Bending Behaviour of Laser formed High Strength Low Alloy (HSLA) Steel Sheet

 

 

A. Els-Botes^I; P.J. McGrath^II; H.J. Pienaar^III

^IMSAIMM, Institute for Advanced Manufacturing and Engineering Research (IAMER), Faculty of Engineering, NMMU, Port Elizabeth, South Africa. E-mail:annelize.els-botes@nmmu.ac.za
^IIMSAIMechE, Department of Mechanical and Industrial Engineering, College of Science, Engineering and Technology, South Africa
^IIIMSAIMechE, Department of Mechanical Engineering, Faculty of Engineering, Tshwane University of Technology, South Africa

 

 

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ABSTRACT

The application of bending/forming steel plate by thermo-mechanical means is extensively and successfully employed in the ship building industry. However, the radius of curvature resulting from this flame line-heating process is considered relatively large compared to the bend radii that can be produced by the application of lasers. The laser technique for bending steel plate to specific bend radii for a given set of laser process parameters is still underdeveloped, requiring further research in order to make this application 'production friendly⁹. The applied approach reported in this paper considers the bending of HSLA sheet samples using a 5 kW Continuous Wave (CW) CO₂ Trumpf Laser in which the radii of curvature of the bend samples produced are characterised with respect to the laser process parameters. An interesting feature when characterising process parameters for producing a bend sample having a specific radius of curvature, is that the indicated trend was found to be linear. A further observation was in characterising the bend angle (or reverse bend angle) against bend height; not only was the indicated trend linear but the equation best describing this trend indicates that when the bend height tends to zero the bend angle tends to 0° (or reverse bend angle tends to 360°).

Additional Keywords: Radius of curvature, laser forming, line heating, laser forming mechanisms

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1. Introduction

Many scientists and engineers have contributed to this technology since its inception during the mid-1980's^1-6. However, the objective of this work was to seek an alternative manufacturing process for producing a spatial shaped component that will not impair the metallurgical structure of sheet material as used by motor-component that the mechanical forming process can decrease the component's fatigue life by as much as 83 % during the first draw operation (forming of a bulge shape). This research project will attempt in forming sheet specimens to the same radius of curvature as that formed during the first draw stage of mechanical forming (of a wheel centre disc). Microstructural analysis of previous research undertaken by the authors⁷ indicated that micro-voids were nucleated in the matrix microstructure as a result of the mechanical forming process. The radius of curvature of this particular spatial shape was approximately 125 mm. The investigation reported in this paper considers the bending of 3.5 mm thick plate by laser to a radius of curvature similar to the spatial shape. As the bending of plate by laser was successful further investigations led to establishing laser and scanning parameters for producing plate samples having a radius of curvature of approximately 120 mm. It should be mentioned that the process effects not only enhance the mechanical properties of this material but would also render this application more scientific than the mechanical forming process through advanced and accurate shape control.

 

2. Material

The sheet material used for the experimental investigations is conventional rim steel (C2) as utilised by wheel manufacturers. It has a carbon content of approximately 0.09 % and has a pearlitic/ferritic microstructure, containing approximately 15 % pearlite by volume fraction. The Yield and Tensile Strengths are 385 MPa and 580 MPa respectively.

 

3. Experimental Set-Up and Procedure

Plate samples were laser-cut from the plane; transverse to the rolling direction of the manufactured sheet to ensure that irradiation of the plate surface would be parallel to the rolling direction of the sheet. No scientific significance for this direction was pre-empted only that the scanning (irradiation) path should be either parallel or transverse to the rolling direction of the sheet. The dimensions of the sheet samples were 200mm long by 48 mm wide and 3.5 mm thick. The purpose of this work was to bend plate samples to similar curvatures using three different laser process settings. The initial curvature required was approximately 125 mm. The original settings were: laser power of 1.5 kW, a beam diameter of 7.5 mm and an interval spacing of 7.5 mm (no overlapping between laser heat lines). Depending upon the curvature obtained from the abovementioned settings it was pre-decided to alter the scanning speed rather than the laser power or beam diameter to achieve this curvature. One other parameter that required establishing was that of the number of scans per position. Consequently, these quantities were found to be 1 m/min and five (5) scans per position respectively.

Five scans per position will from this point forward be referred to as a scanning cycle.

The second set of laser parameters was a laser power of 5 kW, a beam diameter of 20 mm and an interval spacing of 10 mm (i.e. 50 % overlap of the previously irradiation line). The scanning speed was adjusted to produce the desired bend curvature of approximately 125 mm. The third set of laser process parameters considers quantities of intermediate values in accordance with the previous two laser settings. Sheet samples were secured in a purpose built clamping device depicting a cantilever type arrangement as shown in figures 1 and 2.

 

 

 

 

The edge of the sample in the clamped condition was aligned to the traverse directions (X and Y) by using the pilot beam of the laser head. This ensured that the sample surface would be irradiated parallel to the rolling direction of the sheet.

Irradiation of the sheet surface commenced 30 mm from the free end and over a minimum length of 100 mm. This minimum length depended upon the beam diameter and interval spacing employed for the particular process settings indicated above. Upon securing the clamping device in position, the sample was coated with a graphite spray to allow for maximum absorptivity of the sample surface i.e. to reduce the reflectivity of the laser beam. As this was a first order investigation height measurements were taken at the free end of the sample using a vernier-caliper after every scanning cycle. Measurement time took on average approximately 35 seconds. This measuring time must be noted because the sample would have additional time to cool between consecutive scanning cycles that would result in a slight change in bend curvature.

 

4. Experimental Results

Initially a number of samples were bent to different radii of curvature in attempting to achieve the bend curvature required. These bend curvatures were in the first instance graphically assessed and later verified using a Taylor-Hobson instrument. It should be noted that the bend angle and bend height values as indicated in table 1 are with respect to a scanned length of 100 mm. This was done to provide a base length for all bend samples. The results giving the scanning and laser parameters as well as the bending behaviour of the sheet samples are also given in table 1.

 

 

Typically, when bend height (BH) values are plotted against the scanned length (SL), see figure 3, the resulting behaviour was found to be polynomial and of the order:

 

 

Hence, when the scanned length (SL) tends to zero, the bend height resolves to approximately the datum height of the sheet sample prior to 'line heating' process. It should be noted that the scanned length refers to the horizontal distance over which the sample was irradiated (considering the interval spacing and number of positions scanned). Similar expressions were obtained for the subsequent bend samples given in table 1, which also provide the scanning parameters employed. The incremental difference in bend height (IHD) per scanning cycle was observed as having a linear trend, see figure 4, over the scanned length (SL).

 

 

From equation 2 when scanned length tends to zero the IHD is 0.4363 mm. This discrepancy indicates a percentage error of 1.5% and that with improved measuring instrumentation would result in the IHD be equal to zero. A slight twisting was observed along the edge where the beam exits the sheet sample and assessed at approximately 3% of the sheet width in the worst condition. Scully¹ identified this occurrence as a result of the high heat intensity experienced by the plate along the exiting edge. However, this twisting effect was subsequently nullified by allowing the beam to exit the sheet sample at opposite edges during subsequent irradiation positions. Although unable to assess the surface temperatures, an indication of the temperature range reached was possible by conducting microstructural evaluation (considering the 1.5 kW laser parameters). Here the microstructural changes in the centre of the irradiation path (zone 1 - figure 5) indicates a martensitic/ferritic structure implying that the temperature experienced within this zone was above the A₃ temperature i.e. in the ferrite/austenite phase field.

 

 

Surface hardness assessments undertaken on the bend samples produced by the 1.5 kW laser settings indicated an average Vickers hardness value of 200 HV₁₀ at the centre of the irradiated path and 188 HV₁₀ at the centre of the boundary sections see (figure 5). The average hardness value considering the outer bend surface was 171 HV₁₀, while the stock plate samples had a hardness value of 164 HV₁₀. It is envisaged that the increase in hardness and consequently tensile strength of the surface layer of the material will increase the fatigue performance of the material. The compressive stresses set up in the micro-structure due to the martensitic transformation will also be beneficial in terms of fatigue performance.

Characterising the various quantities given in table 1, the most significant was in relating bend height (BH) to bend angle or reverse bend angle (RBA) - see figure 6.

 

 

The equation that best describes this relationship indicates that when bend height tends to zero, reverse bend angle tends to 360° (or bend angle tends to 0°). This would imply that a circular shaped component could be produced.

The laser and scanning parameters to produce a curvature of approximately 120 mm is given in table 2.

 

 

In characterising the above laser and scanning settings, a master diagram as shown in figure 7 was developed. This diagram allow for predetermining the laser and scanning parameters for producing bend curvatures of approximately 122 mm for this C2 material within the limits as indicated in figure 7.

 

 

Using this diagram, subsequent bend samples were produced resulting in similar bend curvatures by interpolating between the lower (0 and 25 % overlap) and upper (25 and 50 % overlap) levels of the diagram. Of further interest was extrapolating laser and scanning parameters above the 50 % overlap level. Two sets of parameters were determined for this exercise in which the radius of curvature was also approximately 120 mm.

 

5. Discussion and Conclusion

□ The linear behaviour observed with respect to figure 7 would tend to make bending prediction more accurate. Repeatability of bend curvatures can be greatly enhanced through installing an intelligent displacement measuring device that is interfaced with the laser system.

□ The significance of bend angle or reverse bend angle versus bend height indicates that complete circular (round) components can be manufactured by routing the laser beam through fibre optic cabling.

□ Laser forming results in precise bending behaviour, unlike mechanical forming where springback phenomenon (elastic recovery) is encountered. This aspect would render this application more easily controllable than the mechanical forming process.

□ The anisotropic behaviour associated with sheet material was found to have no effect on the laser forming process. It was found that the bending response to the 'line heating' process is the same considering sheet samples cut from the parallel, 45° and transverse directions to the rolling direction of the sheet plane.

 

Acknowledgements

The authors wish to thank the following persons and institutions for their contribution:

□ Staff at the National Laser Centre at the CSIR South Africa for their assistance and use of the laser facility.

□ The late Mr C Du Preez of Nelson Mandela Metropolitan University, South Africa, for his technical assistance.

□ Mr C Hughes of Tshwane University of Technology, South Africa, for his technical assistance during 2003.

□ Dorbyl Automotive (Wheels Division), Port Elizabeth, South Africa, for supplying the material used in this investigation.

□ The Nelson Mandela Metropolitan University and the National Research Foundation of South Africa for financial support.

 

References

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3.Abe N, Higashino R, Nakagawa N, Tsukamoto M, Miyake S and Noguchi S, Welding and forming of steel plates with diode laser, Transactions of Joining and Welding Research Institute, 2001, 30 (1), 138 - 141.         [ Links ]

4.Namba Y, Laser forming of metals and alloys, Proceedings of LAMP '87, Osaka, May 1987, 601 - 606.

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7.McGrath PJ, An Investigation of Residual Stresses Induced by Forming Processes on the Fatigue Resistance of Automotive Wheels, Doctoral Thesis, University of Plymouth, Plymouth, United Kingdom, 2001.         [ Links ]