 MPa and the elongation is 19–27%. Due to the overlapping effect of the cladding tracks and the influence of the substrate, tensile properties perpendicular to the scanning speed is somewhat inferior to those in the parallel direction. Both the tensile strength and the elongation are better than those of the substrate. The average impacting energy of the laser-rebuilt samples is 51 J, 130% of the impacting value of the substrate. The microhardness distributed along the cladding layers varies from 276 to 312 HV, as shown in Fig. 8. It can be seen that the microhardness is rather homogeneously distributed throughout the rebuilt layers and much higher than that of the substrate. With the increasing of the distance from the surface, the hardness decreases gradually and there is an abrupt increase around the bonding interface. This result can be attributed to the coarse microstructure at the bottom and finer dendrites at the upper parts of the cladding layers that are caused by the laser beam reheating of the underlying layers and the super-rapid cooling rate at the interface. Fig. 9 shows the scanning electronic microscope morphology (SEM) of the tensile fracture. It is composed of reticularly connected white tearing edges and plastic dimples, which indicate good ductility of the deposited materials. The general better mechanical property is a result of grain size refinement. 4. Conclusions
Fig. 8. Microhardness distribution along the rebuilt cladding layers.
In this study, pre-machined V-grooves on common carbon steel substrates were successfully rebuilt with laser cladding forming method. Microstructure and mechanical properties of the rebuilt V-groove samples were characterized and analyzed. From the experimental results, the following conclusions can be drawn.
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The rebuilt V-groove regions present a strong metallurgical bonding with the substrate. Microstructure of the deposited regions mainly consists of fine and defect-free epitaxial columnar dendrites. Compositions distributed in the cladding layers are rather uniform and interdendritic segregation of the main elements is relatively small, which is attributed to the inherent rapid heating and cooling characters of the laser cladding forming process. Due to the effects of grain size refinement, the comprehensive mechanical properties of the rebuilt layers have greatly exceeded that of the substrate. Tensile strength of the deposited V-groove layers varies from 647 to 752 MPa and the elongation is within the range of 19–32%. The average impacting energy of the laser-rebuilt samples is 130% of the substrate’s, and the laser cladding forming rebuilt regions exhibit good ductility. The microhardness is quite homogeneous throughout the rebuilt layers. With the increase of the distance from the surface, the hardness gradually decreases from 312 to 276 HV. At the location around the bonding interface, there is a sharp decrease. The results are attributed to the reheating of the underlying layers and the grain size variation of the microstructure. Acknowledgements The authors would like to acknowledge the financial support of the National Natural Science Foundation of China (50375096).
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