- Email: [email protected]
Formation of cross-sectional profile of a clad bead in coaxial laser cladding
ARTICLE IN PRESS
Optics & Laser Technology 39 (2007) 1532–1536 www.elsevier.com/locate/optlastec
Formation of cross-sectional profile of a clad bead in coaxial laser cladding Jichang Liua,b a
State Key Laboratory of Advanced Design and Manufacture for Vehicle Body, Hunan University, Changsha, Hunan 410083, P.R. China b Laser Institute, Hunan University, Changsha, Hunan 410082, P.R.China Received 15 May 2006; received in revised form 1 September 2006; accepted 23 December 2006 Available online 20 February 2007
Abstract In order to determine a cross-sectional profile of a clad bead in coaxial laser cladding, its formation mechanism is investigated theoretically and experimentally. In laser cladding, every point at the back edge of a melt pool is contributed to a cross-sectional profile of the clad bead to be formed, and points at the same pool edge but on different cross sections are located at different cross-sectional profiles of the clad bead. A cross-sectional profile of a clad bead is composed of points of intersection between the cross section and a series of pool edges. Model of the cross-sectional clad profile in single-pass coaxial laser cladding is developed. A 500 W CO2 laser is used in the experiment. The experimental result agrees well with the calculated cross-sectional clad profile. r 2007 Elsevier Ltd. All rights reserved. Keywords: Laser cladding; Cross-sectional profile of clad bead; Melt pool
1. Introduction Laser cladding has originally been a surface treatment process, in which powder of an alloy is melted by a laser beam and deposited on a substrate to enhance its resistance against corrosion, abrasion and wear. Since 1980s, laser cladding has been used to form a functional compact metal part directly from CAD model [1–3]. During laser cladding, clad beads are generated, and adjacent beads are overlapped. The effect of overlapping depends on both the interval between the two overlapped beads and their cross-sectional profiles. In laser formation of a part, changes in cross-sectional profiles lead to variations in upper surface of the workpiece, height of the layer and thickness of the formed part. Good knowledge of cross-sectional profile is required to calculate the path of laser scanning and the number of layers needed for fabrication of a part in working-out of the process plan. In a close-loop control system, prior to adjustment of process parameters, the on-coming clad bead profile must be estimated. In laser cladding for surface treatment, cross-sectional profiles of clad beads influence E-mail address: [email protected]. 0030-3992/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2006.12.009
surface quality, thickness and chemical composition of the layer deposited on a substrate. Therefore, it is critical to investigate the cross-sectional profile of a clad bead. Steen et al. expressed the relationship between the width of the clad bead, W, the scanning velocity, v, and the spot diameter, d, of laser beam as: W ¼ d(1av), where a is practical constant [4]. Hu et al. pointed out that width of a clad bead increases with powder feed rate and height of the clad bead, he, increases with increase in specific energy or decrease in speed of laser scanning. The equation devel_ p =v, where m _ p is powder feed rate, oped by them is: he ¼ bm he practical value of clad bead height, and b practical constant [3]. Zhang discovered that with increasing specific energy or irradiation time, the cross-sectional area of the clad bead increases linearly, width and contact angle of the clad bead increase, and increase in powder feed rate accelerates these changes [5]. Hereinbefore, only some aspects or some dimensions of the clad bead were explained roughly, while the actual cross-sectional profile was not investigated. In this study, in order to determine a cross-sectional profile of a clad bead in coaxial laser cladding, its formation mechanism and the relationship between a cross-sectional profile and a melt pool are investigated.
ARTICLE IN PRESS J. Liu / Optics & Laser Technology 39 (2007) 1532–1536
2. Formation of cross-sectional clad profile in coaxial laser cladding In laser cladding, laser irradiates and heats the work piece, which leads to the melting of metal at the surface and formation of a melt pool. Fig. 1 shows the longitudinal section and planview of a pool. With laser traversing, the pool moves. Indeed, the liquid in the area having been irradiated by laser solidifies after the beam passes, resulting in the clad bead being lengthened, and the anterior solid metal, while being irradiated, is being melted and occupied by the pool. In other words, the solid metal at the front of the pool, near the curve BC shown in Fig. 1, is being melted, while the liquid metal in the rear of the pool, near the curve AB shown in Fig. 1, is being solidified, and the interphase boundary of solidification moves ahead. The pool can be considered as a closed system with material being heated and cooled, melted and solidified and a balance between input and output energy. As mentioned above, the liquid metal in the area having been scanned by the laser beam solidifies as the pool moves forward. The interface of solidification is a three-dimensional surface involving the curve AB. Its edge is the boundary of the liquid phase, solid phase and gas phase, of which the back part is shown in Fig. 1 as three-dimensional curve bAd. This back part will contribute to the surface of
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the formed clad bead. If a series of boundaries are identified, the surface of a clad bead can be plotted. If the clad bead surface is exposed, it is certain that the corresponding cross-sectional profiles are disclosed. In Fig. 2, the origin of the coordinates is at the longitudinal center line of the bottom of a clad bead, x-axis is along the orientation of the laser scanning, positive z-axis is above the bottom of a clad bead, y-axis points from the left of the positive x-axis to the right. Assumed that a
Fig. 2. Schematic of formation of a cross-sectional clad profile.
Fig. 1. Schematic of the lognitudinal section and planview of a pool.
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cross-sectional profile is located on plane y–z, which is a cross section of the clad bead, it can be seen that all the points at the cross-sectional profile are the intersection ones between plane y–z and the numerous former pool edges. Given that all the former pool edges are x-axially symmetrical, the edge of which the two intersection points with plane y–z are at y-axis is considered as the first one, B0, and the one touching plane y–z at the top of the cross section of the clad bead is regarded as the final one, Bn. If edge B0 is expressed as function z ¼ f(x,y), every edge contributing to formation of the cross-sectional profile can be expressed as function z ¼ f(x+s,y), where s is defined as a value within the range 0–DS, where DS is distance in x direction from an intersection point of edge B0 with plane x–y to the corresponding one of edge Bn with plane x–y. Therefore, all points at the above-mentioned crosssectional profile can be defined by z ¼ f ðx þ s; yÞ, x ¼ 0.
ð1Þ
For any cross-sectional profile, x ¼ X, the points at it can be expressed as z ¼ f ðx þ s; yÞ, x ¼ X.
ð2Þ
Fig. 3 shows the link between the coordinates of the points at the cross-sectional profile of the clad bead and those of the corresponding points at the existing pool edge in steady-state laser cladding. On the surface of the clad bead, the lines aa0 , bb0 , cc0 , dd 0 , ee0 , ff 0 and gg0 are located on the longitudinal profiles of the clad bead and parallel to each other. The points on the same line have the same y-coordinates. In steady-state laser cladding, the pool shape and size remain stable, the shape of the interface of solidification remains constant, thus the pool edge, i.e., the boundary of the liquid phase, solid phase and gas phase, does not vary. In other words, the z-coordinates of the points at the pool edge, a, b, c, d, e, f and g, do not change. So the z-coordinates of the points at the cross-sectional
profile, a0 , b0 , c0 , d0 , e0 , f 0 and g0 , are equal to those of the points at the pool edge, a, b, c, d, e, f and g, respectively. That is to say yða0 Þ ¼ yðaÞ; zða0 Þ ¼ zðaÞ, yðb0 Þ ¼ yðbÞ; zðb0 Þ ¼ zðbÞ, yðc0 Þ ¼ yðcÞ; zðc0 Þ ¼ zðcÞ, yðd 0 Þ ¼ yðdÞ; zðd 0 Þ ¼ zðdÞ, yðe0 Þ ¼ yðeÞ; zðe0 Þ ¼ zðeÞ, yðf 0 Þ ¼ yðf Þ; zðf 0 Þ ¼ zðf Þ, yðg0 Þ ¼ yðgÞ; zðg0 Þ ¼ zðgÞ.
ð3Þ
For any longitudinal profile of the clad bead formed in steady laser cladding, there is an equation like one of the above shown. Therefore, the z-coordinates of all the points at a cross-sectional profile of the clad bead are equivalent to those of all the corresponding points at the edge of a melt pool generated during laser cladding. As far as variable laser cladding, the cross-sectional profile of a clad bead is varying due to changes in the pool shape and size. Nevertheless, the clad bead is still formed by solidification of the liquid metal within the pool. The liquid metal at the back part of the pool edge will be transformed into the solid metal at the corresponding points at a series of cross-sectional profiles after solidification. Although the z-coordinates of the points with similar y-coordinates at various cross-sectional profiles are different, they are solidly equal to those of the corresponding points at the pool edges, which form these cross-sectional profiles. Therefore, the z-coordinates of the points at the cross-sectional profile of the clad bead can be derived from those of the corresponding points at a series of pool edges. In the following section, cross-sectional profile of the clad bead generated in single-pass coaxial low-power laser cladding, regarded as an example, is presented for demonstration of formation of cross-sectional clad profile. 3. Cross-sectional profile of clad bead generated in singlepass coaxial low-power laser cladding 3.1. Simplified model of cross-sectional profile of clad bead
Fig. 3. Schematic of the relationship between the coordinates of the points at the cross-sectional contour line and those at the common boundary of the three phases (gaseous, solid and liquid phase).
In single-pass coaxial low-power laser cladding, given that the substrate is the same kind of material as the powder and there is no remarkable difference between their melt points, the average temperature in the pool is a little higher than the melt point of the mixture of the substrate and the powder alloy. The fluidity of the liquid in the pool is rather small. The influences of the liquid flow and powder–gas stream’s pressure can be neglected. It is assumed that point (x2, y2, z2) and point (x1, y1, z1) are used to express approximately a point at the back edge of a melt pool, which will become a point at a cross-sectional profile, and that at the front edge of the melt pool with the same y-coordinate, respectively. The coordinates of point (x2, y2, z2) are equal to those of the corresponding point at
ARTICLE IN PRESS J. Liu / Optics & Laser Technology 39 (2007) 1532–1536
the above-mentioned cross-sectional profile. It is proposed that the height of point (x2, y2, z2), i.e., its z-coordinate, at the cross-sectional profile is proportional to the sum of the powder piled from point (x1, y1, z1) to point (x2, y2, z2) on the substrate since point (x2, y2, z2) is, in fact, a certain point at the back part of a pool edge. Provided that the shape of a melt pool is simplified as a disk, whose diameter is equal to the clad bead’s width, if the process is steady, point (x2, y2, z2) at the cross-section clad profile is defined in Ref [6] as " # mp 2y22 Z 2 ¼ pffiffiffiffiffiffi exp 2 Rp 2prvRp Z pffiffi2x1 =Rp 2 pffiffiffi expða2 Þ da p 0 ! Z 0 2 2 þ pffiffiffi pffiffi expða Þ da , p 2x2 =Rp
ð4Þ
where mp is mass powder feed rate, Rp radius of the powder stream on the substrate, v scanning speed, and density of the clad material.
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3.2. Experiment and discussion This section is mainly extracted from Ref [6]. In the experiment, the coaxial laser cladding system consists of a 500 W CO2 laser unit, a metal powder delivery system, and a CNC laser-processing tool. The focal length of the lens in the working head of the laser-processing tool is 127 mm (5 in.). The minimum diameter of the focused laser beam is about 0.1 mm. The substrate material utilized in the experiment is Steel 20 (at 0.20 wt% C), and metal powder material is Steel 63 (at 0.63 wt% C). The diameter of the metal powder is 45–80 mm. The shield gas and carriage gas both are N2. Their flow rates are 0.5 m3/h. The temperature in the lab is 20 1C. The laser power is 135 W. The focus of the beam is located 1.0 mm above the surface of the substrate. The minimum radius of the powder flow, Rpmin, is 1.8 mm, and the half spraying angle is 151. Fig. 4 shows the cross-section profile of the clad bead deposited at scanning speed of 2.5 mm/s and powder feed rate of 1.06 g/min. The width of the clad bead shown in Fig. 4 is measured as 0.3 mm, and the radius of the powder flow on the substrate is computed as Rp ¼ Rpmin+1.0 tan 151E 2.1(mm). The calculated cross-section profile of the clad bead deposited at scanning speed of 2.5 mm/s and powder feed rate of 1.06 g/min is shown in Fig. 5. It is seen that the calculated cross-section clad profile illustrated in Fig. 5 resembles the experimental one shown in Fig. 4. So, in single-pass coaxial low-power laser cladding, the height of point (x, y, z) at the cross-sectional profile is proportional to the sum of the powder piled at point (x, y, 0) on the substrate. Therefore, it is partially testified that during laser cladding, the heights of all the points at cross-sectional profiles of the clad bead are equal to those of the counterpoints at a series of pool edges. 4. Conclusions In laser cladding, every point at the back edge of a melt pool is contributed to a cross-sectional profile of the clad
Fig. 4. Experimental cross-section profile of the clad bead.
height(mm)
Cross-section clad profile 0.10 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.00 -0.20
-0.15
-0.10
-0.05
0.00 y (mm)
0.05
0.10
Fig. 5. Calculated cross-section profile of the clad bead.
0.15
0.20
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bead to be formed, and points at the same pool edge but on different cross sections are located at different crosssectional profiles of the clad bead. A cross-sectional profile consists of points of intersection between a series of pool edges and the cross section on which the cross-sectional profile is. In steady-state laser cladding, the z-coordinates of all the points at a cross-sectional profile of the clad bead are equivalent to those of all the corresponding points at the back edge of a melt pool generated during laser cladding. In single-pass coaxial low-power laser cladding, the height of a point at the cross-sectional profile is proportional to the amount of powder delivered at a certain point on the substrate.
References [1] Mazumder J, Nagarathnam K, Choi J, et al. The direct metal deposition of H13 tool steel for 3-D components. JOM 1997;49(5):55–60. [2] Hofmeister W, Griffith M, Ensz M, et al. Solidification in direct metal deposition by LENS processing. JOM 2001;53(9):30–4. [3] Hu YP, Chen CW, Mukherjee K. Innovative laser–aided manufacturing of patterned stamping and cutting dies: processing parameters. Mater Manuf Process 1998;13(3):369–87. [4] Steen WM, Weerasinghe VM, Monson P. Some aspects of the formation of laser clad tracks. SPIE 1986;650:226–34. [5] Zhang Q-m, Liu X-m, Wang Z-d, et al. Influence of processing parameters on geometrical features of powder feeding laser cladding. Trans Chin Weld Inst 2000;21(2):44–6 (in Chinese). [6] Jichang L, Lijun L. Study on cross-section clad profile in coaxial single-pass cladding with a low-power laser. Opt Laser Technol 2005; 37(6):478–82.
Optics & Laser Technology 39 (2007) 1532–1536 www.elsevier.com/locate/optlastec
Formation of cross-sectional profile of a clad bead in coaxial laser cladding Jichang Liua,b a
State Key Laboratory of Advanced Design and Manufacture for Vehicle Body, Hunan University, Changsha, Hunan 410083, P.R. China b Laser Institute, Hunan University, Changsha, Hunan 410082, P.R.China Received 15 May 2006; received in revised form 1 September 2006; accepted 23 December 2006 Available online 20 February 2007
Abstract In order to determine a cross-sectional profile of a clad bead in coaxial laser cladding, its formation mechanism is investigated theoretically and experimentally. In laser cladding, every point at the back edge of a melt pool is contributed to a cross-sectional profile of the clad bead to be formed, and points at the same pool edge but on different cross sections are located at different cross-sectional profiles of the clad bead. A cross-sectional profile of a clad bead is composed of points of intersection between the cross section and a series of pool edges. Model of the cross-sectional clad profile in single-pass coaxial laser cladding is developed. A 500 W CO2 laser is used in the experiment. The experimental result agrees well with the calculated cross-sectional clad profile. r 2007 Elsevier Ltd. All rights reserved. Keywords: Laser cladding; Cross-sectional profile of clad bead; Melt pool
1. Introduction Laser cladding has originally been a surface treatment process, in which powder of an alloy is melted by a laser beam and deposited on a substrate to enhance its resistance against corrosion, abrasion and wear. Since 1980s, laser cladding has been used to form a functional compact metal part directly from CAD model [1–3]. During laser cladding, clad beads are generated, and adjacent beads are overlapped. The effect of overlapping depends on both the interval between the two overlapped beads and their cross-sectional profiles. In laser formation of a part, changes in cross-sectional profiles lead to variations in upper surface of the workpiece, height of the layer and thickness of the formed part. Good knowledge of cross-sectional profile is required to calculate the path of laser scanning and the number of layers needed for fabrication of a part in working-out of the process plan. In a close-loop control system, prior to adjustment of process parameters, the on-coming clad bead profile must be estimated. In laser cladding for surface treatment, cross-sectional profiles of clad beads influence E-mail address: [email protected]. 0030-3992/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2006.12.009
surface quality, thickness and chemical composition of the layer deposited on a substrate. Therefore, it is critical to investigate the cross-sectional profile of a clad bead. Steen et al. expressed the relationship between the width of the clad bead, W, the scanning velocity, v, and the spot diameter, d, of laser beam as: W ¼ d(1av), where a is practical constant [4]. Hu et al. pointed out that width of a clad bead increases with powder feed rate and height of the clad bead, he, increases with increase in specific energy or decrease in speed of laser scanning. The equation devel_ p =v, where m _ p is powder feed rate, oped by them is: he ¼ bm he practical value of clad bead height, and b practical constant [3]. Zhang discovered that with increasing specific energy or irradiation time, the cross-sectional area of the clad bead increases linearly, width and contact angle of the clad bead increase, and increase in powder feed rate accelerates these changes [5]. Hereinbefore, only some aspects or some dimensions of the clad bead were explained roughly, while the actual cross-sectional profile was not investigated. In this study, in order to determine a cross-sectional profile of a clad bead in coaxial laser cladding, its formation mechanism and the relationship between a cross-sectional profile and a melt pool are investigated.
ARTICLE IN PRESS J. Liu / Optics & Laser Technology 39 (2007) 1532–1536
2. Formation of cross-sectional clad profile in coaxial laser cladding In laser cladding, laser irradiates and heats the work piece, which leads to the melting of metal at the surface and formation of a melt pool. Fig. 1 shows the longitudinal section and planview of a pool. With laser traversing, the pool moves. Indeed, the liquid in the area having been irradiated by laser solidifies after the beam passes, resulting in the clad bead being lengthened, and the anterior solid metal, while being irradiated, is being melted and occupied by the pool. In other words, the solid metal at the front of the pool, near the curve BC shown in Fig. 1, is being melted, while the liquid metal in the rear of the pool, near the curve AB shown in Fig. 1, is being solidified, and the interphase boundary of solidification moves ahead. The pool can be considered as a closed system with material being heated and cooled, melted and solidified and a balance between input and output energy. As mentioned above, the liquid metal in the area having been scanned by the laser beam solidifies as the pool moves forward. The interface of solidification is a three-dimensional surface involving the curve AB. Its edge is the boundary of the liquid phase, solid phase and gas phase, of which the back part is shown in Fig. 1 as three-dimensional curve bAd. This back part will contribute to the surface of
1533
the formed clad bead. If a series of boundaries are identified, the surface of a clad bead can be plotted. If the clad bead surface is exposed, it is certain that the corresponding cross-sectional profiles are disclosed. In Fig. 2, the origin of the coordinates is at the longitudinal center line of the bottom of a clad bead, x-axis is along the orientation of the laser scanning, positive z-axis is above the bottom of a clad bead, y-axis points from the left of the positive x-axis to the right. Assumed that a
Fig. 2. Schematic of formation of a cross-sectional clad profile.
Fig. 1. Schematic of the lognitudinal section and planview of a pool.
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cross-sectional profile is located on plane y–z, which is a cross section of the clad bead, it can be seen that all the points at the cross-sectional profile are the intersection ones between plane y–z and the numerous former pool edges. Given that all the former pool edges are x-axially symmetrical, the edge of which the two intersection points with plane y–z are at y-axis is considered as the first one, B0, and the one touching plane y–z at the top of the cross section of the clad bead is regarded as the final one, Bn. If edge B0 is expressed as function z ¼ f(x,y), every edge contributing to formation of the cross-sectional profile can be expressed as function z ¼ f(x+s,y), where s is defined as a value within the range 0–DS, where DS is distance in x direction from an intersection point of edge B0 with plane x–y to the corresponding one of edge Bn with plane x–y. Therefore, all points at the above-mentioned crosssectional profile can be defined by z ¼ f ðx þ s; yÞ, x ¼ 0.
ð1Þ
For any cross-sectional profile, x ¼ X, the points at it can be expressed as z ¼ f ðx þ s; yÞ, x ¼ X.
ð2Þ
Fig. 3 shows the link between the coordinates of the points at the cross-sectional profile of the clad bead and those of the corresponding points at the existing pool edge in steady-state laser cladding. On the surface of the clad bead, the lines aa0 , bb0 , cc0 , dd 0 , ee0 , ff 0 and gg0 are located on the longitudinal profiles of the clad bead and parallel to each other. The points on the same line have the same y-coordinates. In steady-state laser cladding, the pool shape and size remain stable, the shape of the interface of solidification remains constant, thus the pool edge, i.e., the boundary of the liquid phase, solid phase and gas phase, does not vary. In other words, the z-coordinates of the points at the pool edge, a, b, c, d, e, f and g, do not change. So the z-coordinates of the points at the cross-sectional
profile, a0 , b0 , c0 , d0 , e0 , f 0 and g0 , are equal to those of the points at the pool edge, a, b, c, d, e, f and g, respectively. That is to say yða0 Þ ¼ yðaÞ; zða0 Þ ¼ zðaÞ, yðb0 Þ ¼ yðbÞ; zðb0 Þ ¼ zðbÞ, yðc0 Þ ¼ yðcÞ; zðc0 Þ ¼ zðcÞ, yðd 0 Þ ¼ yðdÞ; zðd 0 Þ ¼ zðdÞ, yðe0 Þ ¼ yðeÞ; zðe0 Þ ¼ zðeÞ, yðf 0 Þ ¼ yðf Þ; zðf 0 Þ ¼ zðf Þ, yðg0 Þ ¼ yðgÞ; zðg0 Þ ¼ zðgÞ.
ð3Þ
For any longitudinal profile of the clad bead formed in steady laser cladding, there is an equation like one of the above shown. Therefore, the z-coordinates of all the points at a cross-sectional profile of the clad bead are equivalent to those of all the corresponding points at the edge of a melt pool generated during laser cladding. As far as variable laser cladding, the cross-sectional profile of a clad bead is varying due to changes in the pool shape and size. Nevertheless, the clad bead is still formed by solidification of the liquid metal within the pool. The liquid metal at the back part of the pool edge will be transformed into the solid metal at the corresponding points at a series of cross-sectional profiles after solidification. Although the z-coordinates of the points with similar y-coordinates at various cross-sectional profiles are different, they are solidly equal to those of the corresponding points at the pool edges, which form these cross-sectional profiles. Therefore, the z-coordinates of the points at the cross-sectional profile of the clad bead can be derived from those of the corresponding points at a series of pool edges. In the following section, cross-sectional profile of the clad bead generated in single-pass coaxial low-power laser cladding, regarded as an example, is presented for demonstration of formation of cross-sectional clad profile. 3. Cross-sectional profile of clad bead generated in singlepass coaxial low-power laser cladding 3.1. Simplified model of cross-sectional profile of clad bead
Fig. 3. Schematic of the relationship between the coordinates of the points at the cross-sectional contour line and those at the common boundary of the three phases (gaseous, solid and liquid phase).
In single-pass coaxial low-power laser cladding, given that the substrate is the same kind of material as the powder and there is no remarkable difference between their melt points, the average temperature in the pool is a little higher than the melt point of the mixture of the substrate and the powder alloy. The fluidity of the liquid in the pool is rather small. The influences of the liquid flow and powder–gas stream’s pressure can be neglected. It is assumed that point (x2, y2, z2) and point (x1, y1, z1) are used to express approximately a point at the back edge of a melt pool, which will become a point at a cross-sectional profile, and that at the front edge of the melt pool with the same y-coordinate, respectively. The coordinates of point (x2, y2, z2) are equal to those of the corresponding point at
ARTICLE IN PRESS J. Liu / Optics & Laser Technology 39 (2007) 1532–1536
the above-mentioned cross-sectional profile. It is proposed that the height of point (x2, y2, z2), i.e., its z-coordinate, at the cross-sectional profile is proportional to the sum of the powder piled from point (x1, y1, z1) to point (x2, y2, z2) on the substrate since point (x2, y2, z2) is, in fact, a certain point at the back part of a pool edge. Provided that the shape of a melt pool is simplified as a disk, whose diameter is equal to the clad bead’s width, if the process is steady, point (x2, y2, z2) at the cross-section clad profile is defined in Ref [6] as " # mp 2y22 Z 2 ¼ pffiffiffiffiffiffi exp 2 Rp 2prvRp Z pffiffi2x1 =Rp 2 pffiffiffi expða2 Þ da p 0 ! Z 0 2 2 þ pffiffiffi pffiffi expða Þ da , p 2x2 =Rp
ð4Þ
where mp is mass powder feed rate, Rp radius of the powder stream on the substrate, v scanning speed, and density of the clad material.
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3.2. Experiment and discussion This section is mainly extracted from Ref [6]. In the experiment, the coaxial laser cladding system consists of a 500 W CO2 laser unit, a metal powder delivery system, and a CNC laser-processing tool. The focal length of the lens in the working head of the laser-processing tool is 127 mm (5 in.). The minimum diameter of the focused laser beam is about 0.1 mm. The substrate material utilized in the experiment is Steel 20 (at 0.20 wt% C), and metal powder material is Steel 63 (at 0.63 wt% C). The diameter of the metal powder is 45–80 mm. The shield gas and carriage gas both are N2. Their flow rates are 0.5 m3/h. The temperature in the lab is 20 1C. The laser power is 135 W. The focus of the beam is located 1.0 mm above the surface of the substrate. The minimum radius of the powder flow, Rpmin, is 1.8 mm, and the half spraying angle is 151. Fig. 4 shows the cross-section profile of the clad bead deposited at scanning speed of 2.5 mm/s and powder feed rate of 1.06 g/min. The width of the clad bead shown in Fig. 4 is measured as 0.3 mm, and the radius of the powder flow on the substrate is computed as Rp ¼ Rpmin+1.0 tan 151E 2.1(mm). The calculated cross-section profile of the clad bead deposited at scanning speed of 2.5 mm/s and powder feed rate of 1.06 g/min is shown in Fig. 5. It is seen that the calculated cross-section clad profile illustrated in Fig. 5 resembles the experimental one shown in Fig. 4. So, in single-pass coaxial low-power laser cladding, the height of point (x, y, z) at the cross-sectional profile is proportional to the sum of the powder piled at point (x, y, 0) on the substrate. Therefore, it is partially testified that during laser cladding, the heights of all the points at cross-sectional profiles of the clad bead are equal to those of the counterpoints at a series of pool edges. 4. Conclusions In laser cladding, every point at the back edge of a melt pool is contributed to a cross-sectional profile of the clad
Fig. 4. Experimental cross-section profile of the clad bead.
height(mm)
Cross-section clad profile 0.10 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.00 -0.20
-0.15
-0.10
-0.05
0.00 y (mm)
0.05
0.10
Fig. 5. Calculated cross-section profile of the clad bead.
0.15
0.20
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bead to be formed, and points at the same pool edge but on different cross sections are located at different crosssectional profiles of the clad bead. A cross-sectional profile consists of points of intersection between a series of pool edges and the cross section on which the cross-sectional profile is. In steady-state laser cladding, the z-coordinates of all the points at a cross-sectional profile of the clad bead are equivalent to those of all the corresponding points at the back edge of a melt pool generated during laser cladding. In single-pass coaxial low-power laser cladding, the height of a point at the cross-sectional profile is proportional to the amount of powder delivered at a certain point on the substrate.
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