This paper presents the summary of a experimental study related to the. surface coating, of zinc or aluminum. The main objective of the study was

Transcription

1 Publication: 97 Meeting of Red Iberoamericana sobre Procesamiento de Materiales por Laser, CYTED Program, Belo Horizonte, MG, Brazil; 05/1997 An Experimental Comparative Study of Laser Cutting of Mild Steel Sheets With and Without Zinc or Aluminum Surface Coating Edgardo Gerck - Lasertech 1 S/A - DEF/FEM/UNICAMP 2 Jorge L. Lima - Lasercam 1 - DEF/FEM/UNICAMP Abstract This paper presents the summary of a experimental study related to the gas-assisted CO 2 laser cutting of mild steel sheets, with and without a surface coating, of zinc or aluminum. The main objective of the study was the comparative analysis of the achieved results of the laser cutting of a complex geometrical part profile, including sharp angles. Other parameters were the sheet thickness and the temporal laser power output mode, with the laser cutting always occurring at maximum speed. The conclusions were that, for a thin steel sheet, the addition of a surface coating layer with few tens of micron can affect more the laser cutting process results than a comparatively much higher increase in the sheet thickness, and that the laser power output regime may remarkably influence the cutting results. Some results tables and photos are presented. Keywords: laser cutting; coated steel sheets; superpulse; CO 2 laser system. (originally written in Portuguese, delivered in Portuguese and Spanish) 1 C.P. 1201; C.E.P.: , Campinas - SP - Brazil; Current ed@gerck.com DEF/FEM/UNICAMP - Campinas - São Paulo - Brazil; jorge@laser.cps.softex.br Copyright by the authors, 1997

2 1. Introduction We present an experimental study related to laser cutting realized on mild steel sheets with or without zinc or aluminum coating. Nowadays, these surface-coated steel sheets types has been widely utilized in several industrial manufacturing activities, then a better comprehension of the laser cutting process of such materials is very important. Here, the major objective was to investigate the influence of the presence of thin anticorrosives metallic surface coatings on mild steel sheets relatively to the results of oxygen gas-assisted CO 2 laser cutting process of a complex geometrical cut path, and to compare with the results obtained on the corresponding uncoated steel sheets. Assuming here that some common criteria for laser cut quality on the parts such as mean roughness, inclination of cut surface, and the HAZ (heat-affected zone) width would not be considered, due to the fact that many adopted experimental process parameters were not the more appropriate ones for a real production case. So, the quality analysis of the parts formed by laser cutting on the steel sheets was concentrate on the geometrical features of the formed part s cutting profile, and it was made through the comparison of the obtained part s shape by laser cutting with the ideal part s shape made by CAD, and through direct measures realized on the laser cut parts, Also, the characteristics of the HAZ formed by the laser cutting in some parts were microscopically observed and compared in this experimental study. As it has been recognized [1], the reactive gas-assisted laser cutting of ferrous metals is still considered a complex phenomenon because, beyond the mechanisms of laser beam energy absorption by the metal surface and thermal conduction through the metal volume, it takes place convective mechanisms and thermochemical reactions

3 between the iron and the O 2 gas cutting into the thin melt metal layer formed in the cut front advance during the laser cutting process [1]. Moreover, the laser cutting, differently of the others thermal cutting processes, involves a extremely fast application and transfer of heat which is accompanied by very high temperatures and thermal gradients. So, the conventional thermodynamic approaches were not adequate to explain fully and correctly the laser cutting process. 2. Experimental Procedure The selected materials for the laser cutting were composed by hot-rolled SAE 1006 steel sheets, which were present in the following nominal thickness: 0.9 mm, 1.5 mm, and 2.0 mm. A percentage of the total number of these mild steel sheets was constituted by zinc-coated sheets and by aluminum-coated sheets. The rest of them was formed by sheets without any metallic coating. The aluminum-coated steel sheets was present in the 1.5-mm thickness only. It was utilized for this experimental procedure of laser cutting a 3-axis CNC (Computer Numerical Control)-controlled CO 2 laser system. This CNC/laser system was integrated with a CAD/CAM (Computer Aided Design / Computer Aided Manufacture) system wherein the drawings of the bidimensional parts designed to laser cutting operation was made by CAD with a final precision of ± mm. The CAD software was integrated with the CAM3, a CAM software optimized for laser processing of materials, and its precision is ± mm for the x-y Cartesian Coordinates of the relative laser head/workpiece movement. With only a few process data, the CAM3 automatically generated a CNC part program with the complete data

4 both of the part laser cutting path, based on the previous CAD drawing data, and of the laser cutting process parameters. The laser machine utilized was the MT-1000 Ô model [2], a CNC-controlled high-power CO 2 laser developed by LASERTECH S/A. The CNC unit of the MT-1000 Ô was commanded through the CNCXY-2 Ô software, which controlled, in real-time, every parameter of the laser cutting process. The functional characteristics of the CNCXY-2 Ô were established in manner of to warrant the laser cutting process repeatability inside a tolerance range superior to mechanical capability of the laser cutting system. The MT-1000 Ô laser source was an axial slow-flow CO 2 laser type. It power output regime occurs both in CW and superpulsed laser modes [2], and it achieves 3,000 watts peak-power in superpulsed mode. The table 1 following presents an informative summary of the MT-1000 Ô machine s principal features. TABLE 1 : Technical features of CNC Laser Cutting Machine MT-1000 Ô. [2] Laser power:...3,000 W Capacity:...up to 12.0 mm steel Cutting area:...2 m X 1.6 m Cutting types:...cw, superpulse Max. speed:...30 mm/minute Smallest drilled hole: mm Precision: mm Z axis:...contactless sensor Weight:...4,500 Kg Structure:...Pre-aligned monoblock Motors:...Servo AC Laser base:...granite 1,000 Kg Linear guides:...dust-proof CNC with 80 bits precision Control pad...hand-held Gases controlled by CNC The following figures show photos of the MT1000 CNC laser system:

5 FIGURE 1 - Photo showing the opened front view of the MT laser machine and the work area, where are seen the laser head block (A), the X-axis movement system of laser head block (B), and the Y-axis movement work table (C), the gas cutting nozzle assembly (D), and the contactless sensor (E). [2] FIGURE 2 - Detail of the MT-1000 laser machine showing the laser head set during a steel sheet laser cutting job. The laser focusing optics lodgment (A), the gases cutting nozzle assembly (B), the contactless inductive sensor (C), the flow tubes for N 2 (T1) and O 2 gases cutting (T2), and for exit (T3) and entry (T4) of pumped water for laser focusing optics cooling, are presented.[2] So, for the experimental procedure, it was initially produced a drawing of a bidimensional test part, which had arbitrary dimensions and a geometrical profile with the presence on it of sharp angles, and the CNC laser cutting program by the

6 CAD/CAM system. Some experimental part and process s parameters determined were: Cutting perimeter = mm Part area = 8420 mm 2 Maximum width = mm Laser beam cutting entries = 3 Maximum height = mm Auxiliary cutting gas = O 2 It was established the use of distinct laser operation parameters sets, with the laser operating both in CW and geometrical superpulse 2 modes. Then, it was performed a initial test series on laser cutting of the mild steel sheets, whose major objective was to find the more appropriate parameters sets for the laser beam output and cutting process for the laser cutting of all experimental steel sheets. Some prefixed process parameters were: Clearance of nozzle tip to sheet surface Þ 1 mm; Gas-cutting jet pressure on the sheet surface Þ around 300 kpa; Drilling time for the laser beam entry on the sheet Þ 1,55 ms. After the initial laser cutting tests, a number of ten parts were laser cut for each workpiece group characterized by its same sheet thickness class, surface coating type, and laser source output temporal mode. Each workpiece group was identified according to an alphanumeric code related to its characteristics, and for each part inside each group was given a identification number. Hence, it was used a XYZ-sequence of variables for the code of each workpiece group with the following meanings: X-variable Þ the type of surface coating present on the sheets: X = AA aluminum-coated steel sheet; X = AC steel sheet without surface coating; X = G zinc-coated steel sheet. Y-variable Þ the steel sheet thickness: Y = mm-thickness 2 A superpulse regime type wherein the pulse frequency was constant relatively to space instead of time.

7 sheets; Y = mm-thickness sheets; Y = mm-thickness sheets. Z-variable Þ the laser operating parameter set: Z = cw set of laser parameters in CW operation; Z = sp set of laser parameters in full superpulse operation. After the realization of the laser cutting of all parts inside each group, it was made the evaluation of the dimensional and shape quality of each obtained part through visual examination and direct measures on them. In terms of quality criterion, it was previously determined here that the quality of the experimental parts should to consider theirs use in a posterior fabrication process wherein previous part quality aspects like kerf roughness and inclination would not be important, and, by other hand, the dimensional and shape precision would be fundamentals, e.g. a welding process. So, for the characterization of shape quality of the parts was adopted here a classification criterion whereby each sample received a graduation value of 1, 2 or 3, corresponding to quality classes. This criterion is concisely showed in the table 2 following. TABLE 2 : Classification key for the quality based on shape deviations of the laser cut parts. Quality Meaning Part geometry Kerf edges Class 1 Refused Severe defects 2 Acceptable Negligible defects Small defects 3 Good No defects No defects The dimensional quality succeeds the shape quality classification of the parts,

8 conform the table 2 above. It was selected some dimensions in the geometrical profile on the parts for a comparative analysis. The part s dimension quality was not defined for the class 1 shape quality (conform the table 2); for the classes 2 and 3 it was established that, taking into account a laser beam spot size of 0.1 mm, the part s dimensional quality should to be considered excellent for the average dimension deviations lesser or equal to 0.1 mm, and acceptable between 0.1 mm and 0.2 mm included. For the microstructural analysis of the parts in their kerf region, the procedure of metallographic samples preparation was made conform the related ASTM A3-80 norm 3. The area of parts chosen for metallographic analysis was the same in all of them, and it was located in their upper-right region, near to a right corner. The metallographic sample s face of interest was corresponding to the inner thickness sheet region located along and subjacent to the kerf wall. For it carry out the chemical etching of the metallographic samples was utilized Nital 2% as reagent, with a application period of 15 seconds on each sample. The samples were identified and then analyzed in a metallographic analysis optical equipment. In sequence, microphotos of the samples were taken. 3. Results and Conclusions The results of measures and data collection realized during and after the experimental procedure of laser cutting of the workpieces are summarized in the tables 3, 4, 5, and 6, forward. Some macrophotos of a laser cut part are showed following. 3 ASTM A3-80

9 FIGURE 3 - A laser cut part G15sp without defects along its geometrical profile. The mild steel sheet is zinc-coated and the laser cutting was performed in laser superpulse regime. FIGURE 4 - A detail of the part in the figure 3, which it is showing one of the sharp angles in the its profile. TABLE 3 : Comparative summary of dimension quality achieved in laser cut parts (all surface coating types; 1.5 mm-thickness steel sheets). Obs.: 1) A, B, C, D e E selected dimensions for measure along the part s perimeter. 2) The figures below are the relative deviations relating to each selected dimension of part. They represent the difference between the measure dimension and the CAD nominal dimension. Part code Dimensional deviation relating to the indicated nominal part dimension (mm) A B C D E AC15cw G15cw

10 AAcw AC15sp G15sp AAsp TABLE 4 : Experimental data and results for the laser cutting of mild steel sheets with aluminum surface coating. Parts distribution due to the laser cutting quality Parts group Sheet thickness (mm) Speed cutting (mm/min) Laser power output regime Class 1 Cf. table 2 Class 2 Cf. table 2 Class 3 Cf. table 2 Cutting time (s) AAcw CW AAsp SP TABLE 5 : Experimental data and results for the laser cutting of mild steel sheets with zinc surface coating. Parts distribution due to the laser cutting quality Parts group Sheet thickness (mm) Speed cutting (mm/min) Laser power output regime Class 1 Cf. table 2 Class 2 Cf. table 2 Class 3 Cf. table 2 Cutting time (s) G10cw CW G10sp SP G15cw CW G15sp SP G20cw CW G20sp SP TABLE 6 : Experimental data and results for the laser cutting of mild steel sheets without surface coating. Parts distribution due to the laser cutting quality Parts group Sheet thickness (mm) Speed cutting (mm/min) Laser power output regime Class 1 Cf. table 2 Class 2 Cf. table 2 Class 3 Cf. table 2 Cutting time (s) AC10cw CW AC10sp SP AC15cw CW

11 AC15sp SP AC20cw CW AC20sp SP According to the results tables previously showed, from a macroscopic point of view the major conclusion is that, for a 1.5-mm-thickness mild steel sheet, the presence of a zinc coating layer with a thickness between 20 mm to 30 mm in each sheet face can change remarkably the final quality characteristic of a workpiece cut by CO 2 laser, with the laser source operating in continuous wave and the same laser cutting parameters, comparatively to a increase of 0.5 mm in the base metal thickness of the sheet without surface coating. This change is less notable when is considered the sheets with 0.9 mm in thickness. Probably, the influence of the zinc and aluminum coatings on the final results may to increase with the sheet thickness as well as the coating thickness, but this assumptions must be verified in a posterior experimental study. It was observed that the thermogeometrical 4 features related to present sharp angles of the part, which, in general, are adverse for continuous wave laser cutting since they provoke overburning [3], sometimes with small explosions on the coated sheets generating defects in the workpiece profile during the laser cutting at these points, can be strongly reduced with the use of a appropriated laser geometrical-superpulse regime, although with the sacrifice of the laser cut speed. As the microstructural observation, the thermal cycle of the laser cutting process produced a heat affected zone (HAZ) in the formed workpiece kerf region, varying from 40 mm to 70 mm in width, with a microstructural phase clearly visible from microscopic magnifications equal or greater than 200X in metallographic samples etched by Nital 4 Ability of the part s geometrical profile to concentrate, or to dissipate, heat during the laser cutting.

12 2%. This microstructure presented irregular globules scattered in a homogeneous matrix. The size, distribution density, and extension of this globules in the HAZ was modified along with all the others parameters that had influenced the laser cutting thermal cycle. The table 7, forward, shows the microscopic measures obtained related to the microscopic aspects of the experimental procedure. It is noted in table 7 that the HAZ extension of the aluminum-coated steel sheets (1.5-mm thickness) was equal or greater than the HAZ extension of the 2-mm-thickness steel sheets (with or without zinc coating). This result was due to the greater CO 2 laser power density necessary to overcome the high reflectivity of the aluminum surface to the CO 2 laser beam wavelenght during the laser cut front advance. TABLE 7 : HAZ width, and zinc and aluminum surface coatings thickness in the metallographic samples, measured by the microphotos and optical microscopy. Part HAZ code dimension Sheet thickness (mm) Thickness of the zinc coating ± 2.5 mm Thickness of the aluminum coating ± 2.5 mm ± 2.5 mm AC20cw AC20sp AC20sp G20cw G20cw G20sp G20sp AAcw AAsp

13 AAsp AAsp Forward, it is showed some microphotos of the metallographic samples, wherein is noted the HAZ microstructural phases. FIGURE 5 - Cut kerf s subjacent inner zone in the G15cw-1 part, wherein is noted the characteristic microstructural phase formed in the HAZ. Etching: Nital 2% FIGURE 6 - Laser beam entry side in the AC20cw-1 part. It is noted the

14 globules of the HAZ microstructure subjacent located to the kerf (left). Etching: Nital 2% FIGURE 7 - Lower cutting edge (left) of the part AAsp-2. It is showed a discrete dross puddle near to the kerf edge. Etching: Nital 2% 4. References [ 1 ] - Hsu, M. J., Molian, P. A., Thermochemical modelling in CO 2 laser cutting of carbon steel; Journal of Materials Science, n.29, 1994, pp [ 2 ] - CO 2 Laser/CNC Machine MT-1000 Ô - Technical Characteristics and Operation Manual, LASERTECH Ò S/A, [ 3 ] - Moryasu, M., Hiramoro, S., Hoshinouchi, S. & Ohmine, M., Adaptative control for high-speed and high-quality laser cutting; In: Laser Welding, Machining and Materials Processing: Proceedings of the International Congress on Applications of Lasers and Electro-optics, 1985, pp [ 4 ] - Mazunder, J., Steen, W. M., Heat transfer model for CW laser material processing, Journal of Applied Physics, v.51, n.2, 1980, pp.941. [ 5 ] - Powell, J., King, T. G., Menzies, I. A., Frass, K., Optimization of pulsed laser cutting of mild steels; In: Proceedings of the 3rd International Conference on Lasers in Manufacturing, 1986, pp [ 6 ] - Powell, J., Frass, K., Menzies, I.A., Fuhr, H., CO 2 laser cutting of non-ferrous metals; In: High Power CO 2 Lasers Systems and Applications, Proceedings of SPIE n.1020, 1989, pp