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Studies on laser bending of stainless steel
Materials Science and Engineering A 385 (2004) 113–122
Studies on laser bending of stainless steel J. Dutta Majumdar a,∗ , A.K. Nath b , I. Manna a a
Department of Metals & Materials Engineering, Indian Institute of Technology, Kharagpur 721302, India b Industrial CO Laser Centre, Centre for Advanced Technology, Indore 452013, India 2 Received 29 January 2004; received in revised form 25 May 2004
Abstract In the present study, laser bending of AISI 304 stainless steel sheet has been attempted with a high power (2 kW) continuous wave CO2 laser. Bending angle was measured as a function of laser/processing parameters including power density, scan speed, number of passes and sheet thickness. Following laser bending, microstructural evolution (using a scanning electron microscope) and phase analysis (by X-ray diffraction technique) were systematically carried out to study the effect of laser irradiation and thermal stress on the microstructure and phase transformation behavior of the sheet. Microhardness of the bent sheet at different position was carefully measured using a Vickers microhardness tester. Bending angle was found to vary from 0.5◦ to 70◦ under different processing conditions. The microhardness of the bend zone was found to increase (from 1.5 to 2 times) as compared to the as-received sample. The improved microhardness is attributed to grain refinement associated with rapid quenching during laser bending. Finally, the optimum processing zone for laser bending of stainless steel was derived. © 2004 Elsevier B.V. All rights reserved. Keywords: Laser; Bending; Stainless steel; Microhardness
1. Introduction Laser bending is a newly developed technique of modifying the curvature of sheet metal by thermal residual stresses generated by laser assisted heating without any externally applied mechanical forces [1–5]. Laser bending may also serve the purpose of straightening thin sheets by a similar laser-based non-contact process without any mechanical forces. The process assumes significance due to the ease and flexibility of non-contact processing, amenability to materials with diverse shape/geometry, properties and chemistry, and high precision/productivity. Laser bending involves a complex interplay between the thermal profile generated by the laser irradiation and physical/thermal properties and dimension of the material/work-piece. In general, the process is influenced by many parameters such as laser parameters (power density and interaction/pulse time), material properties (thermal conductivity, coefficient of thermal expansion, etc.) and target dimensions (thickness, curvature, etc.). In laser bending, bending is induced by a localized laser generated temperature gradient between the irradiated surface and the neighboring material. The process is briefly de∗
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scribed by Namba [6]. The temperature distribution forces the material to expand non-uniformly, which in turn leads to non-uniform thermal stresses. Plastic deformation starts when the thermal stress exceeds yield stress. There are three different mechanism of laser bending; temperature gradient, buckling and upsetting mechanism. The mechanism of laser bending are briefly described by Vollersten [7], Vollersten et al. [8] and Geiger and Vollersten [9]. Recently, a number of numerical [10], analytical [11] and finite element models [12] have been developed to analyze various kinds of laser bending processes. Hunninge et al. [13] and Magee et al. [14] have studied the influence of laser parameters on dimensional accuracy of bent part. Sprenger et al. [15] and Li and Yao [16] studied the effect of externally applied stress and strain on the quality of bent product. Deformation behavior of Al-based metal matrix composite (Al6013/SiC) was reported by Chan and Liang [17]. In this regard, it is relevant to mention that most of the literature concerns the understanding of laser bending and the influence of process parameters on the bending angle. However, a detailed study of the microstructures and properties of bent zone and the influence of laser parameters on it needs due consideration to optimize the process parameters of laser bending. In the present study, laser bending of AISI 304 stainless steel sheet has been attempted using a high power (2 kW)
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continuous wave CO2 laser with a 3 mm beam diameter. The main process variables were incident power density, scan speed, number of passes and sheet thickness. After bending operation, a detailed characterization of the bent sheet was carried out by measuring the bend angle, detailed microstructural study and phase analysis. The microhardness of the bent sheet at different position were carefully measured using a Vickers microhardness tester. Bending angle, microstructure and mechanical properties of the bent sheet were correlated with laser parameters and sheet thickness to derive an optimum processing zone for laser bending.
Table 1 Summary of process parameters employed in laser bending of AISI 304 stainless steel Sheet thickness (mm)
Power density (kW/cm2 )
Scan speed (mm/min)
Number of passes
0.9 1.6
13–55 13–55
4000–6000 1000–2500
5–40 5–40
the top surface and the cross-section) was measured by a Vickers microhardness tester using a 300 g applied load. Finally, bend angle, microstructure, and microhardness of the bent zone was correlated with laser parameters to derive the processing window for laser bending.
2. Experimental In the present study, thin sheet (10 cm × 1 cm) of cold rolled AISI 304 stainless steel (of two different thickness) was used as samples for bending. The thickness of the specimen was 1.6 mm and 0.9 mm, respectively. Laser bending was carried out using a 2 kW continuous wave (CW) CO2 laser with Ar as shrouding environment to avoid oxidation during lasing. Fig. 1 shows the schematic of laser bending setup. One end of the sheet was clamped in a fixture and the assembly was mounted on a CNC controlled stage that was moved at a speed of 100 mm/min–8000 mm/min. Laser focus rested 30 mm above the sample and was traversed along the centerline of the sheet from one end to other across the width. The process variables for the present study were incident laser power density, scan speed and number of passes. Adequate numbers of trials were conducted to correlate the bend zone characteristics with laser parameters (Table 1). The bend samples were digitised using a CCD camera and measured using a image processing software to an accuracy of 0.5◦ . The microstructure of the bent zone (both the laser irradiated zone and the outer surface along depth) and the heat affected zone (HAZ) were characterized by optical microscope (OM) and scanning electron microscope (SEM). Phase analysis of the bent zone was carried out using X-ray diffraction technique with Co K␣ radiation to observe if there is any phase transformation due to laser irradiation and thermal stress. Microhardness of the bent zone (both
Fig. 1. Schematic of experimental setup for laser bending of AISI 304 stainless steel using a continuous wave CO2 laser.
3. Results and discussion 3.1. Bending angle The extent of bending is determined by bending angle. It is essential to know the effect of laser/process parameters on the bending angle to optimize the process parameters. In the present study, bending angle formed under different processing conditions was measured and correlated with laser/process parameters. Fig. 2 shows the effect of scan speed (ν) on the angle of bending for laser bent AISI 304 stainless steel sheet of thickness (t) (a) 1.6 mm and (b) 0.9 mm, respectively. The angle of bending was found to
Fig. 2. Variation of angle of bending with scan speed in laser bent AISI 304 stainless steel sheet with a thickness of (a) 1.6 mm and (b) 0.9 mm, respectively.
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vary inversely with scan speed for both the sheet thickness. The decreased bending angle with increase in scan speed is attributed to decreased absorbed energy with increasing scan speed (because of a lower interaction time at a higher scan speed), leading to a lower thermal stress and hence, a lower bending angle. In this regard, it is relevant to mention that though bending angle increases with decrease in scan speed, application of a very low scan speed leads to evaporation of material from the surface and hence, crater formation. Hence, the range of scan speed shown in Fig. 2 was capable of formation of a defect free bent zone and was used for bending of stainless steel. The effect of number of passes (n) on the angle of bending is also shown in Fig. 2. Careful observation of Fig. 2a and b shows that bend angle increases with increase in number of passes for both the sheet thickness. A detailed investigation of the effect of scan speed on the angle of bending revealed that bending of AISI 304 stainless steel with a 1.6 mm sheet thickness was achieved at a higher interaction time (lower scan speed) and the range of scan speed used for bending without the adverse effect of surface evaporation was 500 mm/min–3000 mm/min, which was found to be significantly lower than the same with a sheet thickness of 0.9 mm (3500 mm/min–7500 mm/min). Furthermore, a higher angle of bending (1◦ –75◦ ) was achieved in case of stainless steel with 0.9 mm sheet thickness as compared to the same for 1.6 mm sheet thickness (1◦ –30◦ ). Lower sheet thickness required a comparatively lower stress for bending and easier to bend than vice versa. Hence, it may be concluded that sheet thickness also plays a role in determining the bending limit and choosing the laser parameters for bending. Fig. 3a and b summarize the variation of bending angle with number of passes. With each pass thermal stress, proportional to thermal gradient, is introduced and hence, bending angle also increases with increasing number of pass. Moreover, following bending after a single pass, there is reduction of material at the bent zone during successive passes due to material flow outside the bent region after each pass. The increase in bending angle with number of passes was however, found to depend on the introduced microstructure by the previous pass. Increase in bend angle with increasing laser power density, P (cf. Fig. 4) is attributed to increased material flow at a higher absorbed energy density. Rate of increase in bend angle with power is however, much lower for 1.6 mm sheet thickness than the same for 0.9 mm sheet thickness. Furthermore, a larger thermal stress is required for bending of the sheet with a higher thickness (1.6 mm) than the same with a lower thickness. Cheng and Lin [18] developed an analytical model to calculate thermal stress required for bending which evidences the increased level of required thermal stress with increasing sheet thickness. As a result of which, a very low bending angle is achieved for 1.6 mm sheet thickness than the same with 0.9 mm sheet thickness. It is relevant to note that though the bend angle increases with increase in power density, application of a very large power density causes excessive melting and evaporation of
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Fig. 3. Variation of angle of bending with number of passes for laser bent AISI 304 stainless steel sheet with a thickness of (a) 1.6 mm and (b) 0.9 mm, respectively.
surface material at the inner side (laser–material interaction site) of the bent region. 3.2. Microstructural evolution Microstructure plays a very crucial role in determining the property of the bent region and hence, determining the optimum laser processing region. In the present study, a detailed investigation of the microstructure developed in laser bending processed under different conditions was under-
Fig. 4. Variation of angle of bending with applied power density for laser bent AISI 304 stainless steel.
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Fig. 5. Scanning electron micrograph of the as-received AISI 304 stainless steel.
taken and correlated with process parameters. Fig. 5 shows the microstructure of the as-received AISI 304 stainless steel sheet consisting of single phase austenitic grains (size varying from 10 m to 20 m). Laser bending was conducted by introducing thermal stress produced by irradiation with a high power laser beam. Fig. 6a and b show the scanning
Fig. 6. Scanning electron micrograph of laser bent AISI 304 stainless steel (of sheet thickness of 0.9 mm) lased with a power density of 19.6 × 103 W/cm2 , scan speed of 6000 mm/min and (a) 40 and (b) 20 number of passes.
electron micrographs (low magnification view) of laser bent AISI 304 stainless steel (of sheet thickness of 0.9 mm) lased with a power density of 19.6 × 103 W/cm2 , scan speed of 6000 mm/min and (a) 40 and (b) 20 number of passes. A close comparison between Fig. 6a with Fig. 6b reveals that angle of bending increases with increasing the number of passes (influence of number of passes on bending angle is also evident from Fig. 3). Moreover, laser irradiation leads to melting at the laser–matter interaction zone and depth of melting at the irradiated zone increases with increase in number of passes. Similar effect of applied power density and scan speed on bending angle was also observed and the results have been summarized in Figs. 2 and 4, respectively. Repeated irradiation with laser though increases the angle of bending, however, increases the depth of melting of the laser–matter interaction region. A detailed microstructural analysis of the different regions of the bend surface and its variation with laser parameters were undertaken to. Fig. 7a–d show the microstructure of the (a) irradiated region, i.e. inner side of bending (inner bent zone, IBZ) (b) solid–liquid interface (c) heat affected zone and (d) reverse side of the irradiated zone, i.e. outer side of bending (outer bend zone, OBZ) of laser bend AISI 304 stainless steel (of sheet thickness of 0.9 mm) lased with a power density of 19.6 × 103 W/cm2 , scan speed of 4000 mm/min and 10 number of passes. From Fig. 7a it is revealed that laser irradiation of the inner surface of the bend specimen causes melting of the near surface region and subsequently, a very high rate of quenching, resulting in formation of very fine-grained (grain size varying from 1.5 m to 2.5 m) microstructure (cf. Fig. 7a). The microstructure is very fine and equiaxed at the near-surface region. Refinement of microstructure achieved in laser bending operation is beneficial in increasing the strength without sacrificing the ductility of the inner side of the laser bent zone. Though there is melting at the irradiated region, subsequent rapid solidification leads to formation of a defect-free and continuous solid–liquid interface (cf. Fig. 7b). From Fig. 7b it is also relevant that the microstructure of the solid–liquid interface mainly consists of very fine dendrites (with primary arm spacing of 0.5 m) growing from the solid–liquid interface. Immediately after the solid–liquid interface, grain coarsening followed for a very shallow region, mainly due to the heating effect from the top surface, termed as heat affected zone. Fig. 7c shows the microstructure of the heat-affected zone of the laser bend AISI 304 stainless steel consisting of coarse grains with size varying from 20 m to 40 m. A close comparison of Fig. 7c with Fig. 5 reveals the grain coarsening effect at the heat-affected zone. The outer surface of the bent zone was deformed due to thermal stress generated because of a large thermal gradient associated with laser irradiation. However, no significant change in grain size was observed in the outer surface of the bent zone as compared to the as-received microstructure (Fig. 7d vis-à-vis Fig. 5). In this regard, it may be noted that though, the microstructure of the melt zone and the solid–liquid interface
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Fig. 8. Scanning electron micrograph of the (a) melt zone and (b) solid–liquid interface of laser bent AISI 304 stainless steel (of sheet thickness of 0.9 mm) lased with a power density of 19.6 × 103 W/cm2 , scan speed of 4000 mm/min and 40 number of passes.
Fig. 7. Scanning electron micrograph of the (a) irradiated region, i.e. IBZ (b) solid–liquid interface (c) heat affected zone and (d) reverse side of the irradiated zone, i.e. OBZ of laser bend AISI 304 stainless steel (of sheet thickness of 0.9 mm) lased with a power density of 19.6 × 103 W/cm2 , scan speed of 4000 mm/min and 10 number of passes.
was found to vary with laser parameters, the same for heat affected zone and outer surface of the bent sample was found unchanged. Fig. 8a and b show the microstructure of the (a) melt zone and (b) solid–liquid interface of laser bent AISI 304 stain-
less steel (of sheet thickness of 0.9 mm) lased with a power density of 19.6 × 103 W/cm2 , scan speed of 4000 mm/min and 40 number of passes. The irradiated zone microstructure consists of very fine columnar dendrites (with a primary arm spacing of 0.5 m) growing in different directions corresponding to the direction of heat flow. A continuous and defect free solid–liquid interface is formed due to laser irradiation (Fig. 8b). In this case, near to the interface, presence of delta ferrite was found to be present along with the presence of austenite (as shown by the arrowhead in Fig. 8b and was confirmed by X-ray diffraction analysis). Increased cooling rate hinders complete transformation of delta ferrite to austenite, and hence, untransformed delta ferrite is present at the solid–liquid interface. Immediately below the solid–liquid interface, presence of carbide precipitates were observed for a very narrow zone for only a few samples lased under unfavorable conditions of lasing. Fig. 9a–c show the microstructure of the irradiated zone formed in laser bent AISI 304 stainless steel (of sheet thickness of 0.9 mm) lased with a power density of (a) 54.3 × 103 W/cm2 , scan speed of 4000 mm/min (b) 54.3 × 103 W/cm2 , scan speed of 6000 mm/min and (c) 19.6 ×
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Fig. 10. Scanning electron micrograph of the irradiated zone formed in laser bent AISI 304 stainless steel (of sheet thickness of 0.9 mm with a power density of 13.4 × 105 W/cm2 , scan speed of 1500 mm/min) and (a) 10 (b) 40 number of passes.
Fig. 9. Scanning electron micrograph of the irradiated zone formed in laser bent AISI 304 stainless steel (of sheet thickness of 0.9 mm) lased with a power density of (a) 54.3 × 103 W/cm2 , scan speed of 4000 mm/min (b) 54.3 × 103 W/cm2 , scan speed of 6000 mm/min and (c) 19.6 × 103 W/cm2 , scan speed of 4000 mm/min with 40 number of passes.
103 W/cm2 , scan speed of 4000 mm/min with 40 number of passes. The microstructure of melt zone consists of solidified dendrites with almost uniform arm size (6 m–9 m) (cf. Fig. 9a). Increase in scan speed was found to refine the microstructure marginally (with a primary arm spacing of 4 m–6 m) probably due to increased cooling rate associated with increase in scan speed (cf. Fig. 9b). On the other hand, decrease in applied power density refines the mi-
crostructure of the melt zone further (cf. Fig. 9c). Fig. 10a and b show the microstructure of the irradiated zone formed in laser bent AISI 304 stainless steel (of sheet thickness of 0.9 mm) with a power density of 13.4 × 105 W/cm2 , scan speed of 1500 mm/min and (a) 10 (b) 40 number of passes. Application of a very low power, though did not cause melting however, led to deformation due to thermal stress and hence, elongated grains were observed due to material flow along the bent direction (cf. Fig. 10a). Increase in number of passes, causes repeated melting and solidification of the irradiated region, resulting in refinement of grains and formation of sub-grains within the grains attributed to recrystallization of the deformed zone. Hence, it may be concluded that the microstructure of the bent zone to vary with laser parameters, and laser parameters should be carefully chosen to optimize the processing zone for laser bending of AISI 304 stainless steel. 3.3. Phase analysis Fig. 11a–c show the X-ray diffraction profile of (a) as-received (b) irradiated surface of laser bent AISI 304 stainless steel (of sheet thickness of 0.9 mm) lased with a
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steel reveals the presence of only ␥-iron. The X-ray diffraction profile of the irradiated surface of laser bent stainless steel lased with 13.4 × 103 W/cm2 and scan speed of 5000 mm/min reveals that there is no significant change in phase however, broadening of peaks attributed to the lattice strain introduced during laser bending (cf. Fig. 11b). Similar effects were observed for the samples laser bent using low power and high scan speed. On the other hand, X-ray diffraction profile of the irradiated surface lased with a power of 54.3 × 103 W/cm2 and scan speed of 4000 mm/min shows the presence of ␣-Fe, ␦-Fe and Fe3 C in addition to ␥-Fe (cf. Fig. 11c). Application of high power and low scan speed rapid quenching followed by melting of the irradiated region and hence, retention of ␦-Fe in the microstructure. 3.4. Microhardness distribution
Fig. 11. X-ray diffraction profile of (a) as-received and irradiated surface of laser bent AISI 304 stainless steel (of sheet thickness of 0.9 mm) lased with a (b) power of 13.4 × 103 W/cm2 W/cm2 and scan speed of 5000 mm/min and (c) power of 54.3 × 103 W/cm2 and scan speed of 4000 mm/min, respectively.
power of 13.4 × 103 W/cm2 and scan speed of 5000 mm/min and (c) irradiated surface of laser bent AISI 304 stainless steel (of sheet thickness of 0.9 mm) lased with a power of 54.3 × 103 W/cm2 and scan speed of 4000 mm/min. The X-ray diffraction profile of as-received AISI 304 stainless
A detailed study of the variation of microhardness at different position of the bent zone and with laser parameters were undertaken to study the effect of laser parameters on the bending characteristics. Fig. 12 shows the schematic of hardness distribution at different regions (along the cross-section) of laser bent AISI 304 stainless steel samples. In this regard, it is relevant to mention that only the microstructure and microhardness of the centerline of the bending sheet (which is in direct contact with laser beam) is modified due to laser bending operation. The microhardness level shown in Fig. 12 is the average of 25–30 readings in each zone for samples lased at different conditions. The microhardness of the melt zone is significantly increased (to a value of 265 VHN) as compared to the base metal microhardness (190 VHN). For a few set of lasing conditions, there is formation of Cr23 C6 precipitates (confirmed by X-ray diffraction study) for a very thin layer near to the solid–liquid interface. This layer possesses a very high microhardness of 375 VHN. The heat affected zone extends for a very shallow depth with a very low microhardness (160 VHN) as compared to the substrate. Following heat affected zone, the microhardness increases due to formation of highly dislocated deformed region and reaches maximum at the extremely other side of the bent zone (to 220 VHN). The correspond-
Fig. 12. Schematic of hardness distribution at different regions (along the cross-section) of laser bent AISI 304 stainless steel samples.
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Fig. 13. Distribution of microhardness with length at the different region of laser bent AISI 304 stainless steel (sheet thickness of 0.9 mm, lased with a power of 54.3 × 103 W/cm2 , scan speed of 5000 mm/min).
ing distribution of microhardness with length at the different regions of a laser bent steel (sheet thickness of 0.9 mm, lased with a power of 54.3 × 103 W/cm2 , scan speed of 5000 mm/min) is graphically shown in Fig. 13. It is revealed that microhardness increases significantly (to 250 VHN as compared to 190 VHN of the substrate one) at the inner bent zone which is irradiated by laser (i.e. melt zone in Fig. 12). Processing at a higher number of passes increases the microhardness (275 VHN at 40 number of passes). The sudden rise in hardness immediately after the melt zone is attributed to the formation of Cr23 C6 precipitates along the zone near to the solid–liquid interface in Fig. 12. Subsequent grain coarsening in the heat affected zone reduces the hardness considerably resulting in a sudden drop in microhardness in Fig. 13. On the other hand, microhardness of the outer bent zone (reverse side of the irradiated zone) is marginally increased due to working effect (as also evident from Fig. 12). The hardness of melt zone and the outer zone along the centerline of the bent sheet was however, found to vary with laser parameters. Increase in number of passes increases the microhardness of the irradiated zone (as evident from Fig. 13). Increasing microhardness of the melt zone with increase in number of passes is attributed to refinement of microstructure with increasing number of passes (Fig. 8a vis-à-vis Fig. 7a). On the other hand, the hardness of the outer bent region also increases with increased number of passes mainly because of a larger degree of deformation induced at an increased thermal stress developed due to repeated irradiation. Hence, laser bending is a unique technique of bending of sheet metals with an improved mechanical property at the centerline of laser bending. Fig. 14 shows the effect of scan speed on the hardness of inner bent zone (IBZ), outer bent zone (OBZ) and heat affected zone (HAZ) of the laser bent (lased with a power of 54.3 × 103 W/cm2 ) AISI 304 stainless steel of sheet thickness 0.9 mm. From Fig. 14 it is relevant that the microhardness of the inner bent zone increases with increase in scan speed. Increasing scan speed decreases laser–material interaction time and hence,
Fig. 14. Variation of the microhardness of inner bent zone (IBZ), outer bent zone (OBZ) and heat affected zone (HAZ) with scan speed for laser bent (lased with a power of 54.3 × 103 W/cm2 ) AISI 304 stainless steel of sheet thickness 0.9 mm.
reduces the time gap between two successive passes. Repeated melting of the top surface causes formation of coarse dendrites with presence of delta-ferrite. Presence of delta ferrite increases the microhardness of the melt zone. On the other hand, the microhardness of the heat affected zone does not vary with scan speed. Improved microhardness of the outer surface is because of strain hardening due to material flow during bending. Higher the scan speed, lower the angle of bending and hence, lower the stress generated. As a result of which the microhardness of the outer bent zone decreases with increase in scan speed. Similar effect of the effect of power density on hardness of outer zone was also noticed.
4. Process optimization diagram From the above mentioned discussions, it is clear that laser parameters (laser power, scan speed, number of passes) play a very crucial role in determining bending characteristics i.e. bending angle, microstructure and microhardness of the bend zone. Though bending angle was found to increase with increase in applied power density, decrease in scan speed and increase in number of passes, proper selection of laser parameters is essential to avoid excessive melting, formation of chromium carbide and grain coarsening of the bend zone. Moreover, inadequate supply of energy does not cause bending. Fig. 15a and b show the processing zone for laser bending of AISI 304 stainless steel (with respect to laser power density and scan speed) with a sheet thickness of (a) 1.6 mm and (b) 0.9 mm for a number of passes of 10. A close comparison between Fig. 15a with Fig. 15b shows that for a particular value of power density, range of scan speed for laser bending is significantly higher for laser bending of 0.9 mm sheet than that of 1.6 mm. The process windows for bending have been divided into different shaded regions with identical properties and the characteristics, which are
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Fig. 15. Processing windows for laser bending of AISI 304 stainless steel (with respect to laser power density and scan speed) with a sheet thickness of (a) 1.6 mm and (b) 0.9 mm for 10 number of passes.
Fig. 16. Processing windows for laser bending of AISI 304 stainless steel (with respect to laser power density and scan speed) with a sheet thickness of (a) 1.6 mm and (b) 0.9 mm for 40 number of passes.
summarized in Table 2. From Fig. 15 and Table 2 it is relevant that bending in zone I (low power, medium scan speed) causes very low bending angle with no melting. Microstructure of the bent zone is austenitic with increase in hardness of the inner bent zone. Processing in zone II causes melting of the inner bent zone, the microstructure however, remains austenitic with substantial improvement in microhardness of the IBZ. Application of a very high power (zone III) causes melting, formation of ␦-ferrite and Fe3 C at the surface and Cr23 C6 at the solid–liquid interface, reducing the toughness of the zone. With increasing the number of passes, angle of bending increases under comparable conditions of lasing. Furthermore, near-irradiated zone melting is unavoidable even at a very low laser power and high scan speed combinations. However, intermediate combinations of laser power density and scan speed leads to the formation of refined microstructure with improved microhardness at the IBZ. In
contrast, increase in laser power causes excessive melting and grain coarsening effect and hence, reduced hardness level. Fig. 16a and b show the processing zone for laser bending of AISI 304 stainless steel (with respect to laser power density and scan speed) with a sheet thickness of (a) 1.6 mm and (b) 0.9 mm for a number of passes of 40. Accordingly, the overall domain of laser assisted bending of AISI 304 stainless steel has been divided into two distinct zone and the microstructure and microhardness distribution in different zones are summarized in Table 3. From Fig. 16 and Table 3 it is relevant that bending in zone I produces a very low bending angle, however, causes melting of the IBZ. Microstructure of the bent zone processed in zone I is austenitic with substantial improvement in microhardness. On the other hand, processing with a higher power (zone II) causes melting of the inner bent zone, formation of a few ␦-ferrite and Cr23 C6 at the solid–liquid interface. However, the microstructure and microhardness of the outer bent zone
Table 2 Characteristics and properties of the shaded regions mentioned in Fig. 15 Sheet thickness (mm)
Position
0.9
Zone I Zone II Zone III
1.6
Zone I Zone II Zone III
Bend angle (◦ )
Microhardness
Phases
2–5 4–9 8–15
240 260 260–350
Austenite Austenite Austenite, ␦-ferrite and a few chromium carbide
2–10 10–40 35–60
240 250–280 240–350
Austenite Austenite Austenite, ␦-ferrite and a few chromium carbide
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Table 3 Characteristics and properties of the shaded regions mentioned in Fig. 16 Sheet thickness (mm)
Position
Bend angle (◦ )
Microhardness
Phases
0.9
Zone I Zone II
5–20 10–30
240 260
Austenite Austenite, ␦-ferrite and a few chromium carbide
1.6
Zone I Zone II
10–35 30–60
240 250–280
Austenite Austenite, ␦-ferrite and a few chromium carbide
does not vary significantly with laser parameters and hence, are not shown in Tables 2 and 3.
region was not found to vary significantly with laser parameters.
5. Summary and conclusions
Acknowledgements
In the present investigation, laser assisted bending of AISI 304 stainless steel of with two different sheet thickness (0.9 mm and 1.6 mm, respectively) has been conducted with a 2 kW continuous wave CO2 laser. From the detailed investigation the following conclusions may be drawn:
A major part of the work was carried out under the FAST TRACK PROJECT SCHEME, DST (SR/FTP/ET-70/2000), New Delhi. Partial financial support from the ISIRD (IIT, Kharagpur), CSIR (22 ( 0356)/02/EMR-II), N. Delhi, BRNS (LPTD/LMG/BRNS/03-04/76/16-1), Bombay and from the DST (SP/S2/K-17/98), New Delhi are gratefully acknowledged.
a. Bending has been achieved with the following laser parameters: power density = 15–55 kW/cm2 ; scan speed = 1000 mm/min–2500 mm/min (for a sheet thickness of 1.6 mm) and power density = 15–55 KW/cm2 ; scan speed = 4000 mm/min–6000 mm/min and 5–40 number of passes (for a sheet thickness of 0.9 mm). b. Bending angle was found to vary from 2◦ –20◦ (for a sheet thickness of 1.6 mm) and 10◦ –60◦ (for a sheet thickness of 0.9 mm), and increased with increase in laser power, decrease in scan speed and increase in number of passes. c. The microstructure of the laser irradiated zone of bending is significantly refined due to surface melting of a thin layer, followed by a very thin heat affected coarse grained zone and finally banded structure at the outer part of bending. d. The microhardness of the bent region varies from that of the substrate, attributed to the heating effect with laser. At the irradiated region the microhardness is maximum due to grain refinement, followed by which the microhardness decreases due to grain coarsening. The outer surface microhardness is higher than that of the substrate due to working effect. e. The processing zone for laser bending of AISI 304 stainless steel (in terms of laser power and scan speed) with sheet thickness and number of passes has been defined. Application of low power causes very low bending with moderate increase in microhardness value. On the other hand, application of a very high power leads to formation of ␦-ferrite, Fe3 C and Cr23 C6 precipitates which are not desirable. However, moderate power and scan speed causes formation of superior microstructure with improved microhardness in the laser irradiated region. The microstructure and microhardness of the outer bent
References [1] K.C. Chan, J. Liang, Comp. Sci. Technol. 61 (2001) 1265. [2] K.C. Chan, C.L. Yau, W.B. Lee, J. Laser Appl. 12 (2000) 34. [3] K. Masubuchi, W.H. Luebke, G.N. Haidemenopoulis (Eds.), in: Proceedings of the Sixth American Welding Society Convention, New Orleans, LA, ASM, Metals Park, OH, 1988, p. 348. [4] F. Vollersten, S. Holzer, VDI-Z 136 (1994) 35. [5] M. van-den-Burg, J.T.M. De-Hosson, Interface Sci. 3 (1995) 107. [6] Y. Namba (Ed.), Proceedings of Laser Advanced Materials Processing (LAMP’87), Osaka, Japan, 1987, p. 601. [7] F. Vollersten, Laser Optoelektronik 31 (1995) 112. [8] F. Vollersten, M. Rodle, in: M. Geiger, F. Vollersten (Eds.), Laser Assisted Net Shape Engineering, Meisenbach, Bamberg, 1994, p. 371. [9] M. Geiger, F. Vollersten, Ann. CIRP 42 (1993) 301. [10] S. Holzer, H. Arnet, M. Geiger, in: M. Geiger, F. Vollersten (Eds.), Laser Assisted Net Shape Engineering, Meisenbach, Bamberg, 1994, p. 379. [11] F. Vollersten, Laser Eng. 2 (1994) 261. [12] F. Vollersten, M. Geiger, W.M. Li, in: Z.R. Wang, Y. He (Eds.), Advanced Technology of Plasticity III, Prentice Hall, New York, 1993, p. 1793. [13] T. Hennige, S. Holzer, F. Vollersten, M. Geiger, J. Mater. Proc. Tech. 71 (1998) 422. [14] J. Magee, K.G. Watkins, W.M. Steen, N.J. Calder, J. Sidhu, J. Kirby, J. Laser App. 10 (1998) 149. [15] A. Sprenger, F. Vollersten, W.M. Steen, K.G. Watkins, in: M. Geiger, F. Vollersten (Eds.), Laser Assisted Net Shape Engineering, Meisenbach, Bamberg, 1994, p. 361. [16] W. Li, Y.L. Yao, in: Proceedings of the International Conference of Lasers and Electro-Optics (ICALEO ’99), San Diego, CA, 1999, p. 102. [17] K.C. Chan, J. Liang, J. Mater. Proc. Technol. 100 (2000) 214. [18] P.J. Cheng, S.C. Lin, J. Mater. Proc. Technol. 108 (2001) 314.
Studies on laser bending of stainless steel J. Dutta Majumdar a,∗ , A.K. Nath b , I. Manna a a
Department of Metals & Materials Engineering, Indian Institute of Technology, Kharagpur 721302, India b Industrial CO Laser Centre, Centre for Advanced Technology, Indore 452013, India 2 Received 29 January 2004; received in revised form 25 May 2004
Abstract In the present study, laser bending of AISI 304 stainless steel sheet has been attempted with a high power (2 kW) continuous wave CO2 laser. Bending angle was measured as a function of laser/processing parameters including power density, scan speed, number of passes and sheet thickness. Following laser bending, microstructural evolution (using a scanning electron microscope) and phase analysis (by X-ray diffraction technique) were systematically carried out to study the effect of laser irradiation and thermal stress on the microstructure and phase transformation behavior of the sheet. Microhardness of the bent sheet at different position was carefully measured using a Vickers microhardness tester. Bending angle was found to vary from 0.5◦ to 70◦ under different processing conditions. The microhardness of the bend zone was found to increase (from 1.5 to 2 times) as compared to the as-received sample. The improved microhardness is attributed to grain refinement associated with rapid quenching during laser bending. Finally, the optimum processing zone for laser bending of stainless steel was derived. © 2004 Elsevier B.V. All rights reserved. Keywords: Laser; Bending; Stainless steel; Microhardness
1. Introduction Laser bending is a newly developed technique of modifying the curvature of sheet metal by thermal residual stresses generated by laser assisted heating without any externally applied mechanical forces [1–5]. Laser bending may also serve the purpose of straightening thin sheets by a similar laser-based non-contact process without any mechanical forces. The process assumes significance due to the ease and flexibility of non-contact processing, amenability to materials with diverse shape/geometry, properties and chemistry, and high precision/productivity. Laser bending involves a complex interplay between the thermal profile generated by the laser irradiation and physical/thermal properties and dimension of the material/work-piece. In general, the process is influenced by many parameters such as laser parameters (power density and interaction/pulse time), material properties (thermal conductivity, coefficient of thermal expansion, etc.) and target dimensions (thickness, curvature, etc.). In laser bending, bending is induced by a localized laser generated temperature gradient between the irradiated surface and the neighboring material. The process is briefly de∗
Corresponding author. Fax: +91 3222 282280. E-mail address: [email protected] (J.D. Majumdar).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.06.009
scribed by Namba [6]. The temperature distribution forces the material to expand non-uniformly, which in turn leads to non-uniform thermal stresses. Plastic deformation starts when the thermal stress exceeds yield stress. There are three different mechanism of laser bending; temperature gradient, buckling and upsetting mechanism. The mechanism of laser bending are briefly described by Vollersten [7], Vollersten et al. [8] and Geiger and Vollersten [9]. Recently, a number of numerical [10], analytical [11] and finite element models [12] have been developed to analyze various kinds of laser bending processes. Hunninge et al. [13] and Magee et al. [14] have studied the influence of laser parameters on dimensional accuracy of bent part. Sprenger et al. [15] and Li and Yao [16] studied the effect of externally applied stress and strain on the quality of bent product. Deformation behavior of Al-based metal matrix composite (Al6013/SiC) was reported by Chan and Liang [17]. In this regard, it is relevant to mention that most of the literature concerns the understanding of laser bending and the influence of process parameters on the bending angle. However, a detailed study of the microstructures and properties of bent zone and the influence of laser parameters on it needs due consideration to optimize the process parameters of laser bending. In the present study, laser bending of AISI 304 stainless steel sheet has been attempted using a high power (2 kW)
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continuous wave CO2 laser with a 3 mm beam diameter. The main process variables were incident power density, scan speed, number of passes and sheet thickness. After bending operation, a detailed characterization of the bent sheet was carried out by measuring the bend angle, detailed microstructural study and phase analysis. The microhardness of the bent sheet at different position were carefully measured using a Vickers microhardness tester. Bending angle, microstructure and mechanical properties of the bent sheet were correlated with laser parameters and sheet thickness to derive an optimum processing zone for laser bending.
Table 1 Summary of process parameters employed in laser bending of AISI 304 stainless steel Sheet thickness (mm)
Power density (kW/cm2 )
Scan speed (mm/min)
Number of passes
0.9 1.6
13–55 13–55
4000–6000 1000–2500
5–40 5–40
the top surface and the cross-section) was measured by a Vickers microhardness tester using a 300 g applied load. Finally, bend angle, microstructure, and microhardness of the bent zone was correlated with laser parameters to derive the processing window for laser bending.
2. Experimental In the present study, thin sheet (10 cm × 1 cm) of cold rolled AISI 304 stainless steel (of two different thickness) was used as samples for bending. The thickness of the specimen was 1.6 mm and 0.9 mm, respectively. Laser bending was carried out using a 2 kW continuous wave (CW) CO2 laser with Ar as shrouding environment to avoid oxidation during lasing. Fig. 1 shows the schematic of laser bending setup. One end of the sheet was clamped in a fixture and the assembly was mounted on a CNC controlled stage that was moved at a speed of 100 mm/min–8000 mm/min. Laser focus rested 30 mm above the sample and was traversed along the centerline of the sheet from one end to other across the width. The process variables for the present study were incident laser power density, scan speed and number of passes. Adequate numbers of trials were conducted to correlate the bend zone characteristics with laser parameters (Table 1). The bend samples were digitised using a CCD camera and measured using a image processing software to an accuracy of 0.5◦ . The microstructure of the bent zone (both the laser irradiated zone and the outer surface along depth) and the heat affected zone (HAZ) were characterized by optical microscope (OM) and scanning electron microscope (SEM). Phase analysis of the bent zone was carried out using X-ray diffraction technique with Co K␣ radiation to observe if there is any phase transformation due to laser irradiation and thermal stress. Microhardness of the bent zone (both
Fig. 1. Schematic of experimental setup for laser bending of AISI 304 stainless steel using a continuous wave CO2 laser.
3. Results and discussion 3.1. Bending angle The extent of bending is determined by bending angle. It is essential to know the effect of laser/process parameters on the bending angle to optimize the process parameters. In the present study, bending angle formed under different processing conditions was measured and correlated with laser/process parameters. Fig. 2 shows the effect of scan speed (ν) on the angle of bending for laser bent AISI 304 stainless steel sheet of thickness (t) (a) 1.6 mm and (b) 0.9 mm, respectively. The angle of bending was found to
Fig. 2. Variation of angle of bending with scan speed in laser bent AISI 304 stainless steel sheet with a thickness of (a) 1.6 mm and (b) 0.9 mm, respectively.
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vary inversely with scan speed for both the sheet thickness. The decreased bending angle with increase in scan speed is attributed to decreased absorbed energy with increasing scan speed (because of a lower interaction time at a higher scan speed), leading to a lower thermal stress and hence, a lower bending angle. In this regard, it is relevant to mention that though bending angle increases with decrease in scan speed, application of a very low scan speed leads to evaporation of material from the surface and hence, crater formation. Hence, the range of scan speed shown in Fig. 2 was capable of formation of a defect free bent zone and was used for bending of stainless steel. The effect of number of passes (n) on the angle of bending is also shown in Fig. 2. Careful observation of Fig. 2a and b shows that bend angle increases with increase in number of passes for both the sheet thickness. A detailed investigation of the effect of scan speed on the angle of bending revealed that bending of AISI 304 stainless steel with a 1.6 mm sheet thickness was achieved at a higher interaction time (lower scan speed) and the range of scan speed used for bending without the adverse effect of surface evaporation was 500 mm/min–3000 mm/min, which was found to be significantly lower than the same with a sheet thickness of 0.9 mm (3500 mm/min–7500 mm/min). Furthermore, a higher angle of bending (1◦ –75◦ ) was achieved in case of stainless steel with 0.9 mm sheet thickness as compared to the same for 1.6 mm sheet thickness (1◦ –30◦ ). Lower sheet thickness required a comparatively lower stress for bending and easier to bend than vice versa. Hence, it may be concluded that sheet thickness also plays a role in determining the bending limit and choosing the laser parameters for bending. Fig. 3a and b summarize the variation of bending angle with number of passes. With each pass thermal stress, proportional to thermal gradient, is introduced and hence, bending angle also increases with increasing number of pass. Moreover, following bending after a single pass, there is reduction of material at the bent zone during successive passes due to material flow outside the bent region after each pass. The increase in bending angle with number of passes was however, found to depend on the introduced microstructure by the previous pass. Increase in bend angle with increasing laser power density, P (cf. Fig. 4) is attributed to increased material flow at a higher absorbed energy density. Rate of increase in bend angle with power is however, much lower for 1.6 mm sheet thickness than the same for 0.9 mm sheet thickness. Furthermore, a larger thermal stress is required for bending of the sheet with a higher thickness (1.6 mm) than the same with a lower thickness. Cheng and Lin [18] developed an analytical model to calculate thermal stress required for bending which evidences the increased level of required thermal stress with increasing sheet thickness. As a result of which, a very low bending angle is achieved for 1.6 mm sheet thickness than the same with 0.9 mm sheet thickness. It is relevant to note that though the bend angle increases with increase in power density, application of a very large power density causes excessive melting and evaporation of
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Fig. 3. Variation of angle of bending with number of passes for laser bent AISI 304 stainless steel sheet with a thickness of (a) 1.6 mm and (b) 0.9 mm, respectively.
surface material at the inner side (laser–material interaction site) of the bent region. 3.2. Microstructural evolution Microstructure plays a very crucial role in determining the property of the bent region and hence, determining the optimum laser processing region. In the present study, a detailed investigation of the microstructure developed in laser bending processed under different conditions was under-
Fig. 4. Variation of angle of bending with applied power density for laser bent AISI 304 stainless steel.
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Fig. 5. Scanning electron micrograph of the as-received AISI 304 stainless steel.
taken and correlated with process parameters. Fig. 5 shows the microstructure of the as-received AISI 304 stainless steel sheet consisting of single phase austenitic grains (size varying from 10 m to 20 m). Laser bending was conducted by introducing thermal stress produced by irradiation with a high power laser beam. Fig. 6a and b show the scanning
Fig. 6. Scanning electron micrograph of laser bent AISI 304 stainless steel (of sheet thickness of 0.9 mm) lased with a power density of 19.6 × 103 W/cm2 , scan speed of 6000 mm/min and (a) 40 and (b) 20 number of passes.
electron micrographs (low magnification view) of laser bent AISI 304 stainless steel (of sheet thickness of 0.9 mm) lased with a power density of 19.6 × 103 W/cm2 , scan speed of 6000 mm/min and (a) 40 and (b) 20 number of passes. A close comparison between Fig. 6a with Fig. 6b reveals that angle of bending increases with increasing the number of passes (influence of number of passes on bending angle is also evident from Fig. 3). Moreover, laser irradiation leads to melting at the laser–matter interaction zone and depth of melting at the irradiated zone increases with increase in number of passes. Similar effect of applied power density and scan speed on bending angle was also observed and the results have been summarized in Figs. 2 and 4, respectively. Repeated irradiation with laser though increases the angle of bending, however, increases the depth of melting of the laser–matter interaction region. A detailed microstructural analysis of the different regions of the bend surface and its variation with laser parameters were undertaken to. Fig. 7a–d show the microstructure of the (a) irradiated region, i.e. inner side of bending (inner bent zone, IBZ) (b) solid–liquid interface (c) heat affected zone and (d) reverse side of the irradiated zone, i.e. outer side of bending (outer bend zone, OBZ) of laser bend AISI 304 stainless steel (of sheet thickness of 0.9 mm) lased with a power density of 19.6 × 103 W/cm2 , scan speed of 4000 mm/min and 10 number of passes. From Fig. 7a it is revealed that laser irradiation of the inner surface of the bend specimen causes melting of the near surface region and subsequently, a very high rate of quenching, resulting in formation of very fine-grained (grain size varying from 1.5 m to 2.5 m) microstructure (cf. Fig. 7a). The microstructure is very fine and equiaxed at the near-surface region. Refinement of microstructure achieved in laser bending operation is beneficial in increasing the strength without sacrificing the ductility of the inner side of the laser bent zone. Though there is melting at the irradiated region, subsequent rapid solidification leads to formation of a defect-free and continuous solid–liquid interface (cf. Fig. 7b). From Fig. 7b it is also relevant that the microstructure of the solid–liquid interface mainly consists of very fine dendrites (with primary arm spacing of 0.5 m) growing from the solid–liquid interface. Immediately after the solid–liquid interface, grain coarsening followed for a very shallow region, mainly due to the heating effect from the top surface, termed as heat affected zone. Fig. 7c shows the microstructure of the heat-affected zone of the laser bend AISI 304 stainless steel consisting of coarse grains with size varying from 20 m to 40 m. A close comparison of Fig. 7c with Fig. 5 reveals the grain coarsening effect at the heat-affected zone. The outer surface of the bent zone was deformed due to thermal stress generated because of a large thermal gradient associated with laser irradiation. However, no significant change in grain size was observed in the outer surface of the bent zone as compared to the as-received microstructure (Fig. 7d vis-à-vis Fig. 5). In this regard, it may be noted that though, the microstructure of the melt zone and the solid–liquid interface
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Fig. 8. Scanning electron micrograph of the (a) melt zone and (b) solid–liquid interface of laser bent AISI 304 stainless steel (of sheet thickness of 0.9 mm) lased with a power density of 19.6 × 103 W/cm2 , scan speed of 4000 mm/min and 40 number of passes.
Fig. 7. Scanning electron micrograph of the (a) irradiated region, i.e. IBZ (b) solid–liquid interface (c) heat affected zone and (d) reverse side of the irradiated zone, i.e. OBZ of laser bend AISI 304 stainless steel (of sheet thickness of 0.9 mm) lased with a power density of 19.6 × 103 W/cm2 , scan speed of 4000 mm/min and 10 number of passes.
was found to vary with laser parameters, the same for heat affected zone and outer surface of the bent sample was found unchanged. Fig. 8a and b show the microstructure of the (a) melt zone and (b) solid–liquid interface of laser bent AISI 304 stain-
less steel (of sheet thickness of 0.9 mm) lased with a power density of 19.6 × 103 W/cm2 , scan speed of 4000 mm/min and 40 number of passes. The irradiated zone microstructure consists of very fine columnar dendrites (with a primary arm spacing of 0.5 m) growing in different directions corresponding to the direction of heat flow. A continuous and defect free solid–liquid interface is formed due to laser irradiation (Fig. 8b). In this case, near to the interface, presence of delta ferrite was found to be present along with the presence of austenite (as shown by the arrowhead in Fig. 8b and was confirmed by X-ray diffraction analysis). Increased cooling rate hinders complete transformation of delta ferrite to austenite, and hence, untransformed delta ferrite is present at the solid–liquid interface. Immediately below the solid–liquid interface, presence of carbide precipitates were observed for a very narrow zone for only a few samples lased under unfavorable conditions of lasing. Fig. 9a–c show the microstructure of the irradiated zone formed in laser bent AISI 304 stainless steel (of sheet thickness of 0.9 mm) lased with a power density of (a) 54.3 × 103 W/cm2 , scan speed of 4000 mm/min (b) 54.3 × 103 W/cm2 , scan speed of 6000 mm/min and (c) 19.6 ×
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Fig. 10. Scanning electron micrograph of the irradiated zone formed in laser bent AISI 304 stainless steel (of sheet thickness of 0.9 mm with a power density of 13.4 × 105 W/cm2 , scan speed of 1500 mm/min) and (a) 10 (b) 40 number of passes.
Fig. 9. Scanning electron micrograph of the irradiated zone formed in laser bent AISI 304 stainless steel (of sheet thickness of 0.9 mm) lased with a power density of (a) 54.3 × 103 W/cm2 , scan speed of 4000 mm/min (b) 54.3 × 103 W/cm2 , scan speed of 6000 mm/min and (c) 19.6 × 103 W/cm2 , scan speed of 4000 mm/min with 40 number of passes.
103 W/cm2 , scan speed of 4000 mm/min with 40 number of passes. The microstructure of melt zone consists of solidified dendrites with almost uniform arm size (6 m–9 m) (cf. Fig. 9a). Increase in scan speed was found to refine the microstructure marginally (with a primary arm spacing of 4 m–6 m) probably due to increased cooling rate associated with increase in scan speed (cf. Fig. 9b). On the other hand, decrease in applied power density refines the mi-
crostructure of the melt zone further (cf. Fig. 9c). Fig. 10a and b show the microstructure of the irradiated zone formed in laser bent AISI 304 stainless steel (of sheet thickness of 0.9 mm) with a power density of 13.4 × 105 W/cm2 , scan speed of 1500 mm/min and (a) 10 (b) 40 number of passes. Application of a very low power, though did not cause melting however, led to deformation due to thermal stress and hence, elongated grains were observed due to material flow along the bent direction (cf. Fig. 10a). Increase in number of passes, causes repeated melting and solidification of the irradiated region, resulting in refinement of grains and formation of sub-grains within the grains attributed to recrystallization of the deformed zone. Hence, it may be concluded that the microstructure of the bent zone to vary with laser parameters, and laser parameters should be carefully chosen to optimize the processing zone for laser bending of AISI 304 stainless steel. 3.3. Phase analysis Fig. 11a–c show the X-ray diffraction profile of (a) as-received (b) irradiated surface of laser bent AISI 304 stainless steel (of sheet thickness of 0.9 mm) lased with a
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steel reveals the presence of only ␥-iron. The X-ray diffraction profile of the irradiated surface of laser bent stainless steel lased with 13.4 × 103 W/cm2 and scan speed of 5000 mm/min reveals that there is no significant change in phase however, broadening of peaks attributed to the lattice strain introduced during laser bending (cf. Fig. 11b). Similar effects were observed for the samples laser bent using low power and high scan speed. On the other hand, X-ray diffraction profile of the irradiated surface lased with a power of 54.3 × 103 W/cm2 and scan speed of 4000 mm/min shows the presence of ␣-Fe, ␦-Fe and Fe3 C in addition to ␥-Fe (cf. Fig. 11c). Application of high power and low scan speed rapid quenching followed by melting of the irradiated region and hence, retention of ␦-Fe in the microstructure. 3.4. Microhardness distribution
Fig. 11. X-ray diffraction profile of (a) as-received and irradiated surface of laser bent AISI 304 stainless steel (of sheet thickness of 0.9 mm) lased with a (b) power of 13.4 × 103 W/cm2 W/cm2 and scan speed of 5000 mm/min and (c) power of 54.3 × 103 W/cm2 and scan speed of 4000 mm/min, respectively.
power of 13.4 × 103 W/cm2 and scan speed of 5000 mm/min and (c) irradiated surface of laser bent AISI 304 stainless steel (of sheet thickness of 0.9 mm) lased with a power of 54.3 × 103 W/cm2 and scan speed of 4000 mm/min. The X-ray diffraction profile of as-received AISI 304 stainless
A detailed study of the variation of microhardness at different position of the bent zone and with laser parameters were undertaken to study the effect of laser parameters on the bending characteristics. Fig. 12 shows the schematic of hardness distribution at different regions (along the cross-section) of laser bent AISI 304 stainless steel samples. In this regard, it is relevant to mention that only the microstructure and microhardness of the centerline of the bending sheet (which is in direct contact with laser beam) is modified due to laser bending operation. The microhardness level shown in Fig. 12 is the average of 25–30 readings in each zone for samples lased at different conditions. The microhardness of the melt zone is significantly increased (to a value of 265 VHN) as compared to the base metal microhardness (190 VHN). For a few set of lasing conditions, there is formation of Cr23 C6 precipitates (confirmed by X-ray diffraction study) for a very thin layer near to the solid–liquid interface. This layer possesses a very high microhardness of 375 VHN. The heat affected zone extends for a very shallow depth with a very low microhardness (160 VHN) as compared to the substrate. Following heat affected zone, the microhardness increases due to formation of highly dislocated deformed region and reaches maximum at the extremely other side of the bent zone (to 220 VHN). The correspond-
Fig. 12. Schematic of hardness distribution at different regions (along the cross-section) of laser bent AISI 304 stainless steel samples.
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Fig. 13. Distribution of microhardness with length at the different region of laser bent AISI 304 stainless steel (sheet thickness of 0.9 mm, lased with a power of 54.3 × 103 W/cm2 , scan speed of 5000 mm/min).
ing distribution of microhardness with length at the different regions of a laser bent steel (sheet thickness of 0.9 mm, lased with a power of 54.3 × 103 W/cm2 , scan speed of 5000 mm/min) is graphically shown in Fig. 13. It is revealed that microhardness increases significantly (to 250 VHN as compared to 190 VHN of the substrate one) at the inner bent zone which is irradiated by laser (i.e. melt zone in Fig. 12). Processing at a higher number of passes increases the microhardness (275 VHN at 40 number of passes). The sudden rise in hardness immediately after the melt zone is attributed to the formation of Cr23 C6 precipitates along the zone near to the solid–liquid interface in Fig. 12. Subsequent grain coarsening in the heat affected zone reduces the hardness considerably resulting in a sudden drop in microhardness in Fig. 13. On the other hand, microhardness of the outer bent zone (reverse side of the irradiated zone) is marginally increased due to working effect (as also evident from Fig. 12). The hardness of melt zone and the outer zone along the centerline of the bent sheet was however, found to vary with laser parameters. Increase in number of passes increases the microhardness of the irradiated zone (as evident from Fig. 13). Increasing microhardness of the melt zone with increase in number of passes is attributed to refinement of microstructure with increasing number of passes (Fig. 8a vis-à-vis Fig. 7a). On the other hand, the hardness of the outer bent region also increases with increased number of passes mainly because of a larger degree of deformation induced at an increased thermal stress developed due to repeated irradiation. Hence, laser bending is a unique technique of bending of sheet metals with an improved mechanical property at the centerline of laser bending. Fig. 14 shows the effect of scan speed on the hardness of inner bent zone (IBZ), outer bent zone (OBZ) and heat affected zone (HAZ) of the laser bent (lased with a power of 54.3 × 103 W/cm2 ) AISI 304 stainless steel of sheet thickness 0.9 mm. From Fig. 14 it is relevant that the microhardness of the inner bent zone increases with increase in scan speed. Increasing scan speed decreases laser–material interaction time and hence,
Fig. 14. Variation of the microhardness of inner bent zone (IBZ), outer bent zone (OBZ) and heat affected zone (HAZ) with scan speed for laser bent (lased with a power of 54.3 × 103 W/cm2 ) AISI 304 stainless steel of sheet thickness 0.9 mm.
reduces the time gap between two successive passes. Repeated melting of the top surface causes formation of coarse dendrites with presence of delta-ferrite. Presence of delta ferrite increases the microhardness of the melt zone. On the other hand, the microhardness of the heat affected zone does not vary with scan speed. Improved microhardness of the outer surface is because of strain hardening due to material flow during bending. Higher the scan speed, lower the angle of bending and hence, lower the stress generated. As a result of which the microhardness of the outer bent zone decreases with increase in scan speed. Similar effect of the effect of power density on hardness of outer zone was also noticed.
4. Process optimization diagram From the above mentioned discussions, it is clear that laser parameters (laser power, scan speed, number of passes) play a very crucial role in determining bending characteristics i.e. bending angle, microstructure and microhardness of the bend zone. Though bending angle was found to increase with increase in applied power density, decrease in scan speed and increase in number of passes, proper selection of laser parameters is essential to avoid excessive melting, formation of chromium carbide and grain coarsening of the bend zone. Moreover, inadequate supply of energy does not cause bending. Fig. 15a and b show the processing zone for laser bending of AISI 304 stainless steel (with respect to laser power density and scan speed) with a sheet thickness of (a) 1.6 mm and (b) 0.9 mm for a number of passes of 10. A close comparison between Fig. 15a with Fig. 15b shows that for a particular value of power density, range of scan speed for laser bending is significantly higher for laser bending of 0.9 mm sheet than that of 1.6 mm. The process windows for bending have been divided into different shaded regions with identical properties and the characteristics, which are
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Fig. 15. Processing windows for laser bending of AISI 304 stainless steel (with respect to laser power density and scan speed) with a sheet thickness of (a) 1.6 mm and (b) 0.9 mm for 10 number of passes.
Fig. 16. Processing windows for laser bending of AISI 304 stainless steel (with respect to laser power density and scan speed) with a sheet thickness of (a) 1.6 mm and (b) 0.9 mm for 40 number of passes.
summarized in Table 2. From Fig. 15 and Table 2 it is relevant that bending in zone I (low power, medium scan speed) causes very low bending angle with no melting. Microstructure of the bent zone is austenitic with increase in hardness of the inner bent zone. Processing in zone II causes melting of the inner bent zone, the microstructure however, remains austenitic with substantial improvement in microhardness of the IBZ. Application of a very high power (zone III) causes melting, formation of ␦-ferrite and Fe3 C at the surface and Cr23 C6 at the solid–liquid interface, reducing the toughness of the zone. With increasing the number of passes, angle of bending increases under comparable conditions of lasing. Furthermore, near-irradiated zone melting is unavoidable even at a very low laser power and high scan speed combinations. However, intermediate combinations of laser power density and scan speed leads to the formation of refined microstructure with improved microhardness at the IBZ. In
contrast, increase in laser power causes excessive melting and grain coarsening effect and hence, reduced hardness level. Fig. 16a and b show the processing zone for laser bending of AISI 304 stainless steel (with respect to laser power density and scan speed) with a sheet thickness of (a) 1.6 mm and (b) 0.9 mm for a number of passes of 40. Accordingly, the overall domain of laser assisted bending of AISI 304 stainless steel has been divided into two distinct zone and the microstructure and microhardness distribution in different zones are summarized in Table 3. From Fig. 16 and Table 3 it is relevant that bending in zone I produces a very low bending angle, however, causes melting of the IBZ. Microstructure of the bent zone processed in zone I is austenitic with substantial improvement in microhardness. On the other hand, processing with a higher power (zone II) causes melting of the inner bent zone, formation of a few ␦-ferrite and Cr23 C6 at the solid–liquid interface. However, the microstructure and microhardness of the outer bent zone
Table 2 Characteristics and properties of the shaded regions mentioned in Fig. 15 Sheet thickness (mm)
Position
0.9
Zone I Zone II Zone III
1.6
Zone I Zone II Zone III
Bend angle (◦ )
Microhardness
Phases
2–5 4–9 8–15
240 260 260–350
Austenite Austenite Austenite, ␦-ferrite and a few chromium carbide
2–10 10–40 35–60
240 250–280 240–350
Austenite Austenite Austenite, ␦-ferrite and a few chromium carbide
122
J.D. Majumdar et al. / Materials Science and Engineering A 385 (2004) 113–122
Table 3 Characteristics and properties of the shaded regions mentioned in Fig. 16 Sheet thickness (mm)
Position
Bend angle (◦ )
Microhardness
Phases
0.9
Zone I Zone II
5–20 10–30
240 260
Austenite Austenite, ␦-ferrite and a few chromium carbide
1.6
Zone I Zone II
10–35 30–60
240 250–280
Austenite Austenite, ␦-ferrite and a few chromium carbide
does not vary significantly with laser parameters and hence, are not shown in Tables 2 and 3.
region was not found to vary significantly with laser parameters.
5. Summary and conclusions
Acknowledgements
In the present investigation, laser assisted bending of AISI 304 stainless steel of with two different sheet thickness (0.9 mm and 1.6 mm, respectively) has been conducted with a 2 kW continuous wave CO2 laser. From the detailed investigation the following conclusions may be drawn:
A major part of the work was carried out under the FAST TRACK PROJECT SCHEME, DST (SR/FTP/ET-70/2000), New Delhi. Partial financial support from the ISIRD (IIT, Kharagpur), CSIR (22 ( 0356)/02/EMR-II), N. Delhi, BRNS (LPTD/LMG/BRNS/03-04/76/16-1), Bombay and from the DST (SP/S2/K-17/98), New Delhi are gratefully acknowledged.
a. Bending has been achieved with the following laser parameters: power density = 15–55 kW/cm2 ; scan speed = 1000 mm/min–2500 mm/min (for a sheet thickness of 1.6 mm) and power density = 15–55 KW/cm2 ; scan speed = 4000 mm/min–6000 mm/min and 5–40 number of passes (for a sheet thickness of 0.9 mm). b. Bending angle was found to vary from 2◦ –20◦ (for a sheet thickness of 1.6 mm) and 10◦ –60◦ (for a sheet thickness of 0.9 mm), and increased with increase in laser power, decrease in scan speed and increase in number of passes. c. The microstructure of the laser irradiated zone of bending is significantly refined due to surface melting of a thin layer, followed by a very thin heat affected coarse grained zone and finally banded structure at the outer part of bending. d. The microhardness of the bent region varies from that of the substrate, attributed to the heating effect with laser. At the irradiated region the microhardness is maximum due to grain refinement, followed by which the microhardness decreases due to grain coarsening. The outer surface microhardness is higher than that of the substrate due to working effect. e. The processing zone for laser bending of AISI 304 stainless steel (in terms of laser power and scan speed) with sheet thickness and number of passes has been defined. Application of low power causes very low bending with moderate increase in microhardness value. On the other hand, application of a very high power leads to formation of ␦-ferrite, Fe3 C and Cr23 C6 precipitates which are not desirable. However, moderate power and scan speed causes formation of superior microstructure with improved microhardness in the laser irradiated region. The microstructure and microhardness of the outer bent
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