IMPROVEMENT OF TECHNOLOGICAL PARAMETERS AT SURFACE FINISHING THROUGH LASER BEAM MACHINING

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1 6th International DAAAM Baltic Conference INDUSTRIAL ENGINEERING April 2008, Tallinn, Estonia IMPROVEMENT OF TECHNOLOGICAL PARAMETERS AT SURFACE FINISHING THROUGH LASER BEAM MACHINING Marinescu, N. I.; Ghiculescu, D., Jitianu, G. & Seritan, G. Abstract: The paper deals with improvement of technological parameters in order to obtain a superior quality of surface through laser beam machining (LBM) finishing. The process is based on superficial layer melting, i.e. micropeaks belonging to surface profile, which fill the microvalleys, contributing to roughness decreasing. Some experimental data concerning machining of different materials, illustrating the technological solutions about the improvement of working parameters are also presented. On this basis, the roughness could be improvement with up to 70% at a single passage of laser spot on machined surface. Key words: laser beam machining, surface, finishing. 1. INTRODUCTION Surface finishing through Laser Beam Machining (LBM) solves the problem of difficult to machine materials, with high resistance characteristics, based on thermal superficial melting. Essentially, during LBM finishing process, thermal energy must affect only the superficial zone of the workpiece to be machined, respectively, micropeaks of the profile, because it is not intended to modify the workpiece macrogeometry. In this respect, all technological parameters must concur to create a low density of energy within laser spot on surface to be machined. Smoothening of micro geometry can be achieved when working with improved values of technological main parameters as: power density, feed rate of laser beam, and beam dimension on machined surface. 2. BASIC MATERIAL REMOVAL MECHANISM The laser spot, covering the machined surface, firstly attacks the micropeaks of surface profile, determining their rapid melting. Thus, the corresponding molten material flows towards microvalleys. Therefore, if initially, the surface is characterised by high roughness (fig. 1.a), through very superficial melting of material, smoothening of microgeometry can be achieved (fig. 1.b) [ 1 ], [ 2 ]. Currently, there are two mechanisms, with corresponding phenomena, and different values of technological parameters, modifying the machined surface during laser processing: Surface Shallow Melting (SSM) and Surface Over Melting (SOM). The principle of each mechanism is determined by different levels of power and feed rate [ 2 ], [ 3 ]. a) b) a) before; b) after laser finishing Fig. 1. Surface profile

2 2.1 Surface Shallow Melting In case of SSM, when machining with high feed rate and low power, then only thin material layer is melted. This molten volume flows toward microvalleys due to gradient of superficial tension and gravity force. The SSM mechanism has particular deployment in case of metallic sinterised surface which nowadays, it is frequently encountered. This specific structure can be considered an aggregate of spheres. A micropeak corresponds to the upper part of a sphere and a microdepression matches the space formed by three tangent spheres. During finishing process, the laser beam attacks the upper parts of the spheres and these surfaces rapidly arrive to the melting temperature [ 4 ]. 2.2 Surface Over Melting When the feed rate of laser beam is decreased, then an overheating of the surface occurs. For that reason, complete melting of contact surface with laser beam is produced, resulting increasing of initial roughness; this is the essence of SOM. Thus, laser spot movement determines flowing of molten material along the tension gradient direction, till solidification front, creating a wave at its proximity. 3. TECHNOLOGICAL PARAMETERS All input technological parameters [ 4 ] contribute to decrease energy density (q E ), which is the parameter characterising the high-quality deployment of LBM finishing. The variables interfering in calculus of energy density are feed rate (v a ), power (P) and laser beam diameter (d), according to the following relations: [v a ]=[mm/s] [P]=[W]=[J/s] (1) [d]=[mm] As a result, energy density (q E ) is calculated with relation: q [ P] [ J / s] [ J ] = = (2) 2 [ d] [ v ] [ mm] [ mm / s] [ mm ] E = a Focal point position (x) is another parameter, which has to be considered in order to control the energy density, which arrives on surface to be machined. This means the distance [mm], from the workpiece surface till focal point where laser beam is convergent. Varying the distance between workpiece to be machined and working head (standoff distance), the spot dimension can be adjusted, resulting spot diameters greater than 2 mm (fig. 2). Therefore, the machined surface at each passage of laser beam has greater width than the one obtained at cutting for example. For different machining processes, the focal point can be situated in three distinct positions, as it is shown in fig. 2: a) focalised (just on the surface to be machined); b) above the surface to be machined (defocalised) the most used variant; c) beyond the surface to be machined (prefocalised). When laser beam is focused just on the surface, the energy density delivered on the workpiece surface has maximum value and the machined area is minimal. In case b), the energy density lowers adjusting the standoff distance. The case c) has to be avoided because of inner damages that could be produced due to energy concentration within material to be machined. position b) position a) position c) x working head workpiece Fig. 2. Spot diameter adjusting by variation of standoff distance d standoff distance

3 Fig. 3. LBM finishing with two different laser spot diameters Another important advantage is obtained by increasing the spot diameter. A greater width of workpiece surface can be finished by a single passage. Thus, the greater is the spot diameter, the fewer passages on machined surface are necessary, as it is illustrated in fig. 3. A correct value of spot diameter also solves two technological problems: the superposition of beam passages for complete polishing of a surface; the uniform distribution of energy density on laser spot. In these conditions, CO 2 laser (which is used within these researches) polishing has not the desired degree of uniformity due to energy distribution, which is defined by Gauss curve. From this point of view, the using of a laser with diodes it is more advantageous if the power provided is sufficient. The necessary energy applied to machined surface for the finishing process also depends on absorption capacity of material, related to its structure and thermo-physical characteristics as well as on initial roughness of surface, as obtained experimental data will emphasise. The final technological parameter which has to be considered is the nature and the pressure (p) of aiding gas. Every laser processing uses different gas types, which have distinctive functions within machining mechanism. The aiding gas or process gas flows along the laser beam. Depending on process and material to be processed, the gas can be active, like oxygen, or inert gases, as argon or helium. The oxygen, based on exothermal reaction, facilitates the metal melting. In this case, the gas pressure influences the evacuation of molten metal [ 5 ]. In other cases, an inert gas can be used in order to obtain a separation between fusion zone and external medium and consequently, controlled machining conditions. This is the case of laser finishing of sinterised material, LaserForm ST-100, machined during our experimental researches, when it is used argon. In order to avoid melting a too deep layer, the laser energy density on machined surface, has to be diminished. Therefore, firstly, the feed rate is maximized, leading to working time decrease. Once the operating mode is established, two possibilities emerge: power decreasing or spot diameter increasing. These two options should have the same effect, i.e. the energy density decreasing, taking also into account the practical aspects of machining. It must be mentioned that when machining with small laser spot, the trend to produce SOM mechanism is also obvious. 4. EXPERIMENTAL RESULTS The experimental research was completed on different materials, using a Robotiker installation, which comprised a CO 2 laser with exit maximum power of 2500 W, part of the Technological Park of Zamudio, Spain. The laser device had a Weidmüller 5 KN/MG working head with a ZnSe focalisation convex lens, with 200 mm focal distance, and 0.4 mm minimum spot diameter [ 4 ].

4 The thermophysical characteristics of machined materials, i.e. absorption capacity of laser radiation, were correlated with technological parameters obtaining an improvement of surface quality by LBM. The roughness obtained on the samples was measured through an electronic device and then computer aided processed in order to obtain 3D microtopography. 4.1 LBM Finishing of ORVAR Steel Orvar Supreme was one of machined material, alloy steel for tools with high temperatures and pressures resistance. The chemical composition of this steel is: C 0.3; Si 1.0; Mn 1.4; Cr 5.2; Mo 1.4; V 0.9 [%]. The Orvar density lowers from 7800 to 7600 kg/m 3 and thermal conductivity (K) grows from 25 to 30 W/m o C when temperature varies between 20 and 700 o C. It can be noticed, analysing entirely material physical properties, that Orvar steel has relative good absorption capacity of laser thermal energy mainly, due to high metal carbides content; their thermal conductivity is usually in the range of W/m o C. The high initial roughness of surface resulted from previous machining also increases the thermal absorption capacity of material. In order to emphasise, the effect of laser finishing, the workpiece was machined through LBM only on the centre, the sample margins remaining in the initial state after milling (fig. 4). The results were obtained by linear crossings of the laser beam on the surface. The laser paths were oriented perpendicular on previous machining traces with widths between mm and over positioned for a complete finishing of entire area. Considering the good absorption capacity of Orvar steel, we avoided to degenerate the finishing process to SOM mechanism, correlating material thermo-physical properties with appropriate technological parameters. Consequently, we increased the working power to 1200 W (at high initial roughness), the standoff distances to 27 mm, the laser traces overposition up to 0.4 mm, obtaining R a improvements of up to 55% at single passages over the surface. In case of this material, LBM finishing is usually able to reduce initial roughness with more than 50% if it is aimed to diminish the Gaussian distribution through overposition and defocalisation by decreasing the energy density on laser spot. 4.2 LBM Finishing of F114 Steel The second material finished through LBM was F114 steel with following chemical composition: C 0.18; Mn ; Si 0.3; P max 0.04; S max 0.05 [%]. The main physical properties related to LBM thermal mechanism of material removal are: density 7850 [kg/m 3 ] and thermal conductivity 49.8 [W/mºC]. In comparison with the previous machined steel, F114 has a lower absorption capacity due, in principal, to higher content of Feα constituent, which has relative high value of K= W/mºC. An example of machined F114 steel 3D microtopography is presented in fig. 5. R a =0.583 µm LBM R a =1.299µm Milling R a =0.282 µm LBM R a =0.938 µm EDM Material: Orvar steel; Power=1200 W; Feed rate= 1300mm/min; Standoff-distance= 27 mm Traces overposition=0.4 mm Fig. 4. Results obtained on Orvar steel Material: F114 steel; Power = 1700 W; Feed rate =1500mm/min; Standoff-distance=40 mm Fig. 5. Results obtained on F114 steel

5 As in previous case, the initial state of surface resulted from electrodischarge machining (EDM) was kept on sample margins and for comparison, the middle was finished through LBM. The process efficiency was more perceptible in case of previously EDM-ed surfaces (R a around 0.93 μm) than in case of milled ones (R a around 0.8 μm), under appropriate working conditions because of higher thermal absorption capacity. Therefore, we used laser powers within W range and standoff-distances between mm in case of milled surfaces, thus decreasing energy density through increasing spot diameter. In comparison, at laser finishing of EDMed surfaces, the laser power was higher ( W) and reduced standoff distances (20-40 mm). As a result, at milled surfaces finishing, the energy density was lower through effective laser power diminishing. As it can be noticed, in case of this material, we correlated its lower absorption capacity with greater power. The other technological parameters as feed rate and standoff distance contributed to produce SSM through decreasing of laser energy density on spot. From these preliminary researches, it results that LBM finishing has the capacity to reduce initial roughness with up to 70% after single passages. Additionally, LBM finishing process is more efficient, i.e. improves more the surface roughness, when the initial roughness is higher. 4.3 LBM Finishing of LaserFormST-100 The third material machined through laser finishing was LaserForm ST-100 obtained through Selective Laser Sintering (SLS) and based on the mix of stainless steel and bronze powders. The main physical properties of this composite material, influencing the laser thermal mechanism are: density 7700 kg/m 3 (at 23 o C), thermal conductivity 49 W/mºC (at 100 o C) and 56 W/mºC (at 200 o C). Apparently, these characteristics are not far from those of previous material. But in this case, LBM finishing encounters a major problem, consisting in great difference between melting points of bronze and stainless steel. This material characteristic leads to SOM mechanism in case of bronze particles, having lower melting point. In contrast, stainless steel particles are affected much less. Taking into account the materials properties mentioned above, the choice of working parameters becomes very difficult. Additionally, an aiding gas like argon is necessary due to strong oxidation of machined surface. Working with 1200 W laser power, 22 mm standoff-distance and feed rate between mm/s, surface roughness (R a ) was obtained in the range of μm, starting from R a = 7.51 μm, initial roughness. Some preliminary results of LaserForm ST-100 LBM finishing are presented in figure 6. Comparatively, the energy density used at this current material, by far more difficult to be machined, was higher than in earlier case because stainless steel component itself has low absorption capacity. This leads to SOM mechanism in lower melting points volumes (bronze component). In order to avoid SOM degeneration of the surface smoothening process due to composite material sensibility to over melting, we adopted the following solutions for technological parameters: low values for standoff distance and feed rate and medium values for power. R a =0.523 µm LBM R a =7.51 µm SLS Material: LaserForm ST-100; Power = 1500 W; Feed rate =1300mm/min; Standoff-distance=22 mm Fig. 6. Results obtained on LaserForm ST-100 µm

6 5. FUTURE RESEARCHES The objective of our researches is significant increase of surface smoothening level and obtaining nanometric values for R a surface roughness. This could be achieved by an accurate computer control of the process focused on correlation of thermal absorption capacity with technological parameters and, on the other hand, by elaboration of specialised software, necessary to generate the optimum trajectory of laser spot. The software is based on an iterative algorithm to obtain a maximum decreasing of surface roughness at each passage of laser spot over the surface. At each step, the parts of surfaces with high protrusion are identified by sectioning the 3D microtopography - obtained at previous machining and determined by microgeometry scanning device - with a section plane, parallel with the horizontal one. The positions of sectioning horizontal plane are deeper until the required level of smoothening is obtained. Thus, high level surface zones are identified, which must be covered by laser spot by transferring their coordinates to machine code instructions followed by laser installation CNC. 6. CONCLUSIONS The present state of researches concerning LBM finishing highlights that significant roughness improvement of up to 70 % of machined areas can be achieved by adjusting the technological parameters of laser finishing process, taking into account material absorption capacity. As in other cases of thermal energy based finishing technologies, e.g. EDM, better results are obtained on homogenous materials with lower absorption capacity. To avoid an uneven energy density on laser spot due to Gaussian distribution, the overposition of linear polishing traces is essential, achieving thus, the entire surface finishing. A possible surface smoothening at nanometer scale could be accomplished through iterative stages. On the basis of microgeometry scanning, the laser beam could follow, using a CAM system, the microelevations traces resulted from earlier machining, gradually levelling the surface. 7. REFERENCES 1. Marinescu, N. I., Machining Technologies with Beams and Oscillations, Bren, Bucharest, Burakowski, T. Wierzchrow, T. Surface Engineering of Metals, Boston, CRC Press, New York, USA, Ramos, J.A., Bourell, D.L., Beaman, J.J. Surface Over-melt During Laser Polishing of Indirect-SLS Metal Parts, Materials Research Society Symposium Proceedings, 2003, 758, Marinescu, N. I., Ghiculescu, D., Anger C. Some results concerning laser surface finishing, Proceedings of 6th European Space Agency Round Table on Micro & Nano Technologies for Space Applications, Noordwijk, The Netherlands, 2007, Ghiculescu, D., Nonconventional Machinings, Printech, Bucharest, ADDITIONAL DATA ABOUT AUTHORS Marinescu, Niculae, Prof., Ph.D; niculae.marinescu@yahoo.com Ghiculescu, Daniel, Assoc. Prof., Ph.D.; liviudanielghiculescu@yahoo.com Seritan, George, Lecturer, Ph.D., george@seritan.ro University Politehnica of Bucharest; Splaiul Independentei 313, sector 6, Bucharest, Romania, phone: Jitianu, Gheorghe, Eng., SC EDMING SERV CONSULT SRL, Al. Lunca Siretului nr.4, sector 6, Bucharest, Romania, gelu.jitianu@gmail.com, phone: