Laser Annealing of Cold Rolled Sheets: A Literature Review

Transcription

1 Delrapport 1 Laser Annealing of Cold Rolled Sheets: A Literature Review Jozefa Zajac, IM ACCRA Teknik AB AK-Konsult Amada/Promecam AB AvestaPolarit AB Bendiro AB Chalmers Tekniska Högskola -Institutionen för byggnadsmekanik Ferruform AB Kanthal AB IM Institutet för Metallforskning AB IVF Industriforskning och utveckling AB ORTIC AB PRESS & PLÅTINDUSTRI AB Scandinavian CAD AB SSAB Tunnplåt AB Volvo Personvagnar AB VINNOVA

2 LASER ANNEALING OF COLD ROLLED SHEETS A Literature Review Jozefa Zajac Swedish Institute for Metals Research Drottning Kristinas väg 48 S STOCKHOLM SWEDEN Keywords: Nd:YAG laser, surface treatment, laser annealing, laser forming, hardness, microstructure, Abstract Localised heat treatment by laser may be utilised to soften certain areas of cold rolled sheets prior to forming operations. Softening can be achieved by recovery, recrystalisation or soft annealing. Laser enables not only a separate local softening but also allows to perform forming operation at elevated temperatures, simultaneously with laser heating. Localised annealing combined with bending or drawing offers a variety of both applications and advantages, which have not yet been evaluated. Therefore, intensive investigations have been in progress. In most cases their results are covered by patents in which the main role is attributed to lasers which generate a suitable temperature field. The present review outlines examples of laser annealing, hot bending and drawing, non-contact bending, laser bending, welding and annealing as well as complex treatment comprising laser cutting + bending + welding + final heat treatment. They show the possibilities of laser sheet forming and challenge to innovative solution in plastic deformation. Some annealing trials of cold rolled sheets were performed using the IM's Nd:YAG laser. They confirmed that pulsed Nd:YAG laser can be successfully used for local recrystallisation and softening. Mathematical modelling of heat flow was performed in order to calculate the temperature profile during annealing trials. Both simple models and complex FEM models were tested and compared with experimental results.

3 1 CONTENTS 1. INTRODUCTION SOFT ANNEALING BY LASER TREATMENT SHEET METAL ANNEALING LASER SURFACE ANNEALING OF STEEL WIRES FOR AUTOMOTIVE TIRES LASER ANNEALING OF NON-FERROUS SHEETS LASER FORMING HOT FORMING OF SHEET MATERIALS IMPROVING COLD ROLLABILITY AND/OR STRIP ANNEALABILITY LASER BENDING OTHER APPLICATIONS SOLUTION ANNEALING HEAT TREATMENT INCREASING THE DRAWABILITY OF AL-CU ALLOYS BY DIFFERENTIAL HEAT TREATMENT LASER WELDING AND ANNEALING HEAT FLOW THROUGH THE LASER TREATED SHEET DETERMINATION OF PROCESS PARAMETERS MATHEMATICAL MODEL OF HEAT FLOW PRELIMINARY INVESTIGATION OF LASER ANNEALING OF COLD ROLLED SHEETS SUMMARY ACKNOWLEDGEMENTS REFERENCES...37

4 2 1. INTRODUCTION Laser radiation can be used in several ways to modify surface properties of metals and alloys. Laser transformation hardening, remelting or alloying are established methods for producing hard, wear resistant cases on selected surface areas of engineering components. Laser radiation can also be used to perform a simple heating for solution annealing, soft annealing or ageing. This review is focused on soft annealing of cold rolled sheets or final bending or drawing. When soft annealing or annealing of hardened components are used as bulk treatments conventional heat treatments are generally considered as being cheaper 1. Laser, on the other hand, is very efficient in heating a localised area. It is known that localised heat treatment may be utilised to soften portions of components for a variety of purposes such as stress relief or creation of components with different physical properties in different areas. This is especially true for processing of sheet metals. Consider a cold rolled sheet, which should be deformed by bending. Laser enables not only a local softening but also application of non-contact bending without forming dies. The sheet can be bent into a V or C or other shape by repetitively scanning the beam over the sheet at particular zones. In conventional sheet metal processing, bending or drawing is performed by a sheet metal bending or drawing machine using various special dies. The forming operation can also be performed simultaneously with laser heating. Such a forming process is often called as superplastic forming. The yield strength of a metal material is significantly reduced at elevated temperatures and even a hard metal sheet can be easily hot formed into complex shapes. The technique of local softening, recrystalisation and forming of sheet materials has obviously a number of possible applications and advantages, which have not yet been evaluated. Therefore, intensive research activities have been in progress. Most of them have resulted in specific patents in which the main role is played by the lasers, which generate a suitable temperature field. Considering claims from these patents one may have the impression that even normal laser heating is restricted. Obviously they pertain to a specific part or process, which leaves a broad area for new solutions and inventions. If a new laser treatment or forming has to be applied, a careful experimental optimisation of the process parameters must be carried out in order to obtain maximum processing speed and optimum quality. In most cases, there are no systematic investigations, which might help design and simplify the step from experimental state into a manufacturing line, despite a growing area of mathematical modelling of the interaction between laser radiation and heated surface, heat transfer and the thermal field in the treated materials. The present review is aimed at summarising the possibilities for laser processing of sheet materials. Especially, a soft annealing for bending and drawing is highlighted. Mathematical modelling of heat flow through the sheet based on heat flow by conduction is also described.

5 3 2. SOFT ANNEALING BY LASER TREATMENT A complete soft annealing of the hot rolled sheet before final forming is often undesirable as it reduces the strength and may cause distortion problems. The optimum solution is local, soft annealing only in these regions where the sheet has to be deformed. More generally, if a hardened part has to undergo a final shaping, localised soft annealing can be performed with laser heating systems Sheet metal annealing The possibilities of local soft annealing of different sheet materials using CO 2 -lasers were analysed by Bergman 1. The experiments were carried out with CO 2- lasers of 0,6 and 4 kw output power. Figure 1 gives a schematic drawing of the experimental set up, for experiments carried out with 0,6 kw laser. The absorption was simply increased by drawing a black line where the laser treatments were applied. The optimum treatment conditions were found by varying the power density and feed rate, ensuring that ripple formation did not occur and the sheets were soft annealed throughout the whole thickness. The materials investigated and the treatment conditions for laser power of 600W are listed in Table 1 and 2. Sheets of austenitic stainless steel and of a typical deep drawing grade were used as reference materials. Table 1. Investigated materials and obtained microstructure after treatment with 600W laser power 1. No. Material Degree of deformation % Sheet thickness Microstructure after laser annealing 1. CuNi4,5Al4,5Mn2 Cr0,8 75 0,5 50% of the cross section molten, showing dendritic structure, rest recrystallised with equated grains. 2. CuNi78Zn ,25 As above, 30% molten 3. AlMgSi2 (0) age hardened 1,5 Differences in microstructures with optical microscopy not detectable. 4. X12Cr18Ni8 33 1,0 Partially recrystallised microstructure, no melting. 5. C ,0 Partially recrystallised microstructure in the heat affected zone.

6 4 Fig. 1. Schematic drawing of the experimental set up 1. B A a) b) c) Fig. 2 Microstructures of soft annealed C10 steel a) general view b) transition zone c) recrystallized region A a) b) Fig. 3 Microstructure of partially melted CuNi 4,5 Al 4,5 Mn 2,5 Cr 0,8 sheet. a) General view and b) enlargement.

7 5 Table 2. Laser parameters and hardness values No. Power Focus Power density Feed rate Vickers Hardness (HV0,5) W cm W/cm 2 m/min Starting material Laser annealed After bending ,0 4,8*10 5 5, ,5 2,3*10 4 9, ,5 2,3*10 4 0, ,5 3,4*10 4 0, ,5 2,3*10 4 0, The final optimisation of processing parameters for austenitic stainless steel and a typical deep drawing steel was performed using a 4 kw laser, Table 3. In this case the intensity distribution of the output energy was modified using a BIAS optic in order to reach a constant energy distribution throughout a rectangular beam of 5mm x 14mm in crosssection. Table 3. Optimum processing parameters for obtaining full recrystallisation of cold rolled stainless and C-Mn sheets using 4kW laser. Material Thickness mm Power kw Feed rate m/min 0,7 2,5 6,5-7 X12Cr18Ni8 1 2,5 3,5-4 1,5 2,5 1,5 1,5 1, ,0 2,5 5-6 C10 1,5 2,5 3 1,5 1,7 2-2,5 The experimental results confirmed that it is possible to obtain fully recrystallised soft microstructure, Fig. 2. The recrystallised region contains eqiaxed grains and is clearly distinguisable from the cold rolled structure. However, if the laser parameters are not optimised a partial melting can occur, Fig. 3. Hardness profiles across the laser annealed zones on steel sheet and aluminium sheet are shown in Fig. 4(a) and (b) respectively. It was noticed that the width of the softened zone on the steel sheet was similar to the diameter of the laser beam with a narrow heat affected zone. However, for aluminium sheet the width of the softened zone was much wider due to a higher thermal conductivity. After laser softening treatments, all sheets could easily be bent to a rectangular edge. The bending followed exactly the treated track and no cracks were found.

8 6 a) cold rolled steel C10 b) aged hardened AlMgSi 2 Fig.4. Hardness profiles of soft annealed materials. Fig. 5 Laser surface annealing of steel wires for automotive tires- the schematic diagram of the method.

9 7 The following conclusions can be consider as general findings from the above investigation: It was demonstrated that local soft annealing of cold rolled materials is possible. The required depth in which a surface layer has to be modified depends on power density and interaction time. The width of the softened track is not only determined by the laser parameters but also by thermal conductivity of the material and the thickness of the sheet. Laser power and feed rate should be adjusted in order to ensure complete soft annealing throughout the sheet thickness and avoid local melting of the surface. For a low thermal conductivity, the feed rate has to be reduced to achieve soft annealing over the whole sheet thickness and avoid local melting. The reduction in hardness can be due to the recrystalisation or overaging, or possibly, homogenisation and subsequent ageing might have occurred. The result of the treatment with a rectangular beam confirm that it is possible to obtain fully recrystallised traces even for material with low thermal conductivity (austenitic stainless steel). Treatments of thicker sheets are also possible without local melting Laser surface annealing of steel wires for automotive tires The application of tire rod wire with diameter <300µm and tensile strength above 3000 MPa is limited by their insufficient fatigue strength. A new method for improvement of the fatigue strength based on laser surface annealing of a thin surface layer was developed at Nippon Steel Corporation 2. In this method, the coaxially focused laser beam of high energy irradiates the wire to anneal uniformly a thin surface layer. Using optimum parameters, up to 3 µm depth can be uniformly annealed. After laser annealing the fatigue strength of the test wire was increased four times without any deterioration of the tensile strength. The schematic diagram of the method is shown in Fig 5. The laser beam was introduced by the bending mirror into the cone shaped focusing mirror. When the steel wire passes through the laser cloud zone, the surface layer of the wire is heated. Heat input into the wire was controlled by regulating the wire feeding speed and the laser power. The surface was annealed because the wire is so thin in diameter that the cooling velocity by heat conduction is slower than that of typical laser transformation hardening. In processing line, the feeding speed of the wire was in the range of m/min. Microstructural investigation confirmed that the fatigue strength of the wire is improved when the fine columnar cementite is formed at the surface layer as the result of annealing. Martensite structure was not observed despite a high carbon content of 0.82%C. The improvement of fatigue strength after laser annealing was somewhat contradictory to the traditional view that fatigue strength increases as a result of laser hardening of the surface layer. It was suggested that the annealed surface of the wire may be less sensitive for initiation of fatigue cracks as laser annealing produces a smooth surface without melting or oxidation defects. This seems to be true for bending loading of tire rods.

10 8 Fig.6 Annealing method of the part of the spring member made of non-ferrous metal by pulsed Nd-YAG laser Vickers DPH 500g 250 Back Front ,2 7,4 7,6 7,8 8 8,2 8,4 8,6 8,8 Distance from reference, mm Fig.7 The hardness across the irradiated zone on both the irradiated and reverse sides.

11 Laser annealing of non-ferrous sheets A method for annealing a selected part of non-ferrous, metallic workpieces to a controlled degree of temper was invented by Charschan and Tice (US patent No 4,151,014) 3. In this invention the annealed part is treated by irradiation with a pulsed laser beam. The intensity and /or pulse duration of the beam, is controlled to obtain a required degree of temper. The method was developed for a phosphor-bronze or beryllium-copper connector contact springs hardened to extra-spring hardness. Fig. 6 shows an annealing method of a part of a spring. A pulsed Nd-YAG laser is used in this case to anneal the selected area (13) of the spring (11). The hardness across the irradiated zone on both the irradiated and reverse sides is shown in Fig. 7. It can be note that on the irradiated (front) side the heat-affected zone was only 1,4 mm wide with an effective spot size of 0,7mm. A metallographic analysis revealed that the heat-affected zone did not show the effect of recrystalisation usually associated with annealing. Therefore, it was suggested that softening was due to recovery of strain, induced during a rolling operation of the spring. This invention was restricted for nonferrous metals, although other materials might also be treated in this way. By annealing only the localised, selected region of the workpiece in a controlled manner, other operations, which are enhanced by the presence of a localised annealed region, may be performed e.g. bending after the irradiation. 3. LASER FORMING 3.1. Hot forming of sheet materials A different method of laser bending was described by Nashiki from Okuma Co. (US Patent No.5,359,872). 4 In this case, the portion of a metal sheet is heated by irradiating it with a laser beam and when the portion to be deformed has attained a given temperature, a bending operation is performed. As the yield strength of a steel is significantly reduced when it is heated Figs. 8 and 9 the author claims that no dies are necessary for performing complex forming operations. This patent cover a method and apparatus for metal-sheet processing for performing cutting, bending, drawing, welding and modifying of a sheet metal. In this method a laser can be also used to perform heating of large areas by scanning a laser beam at high speed, for example for drawing operation, heating of narrow line or optional contour, mainly for bending operation and heating in order to perform heat treatment. Bending and drawing of a metal sheet that conventionally requires many types of special dies may be replaced by a sheet metal processing method, which is based on this simple principle. Examples of this method for sheet metal bending are shown in Figs When the line segment CD of a metal sheet, shown in Fig. 10a is irradiated with a laser beam and reach a temperature of around 800 C the elastic limit for this line is 1/10 or less of that for the other portions as shown in Fig. 9 and the sheet may be easily bent. A bending process of a more smoothly curved surface is also possible as shown in Fig.10b. The curvature of the bending part can be controlled by adjusting the diameter of the laser beam to a suitable value as shown in Figs 11a and 11b. By repeating the heating and bending operations, various shapes of processed metal sheet can be obtained Fig.11c and 11d.

12 10 Fig. 8. Relation between a tensile strength P/S and a rate of elongation l/l of a sheet material. Fig. 9. Temperature dependency of tensile load in the elastic region a) b) Fig.10 Laser sheet metal heating of selected parts of sheet which require further bending. a) b) Fig. 11 Controlling a bending curvature by adjusting the laser beam diameter.

13 11 c) d) Fig. 11 Possibilities of controlling a bending curvature by adjusting the laser beam diameter and the position of laser tracks. Fig. 12 Laser equipment for laser bending.

14 12 a) b) c) Fig. 13 Laser drawing applications a) b) c) Fig. 14 Example of laser sheet metal drawing d) Fig. 15 Complex laser sheet metal processing consisting of laser cutting, bending after irradiating to decrease yield strength and laser welding the shown configuration.

15 13 Laser equipment for performing the above bending operations is shown in Fig. 12. Another application which pertain to drawing is shown in Figs. 13 and 14. In these cases the circumference IJKL is heated to a suitable temperature and may be drawn into a convex shape by applying a force. Moreover, as shown in Figs. 13b and 13c the area within the disk IJKL can be heated and drawn sequentially to a stepped or curved surface. In order to accurately obtain desired shapes in curved surface it is necessary to obtain a proper temperature distribution. Therefore, a temperature detector is used to control the sheet metal processing process. It is also possible that the reaction force of the metal sheet control the output power of laser beam. An example of complex sheet metal processing such as cutting, bending, welding and heat treating is shown in Fig. 15. First a sheet is cut by means of the laser then heated along the line H3-H4-H5-H6 and bend and finally welded. In addition, quenching by means of the laser for improving the strength of the box can be performed. Another method and device for shaping details by means of superplastic forming is described in US patent No. 5,592, The applicant of this patent is Electrolux AB. In the method shown in Fig.16 the foil or plate material is heated by laser over a selected area to a suitable temperature and simultaneously exposed to a fluid pressure so that the material is slowly deformed to a predetermined desired shape. The purpose of this invention is also eliminating the need for a mould having a form corresponding to the shape of the detail. The heating and hence the deformation of the plate takes place locally by guiding a laser beam (17) over the surface of the foil or plate (12). The device shown in the figure operates in the following way. A fluid under pressure is applied into the chamber (13) at the same time as a laser beam (17) heats a particular area of the plate to a suitable temperature. By choosing suitable time periods for the influence of the laser beam on different areas of the plate and at the same time having a suitable pressure in the chamber (13) the process can be controlled in such a way that the plate is deformed to a predetermined shape. The methods of forming metal sheets by heating the alloy to a temperature at which it is superplastic and then forming has been known for many years (for example US Patents No , 4,181,000 7 ). The laser as the heat source in forming operations has became accepted since a few years. It is engineer skill and imagination how to use this heat source for forming superplastic metal Improving cold rollability and/or strip annealability. Mravic and Shapiro (U.S. Pat. 4,405,386) 8 developed a process for improving cold rollability and/or strip annealability of metals and metals alloys. In their invention, a selective, on-line heating of edge and near-edge portions of a strip is performed to increase the ductility of the edges during cold rolling. In most metals and metal alloys, the maximum degree of cold reduction that can be imposed on a strip material before an interanneal is required, is determined by the onset of significant edge cracking. In the past, this problem has been overcome by limiting the degree of cold reduction. A method of heat treating edge portions of a strip material in coil form was described by Lamb (US. Pat. No.1,870,577). In this method, heated metal plates are placed in contact with the top or bottom portions of the coil.

16 14 Fig. 16 Fluid pressure forming of the plate being laser heated. 14- spray nozzle to supply a coating material 16- spray nozzles to supply protective gas 18-heating by laser beam Fig. 17 Laser heating the edge and near edge regions of the strip material to improve the cold rollability.

17 15 The width of the edge strip, which is heat treated or annealed, may be controlled by controlling the amount of heat applied. A separate annealing imposes, however, additional processing costs which are proportional to the number of such anneals that are required. It is therefore highly desirable that the total reduction taken before complete soft annealing be as great as possible. The degree of reduction that can be sustained before the onset of significant edge cracking depends on such factors as the initial edge shape of the strip material, the degree of front and back tension applied during the working process, and the intrinsic ductility of the metal being processed. Usually, when the ductility is exhausted for a given edge shape and given working conditions, the metal or metal alloy strip material is subjected to a full anneal to restore the ductility. If such a full anneal was not performed, edge cracks could develop and propagate sufficiently to require extensive edge shearing or if the cracks become extensive enough, fracture of the strip material during rolling. According to the invention by Mravic and Shapiro selected areas of the strip edges are annealed using, for example laser, to increase the ductility in those areas. A schematic representation of an apparatus for a continuos annealing of selected areas of a steel strip is shown in Fig. 17. Heating is carried out without significantly annealing the bulk of the strip. The imposed thermal energy is sufficient to cause appreciable softening through such processes as disordering, recovery, recrystallisation, and grain growth. Laser input power should be controlled to insure desired softening without overheating in edge and near-edge. Overheating can result in such undesirable effects as excessive oxidation, melting and phase transformation. When it is desirable to prevent substantially any oxidation during heating a protective atmosphere such as an inert gas may be utilised. The heated area may be cooled to prevent oxidation of the strip material as it emerges from the protective gases. Only edge and near-edge portions of strip material are heated. By using the described method larger reduction in strip material thickness may be taken before substantial edge cracks nucleate Laser bending Although this literature review is limited to laser annealing processes, some information on laser beam bending is also included as non-contact bending can be performed during laser annealing. Laser beam bending was invented more than decade ago, and a few applications have been found and were realised in manufacturing processes. In this process, a metal sheet is irradiated with a very fast scanned laser beam. The sheet can be bent into a V or C or other shape by repetitively scanning the beam over the sheet at particular zones. It was found that the final bending angle of the irradiated sheet is almost in proportion to the number of laser beam scans. This bending action is caused by thermal stresses in the sheet due to the extremely rapid heating and cooling process during the laser irradiation 9. The total bending angle is also affected by the mechanical and thermal properties of the treated material as well as the properties of the input laser energy. Depending on the material, the geometry of the parts and the position where the interaction takes place the achievable bending angle varies and torsion occurs during the material interaction. Despite a significant progress in understanding of the laser bending there is still a lack of systematic investigation of this process. The most important questions pertain to the

18 16 influence of the geometry, the heat input and the pretreatment of the material. Smallest bending angles but largest torsion angles have been found for laser treatment with scans across the whole width of smaller specimens. When the bending angles increase with the width, the torsion angles decrease. The largest bending angles can be achieved when a laser beam (pulses or scan) is positioned in the middle of a metal strip. To obtain maximum bending angles several scans across the specimen should be performed one upon the other. A pretreatment of the material like cold forming or annealing causes different bending angles. Larger bending angles can be obtained for cold formed and not annealed specimens due to the residual stress in the material, which enhance the bending process. The bending of a material strip can also be achieved by single spot-welding or a complete seam welding across the specimen or any laser process which induce thermal stresses. 4. OTHER APPLICATIONS 4.1. Solution annealing heat treatment. A relatively new technique is the application of lasers for solution heat treatment of heat affected zones on austenitic stainless steels (304). Austenitic stainless steel can become sensitised in heat affected zones in conventionally welded material. During this process, Cr-rich carbides can precipitate along grain boundaries producing chromium-depleted zones. Such areas are sensitive to stress corrosion cracking (SCC). To prevent cracking the welded parts are normalised. 10 Kimura at al. have developed a method for surface solution heat treatment, which gives similarly good protection against SCC as bulk solution heat treatment. This method is based on a high-power Nd;YAG laser which is used to completely solutionise the surface layer of welded areas on austenitic stainless steels. The schematic diagram of equipment is shown in Fig. 18. The identified solutionannealed zone reveals Fig. 19. Used laser parameter as well as depth of the zones can be seen in Fig Increasing the drawability of Al-Cu alloys by differential heat treatment Because of the types of crystallographic texture and plastic anisotropy aluminium alloys generally have poorer drawing performance than low carbon steel. Thus, methods of improving the limiting drawing ratio (LDR) are subject to intensive investigations 11. It has been recognised for many years that there is a strong correlation between the plastic anisotropy and the limiting drawing ratio (LDR-the ratio of the maximum blank diameter to the cylindrical diameter of the punch). For a given friction and die profile the plastic anisotropy, usually represented by the mean value of r, (the ratio of width to thickness strain in tensile tests in the plane of the sheet) has a determining influence on LDR. Plastic anisotropy has an effect on LDR because it influences the degree of strength differential between the material being drawn in the flange and that in the wall of the cup, which is transmitting the load to the flange. There are material factors in additions to plastic anisotropy, which can influence the LDR. Temperature gradients can have a very large effect and a further factor is the presence of radial strength gradients in the blank prior to deep drawing.

19 17 Fig. 18 Schematic sketch of the equipment used for solution annealing treatment. a) Macrostructure of solution annealing zone. b) Microstructure of solution annealing zone c) Microstructure of sensitized area. Fig. 19 Cross section of solution annealing zone. Fig.20 Solution heat-treatment zone depth

20 18 Bates and co-workers 12 has shown that the LDR of an Al-4.5%Cu alloy can be significantly improved by the use of differential heat treatment. The differential heat treatment involved setting up a transient thermal gradient to give a gradient of solid solution, which then lead to a large strength gradient after quenching and ageing. They used a very simple method of introducing the temperature gradients in Al-Cu blanks. The sheets were clamped between heated aluminium platoons. After a certain time, it was quickly removed and water quenched prior to ageing at 180 C. By using the differential heat treatment it was possible to produce strength differences in AA2014 sheet from below 150MPa in annealed condition to above 450MPa after solutionising and ageing. The introduction of strength gradient led to a significant increase in the LDR of the AA2114 sheet. An increase from 1.8 for uniformly annealed material to 2.6 for differentially heat-treated was reported. This value was slightly better than that for the IF steel. The authors concluded that although this is an impressive value for an aluminium alloy the differential solution treatments are difficult to apply in practice. The results give, however, a remarkable demonstration of the effect that plastic anisotropy can have on the formability of sheet materials. It is worth to add that such differential heat treatment could be performed using a laser radiation as described above Laser welding and annealing Lasers have long been used to perform precision welding operations due to the highly focused nature of the laser beam. After welding, the welds and HAZ s must often be annealed in order to obtain the desired metallurgical characteristics in the welded area. In many laser-welding applications the traditional annealing by heating the entire workpiece is performed. A laser can be used to perform this annealing operation. There are two possible configurations by using two separate lasers, one high power laser for welding and one low power laser for annealing, or by using a single laser for sequential welding and annealing. The apparatus for laser welding and annealing of printer bands from impact printers is described by Folger and Witerski in U.S. Pat. No 4,879, Fig.21 illustrates a top view of the welding and annealing apparatus. Once the welding is completed, the weld on printer bands is annealed using a defocused laser beam, which cover both the weld and HAZ. Fig. 21 Top view of the welding and annealing apparatus

21 19 5. HEAT FLOW THROUGH THE LASER TREATED SHEET Determination of process parameters. Successful surface treatment requires knowledge of all machining parameters to obtain the specified properties of the laser treated workpiece. These properties depend on the thermal field induced by laser heating which is influenced by processing parameters such as laser power, beam diameter and feed rate as well as the thermal characteristics of the treated material. The significance of thermal properties is that it determines how fast a material will take up and conduct thermal energy. Because the laser heats only the surface and all subsurface heating is accomplished by conduction, different parts across the sheet thickness undergoes discrete thermal cycles. It can be assumed that a certain temperature level can be obtained by higher power density and fast feed rate or lower power density and slow feed rate. These two ways of laser treatment, even if they give the same temperature on the surface, produces different temperature distribution across the sheet thickness. The combination of low power densities and slow feed rates produce more stable temperature profiles. In order to obtain the desired properties across the sheet thickness both the power density and the interaction time must be carefully adjusted for a given sheet thickness. Besides the proper thermal profile an important requirement for the laser treatment is such an optimisation as to give a maximum processing speed and optimum quality. Experimental data available from literature pertain in most cases to a specific laser system with a given heating configuration and are not directly applicable to other systems. These configurations might alter the results when experiments are replicated at other laser systems. A significant problem is also the high sensitivity of laser treatment to process conditions and to small changes in processing parameters. A change of only 10% in absorbed laser power, for example, can cause a significant change in properties of the heat-treated area. Very important are also the size of the heated workpiece (sheet thickness) and the position of the treated zone on the workpiece. Optimisation of laser processing parameters can to a high degree be supported by mathematical modelling. Various mathematical models have been developed which describe the phenomena of heat transmission by conduction in metal subject to laser radiation. These models are used to analyse the relationship between temperature distributions and process parameters such as scanning velocity, laser spot size and laser power Mathematical model of heat flow The basis of any process of laser treatment is the ability of a laser beam to create a heat flux of high density on the target surface. The main share of laser-induced heat propagates into the metal bulk by way of electron conduction and so the laser-induced thermal processes are similar in nature to conventional processes of metal heating. This suggests that heat transfer in laser treated metals can be dealt with the classical theory of heat conduction. To take advantage of the mathematical description of the heat conduction, a heat source created on the target surface must first be adequately characterised.

22 20 When a heat source is applied to the surface of a solid, the temperature field within it must satisfy the equations of heat flow. If the thermal properties of the material do not themselves depend on the temperature, the governing differential equation is: 2 1 T k dt dt Qr + K = 0 where k is the thermal diffusivity, K is the conductivity and Q r the rate at which heat is supplied to the solid per unite time and volume. Solutions to this equation for different configurations of heating sources and boundary conditions were described by Carslaw and Jaeger 14. A comprehensive review on mathematical models of high power laser material processing can be found in Ref. 15. Solutions to eq (1) are typically given in the form of complex integrals, which are too difficult to allow the analysis we attempt in this rapport. We will overcome this by presenting a simplified model for heat transfer, described by Grigoryant 16. This model applies to a uniform heating of thin sheets across their thickness. In the analysis of heat treatment with a pulsed laser or a continuous laser that interacts with a target over shorttime periods, the laser source is considered as an instantaneous heat source. When a linear heat source which emits heat Q at time t=0, heats a thin plate across its thickness δ the temperature at point (x,y) can be expressed by: T( r,t ) Q = exp 4πKδt 2 2 ( r ) 4kt α' t where r 2 =x 2 +y 2 is the square of the distance from the heat source to a point (x,y); α'=2k/c p ρδ is the factor accounting for the surface heat transfer to the environment, c p is the specific heat and ρ is the mass density. The above formula applies for the approximate calculation of temperatures at a spot heated for a short time for example in spot welding of plates. If a linear source heats the target for a longer time t, the temperature can be found using the principle of temperature superposition that comes down to the integration of equation (2) with respect to t. Examples of the calculated temperatures as a function of the distance from the heat source obtained from equation (2) are shown in Fig. 22 under assumption that the factor accounting for the surface heat transfer to the environment can be neglected. The calculated peak temperatures versus the sheet thickness are shown in Fig. 23. The effect of the surface absorption on the temperature distribution can be determined from Fig.24. (2) (1)

23 21 Calculated temperature profiles for 1mm thick sheet. Absorption coefficient 0, Temperature o C W 500W 420W 0 0 0,002 0,004 0,006 0,008 0,01 0,012 0,014 Distance, mm x 1000 Fig. 22 Calculated temperature distribution around a liner heat source of different power, for 1mm thick sheet. Absorption coefficient Temperature, C ,5 2 2,5 3 3,5 Sheet thickness, mm Fig. 23 Influence of sheet thickness on the peak temperature for laser power of 420W. Absorption coefficient 0.7.

24 22 Temperature profile around the linear heat source for laser power of 420W and different absorption coefficient Temperature o C ,8 0,7 0,6 0,5 0,45 0,4 0 0,002 0,004 0,006 0,008 0,01 0,012 0,014 Distance,mm x 1000 Fig. 24 Influence of the surface absorption on the temperature distribution. The calculated results were compared with experimentally determined temperatures. In all cases, the agreement was good, provided that the absorption coefficient was properly adjusted. For calculations of heat flow in a thin sheet the most important is the infinite dimension of the sheet thickness. For a given thermal conductivity and feed rate there exists a critical thickness below which the finite dimension of the sheet has to be considered for reliable predictions. Roughly speaking when the length of the thermal diffusivity characteristic of the material is comparable to or greater than the thickness of the workpiece, the lower surface of the workpiece reaches a considerable temperature and as no lower layers exist, the heat diffuses toward the top, thereby modifying the temperature profile. The influence of finite sheet thickness on the calculated temperature field is shown in Fig.25 1.

25 23 a) b) c) f) d) e) Fig. 25 Calculated isotherms for a sheet with thickness 1,5mm (a), (b), (c) and 3mm (d), (e), (f) and a rectangular source 1 a), b), d), e) sections with y = constant c), f) section x=constant = 0 level lines 2=400 C, 4=800 C, 8=1600 On the basis of the above results the following remarks can be given pertaining to laser heating of thin sheets: - the temperature depends strongly on the sheet thickness - small variations in laser power affects significantly the temperature field. The same applies to the feed rate - surface absorption is the most uncertain parameter in the mathematical calculations - the final tuning of laser processing parameters for a given configuration can only by made by experimental trials. In the present investigation, the heat propagation in the sheet metal was also simulated by numerical calculations using the finite element method. We have used the PDEase software to analyse the temperature field for a moving Gaussian energy profile on a sheet of 1mm thickness. As the SIMR Nd:YAG generates a multi-mode energy distribution, we consider that the moving heat flux, q, induced by such a laser radiation can be expressed, with a good approximation, by the following equation,

26 24 2 2QA ( 2( x Vt ) q = exp (3) π 2 2 r r where Q is the laser power in W, A is the absorption coefficient, r is the radius of the beam, v is the traverse speed, t is the time. Figure 26 shows the power density distribution across the laser beam calculated from the above equation for laser power 420W and beam diameter 6mm. Gaussian energy distribution by the laser beam Flux, W/m Laser power 420W, beam diameter φ=6mm 0-0,008-0,006-0,004-0, ,002 0,004 0,006 0,008 Distance, mm x10-3 Fig. 26 Power density distribution across the laser beam Flux, W/m position of x for the calculated fields 0 0,5 1 1,5 2 2,5 3 3,5 Time, s Fig. 27 Changes of the flux in the constant x position as a function of time.

27 25 Changes of the flux with time in the constant x position used in finite element calculations are shown in Fig. 27. This point, x, which can be considered as representative for the heated piece of 1 mm thick, 200 mm bred and 300 mm long, was chosen for the complete calculation of the flux and temperature fields. The maximum flux in this point was obtained after 2.2 seconds for the applied processing parameters. The finite element calculations were then performed for 1.8, 2.0, 2,2, 2.4 and 2.6 seconds. a) b) Fig. 28 Temperature at the position "x" as a function of time. a) the upper surface b) the lower surface Figs 28(a) and 28(b) show the temperature distribution at the point x as a function of time for the upper and lower surface respectively. It is clear from the figures that the temperature on both irradiated and lower sides increases rapidly to above 700 C-900 C in less than 1 second. The cooling profile is, on the other hand, much longer and after 1 second cooling the temperature is still above 600 C. It can be noted also that the temperature on the irradiated surface is about 200 C higher than on the back side. Distribution of fluxes in the zy plane and the respective temperature profiles are shown in Figs 29 and 30. It can be seen that at the beginning of the heating processes the flux is penetrating rapidly through the thickness and then change direction towards the edges of the sheet.

28 26 Flux X,FluxY a) max 9,4751 min 0,4747 Scale=10 6 Time 1,8s b) max 16,963 min 1,814 Scale=10 6 Time 2,0s c) max 11,996 min 2,14 Scale=10 6 Time 2,2s d) max 8,0461 min 1,4067 Scale=10 6 Time 2,4s e) max 1,1996 min 0,214 Scale=10 6 Time 2,6s Fig. 29 The flux flow at the same position of the X direction as a function of time. At the time of 2,2s the maximum power density passes over the considered point.

29 27 Time 1,8s a) Max temperature=287,2 C m-287,2 l-260,0 k-240,0 j-220,0 i-200,0 h-180,0 g-160,0 f-140,0 e-120,0 d-100,0 c-80,0 b-60,0 a-40,0 Time 2,0s b) Max temperature=775,2 C g-700,0 f-600,0 e-500,0 d-400,0 c-300,0 b-200,0 a-100,0 Time 2,2s c) Max temperature=1020 C i-1020 h-918 g-810 f-714 e-612 d-510 c-408 b-306 a-204 Time 2,4s d) Max temperature=907,8 C n-900 m-850 l-800 k-750 j-700 i-650 h-600 g-550 f-500 e-450 d-400 c-350 b-300 a-250 Time 2,6s e) Max temperature=287,2 C i-750 h-700 g-650 f-600 e-550 d-500 c-450 b-400 a-350 Fig. 30 Temperature distribution across the sheet thickness in the position x after processing for 1.8, 2.0, 2.2, 2.4 and 2.6 seconds.

30 Fig.31 Temperature and flux profiles at the maximum power density (this is equivalent to the maximum temperature in the considered position). 28

31 29 The temperature profiles for maximum temperatures in the considered x position are given in Fig. 31 along with flux size and direction. The maximum temperature for the applied laser parameters (laser power = 420 W, traverse speed = 8 mm/s and sheet thickness = 1mm) was about 1000 o C. This temperature seems to be higher than the temperature range estimated from the resulting microstructures. The metallographic observations of the treated tracks (Fig. 32) suggest that the Ac 3 temperature was not exceeded, as no α-γ-α transformation was observed. The higher calculated temperature may be a direct result of the assumed absorption coefficient on a level of 0,6 for the cold rolled steel surface and Nd:YAG laser beam. This value was probably too high. Another aspect is that, when the heating rate is high the system is far from equilibrium conditions and the A 3 line will tend to be displaced upward to higher temperature. In addition, the system remains at high temperatures for less than 1 sec. Despite the discrepancy in temperature advanced finite element calculations are very helpful in predicting both the heat flow and temperature distribution in laser heated thin sheets. 400X 25X Fig.32 Microstructureof the laser recrystallised cold rolled DOC 280YP sheet with the following laser parameters; power- 420W, feed speed-8mm/s).

32 30 As was shown early the flux penetration through the thickness depend mainly on the interaction time. The microstructure changes across the laser heated DOC 1000PD sheet steel treated with the same power density and different traverse speed are shown in Fig.33. The microstructure changes (the dark etched region) are influenced by the temperature profile obtained during interaction time 1,2, 1,and 0,83 secs. respectively. 250W(2ms/100Hz/2,5J) φ=5mm, V=5mm/s 250W(2ms/100Hz/2,5J) φ=5mm, V=6mm/s 250W(2ms/100Hz/2,5J) φ=5mm, V=7mm/s Fig.33 Penetration of heat through the sheet thickness for interaction time 1,2; 1; and 0,8 secs.

33 31 Another problem, which was also evaluated by computer simulations, is the influence of edges on the flux flow and temperature. It is obvious that when the laser beam is used to heat areas close to the sheet edge the heat flux becomes unsymmetrical, as there is no heat sink in one direction. This results in unsymmetrical temperature profile and may cause softening of the large areas of the sheet and/or deterioration of properties by a too high temperature. This phenomenon is shown in Figs 34 to 36. The comparison of the temperature distribution for the laser track positioned in the midline and close to the edge clearly illustrates the accumulation of heat in the edge and equivalent increase in temperature. a) b) c) Fig.34. The flux flow (a), and surface temperature (b) and (c) for the laser track moving in the X direction in a midline of a 200x300mm sheet.

34 32 a) a) b) b) c) c) Fig.35 Fig.36 Figs The flux flow (a) and temperature distributions (b) and (c) for the laser track moving in the X direction close to the edge of a 200x300 mm sheet. (Fig. 35 show the position of 10 mm from the opposite side and Fig 36 show the situation when the laser beam reach the opposite side).

35 33 A significant improvement of the quality and efficiency of laser annealing and/or softening treatment is achieved by using a beam of square or rectangular cross section, which gives a uniform temperature distribution (see Fig.37) a) b) Fig.37 Temperature profiles at the surface of a steel sheet produced by single-track treatments with a) Gaussian laser beam and b) rectangular laser beam. The optical systems to be used for obtaining a proper laser treatment have been thoroughly studied and analysed, in order to obtain the required quality of the process and to avoid local melting or local inadequate treatment. The main optical systems studied can be divided into four groups: 1. oscillating mirror systems 2. rotating mirror systems 3. wave guide system 4. beam integrating systems All system have advantages and disadvantages, and the type of a system to be used depends greatly on the overall dimensions of the entire system and primarily on the result required from the treatment. 6. Preliminary investigation of laser annealing of cold rolled sheets In order to check if a pulsed Nd:YAG laser can be used for softening and recrystallisation of cold rolled sheets some preliminary annealing trials were performed with IM s laser. This laser can generate high energy pulses at low repetition rate or low energy pulses at high repetition rate. It was considered that for sheet annealing the pulsed laser should simulate a continuous laser i.e. generate short pulses with high repetition rate. After calculation, the following laser parameters were chosen; 1-2ms pulse duration and 100Hz repetition rate. The laser power density and the interaction time were varied in order to obtain different temperatures. Figs show the results of laser annealing trials. Fig. 42 shows hardness values measured on the irradiated surface of sheet steel across the laser tracks. The laser parameters and traverse speeds are given below the curves. It is clearly seen from both the microstructures and hardness profiles that the best result was obtained after treatment with a laser power of 560W(2ms/100Hz/5,6J) *, beam diameter 5mm and traverse speed 20mm/s. * (pulse length / frequency / energy per pulse)

36 34 a) b) c) Fig.38 Cross-section of laser soft annealing zone. Used laser parameters: 560W(2ms/100Hz/5,6J), traverse speed 15mm/s. a) Microstructure of annealed area. b) Microstructure of heat affected area c) cold deformed steel Fig.39 Microstructure along the laser soft annealing track. Used laser parameters: 560W(2ms/100Hz/5,6J), traverse speed 15mm/s

37 35 Fig.40 Cross-section of laser soft annealing zone. Used laser parameters: 560W(2ms/100Hz/5,6J), traverse speed 20 mm/s. a) Microstructure of annealed area. b) Microstructure of heat affected area c) cold deformed steel Fig.41 Microstructure along the laser soft annealing track. Used laser parameters: 560W(2ms/100Hz/5,6J), traverse speed 20 mm/s

38 HV0, Distance, mm 560W(2ms/100Hz/5,6J), 15mm/s, 560W(2ms/100Hz/5,6J), 20mm/s, Fig. 42 Hardness values measured on the irradiated surface of sheet steel across the laser tracks. The laser parameters and traverse speeds are given below the curves. The following conclusion can be drawn from the initial test: pulsed Nd-YAG laser can be successfully used for local soft annealing of sheet steels no absorption coating is necessary for softening treatment the treated sheet was annealed approximately uniformly throughout the thickness. The max width of the softened track was determined by the maximum output power 7. Summary Laser annealing of cold rolled sheets is a very promising process for localised softening of a certain area before bending or drawing. This treatment is performed by moving a laser beam with a proper power density and scanning velocity over the sheet surface to heat a localised area to the predetermined temperature. Bending operations can be performed at room temperature after softening or simultaneously with heating ( hot bending ) Examples of laser annealing, hot bending and drawing, non-contact bending, laser bending, welding and annealing as well as complex treatment comprising laser cutting + bending + welding + final heat treatment are described. They show the possibilities of laser sheet forming and challenge to innovative solutions in plastic deformation. Preliminary annealing trials on cold rolled sheets were performed using IM's Nd-YAG laser. They confirmed that pulsed Nd:YAG laser can be successfully used for local recrystallisation and softening. It was also shown that no absorption coating is needed for 1.06µm radiation. These results are important as Nd-YAG lasers are easy to integrate into production systems using flexible fibre optics. Mathematical modelling of heat flow was performed in order to calculate the temperature profile during annealing trials. Thermal simulation using the finite element method was also presented.