The study of the roughness established in laser beam machining of glass reinforced composite polymeric materials.

Ionescu, Nicolae ; Visan, Aurelian ; Doicin, Cristian 等

1. INTRODUCTION

The laser beam cutting work- through has lately undergone a major interest, from both an industrial and from a research point of view. As far as the composite materials work-through is concerned, the main strong points in favour of using this procedure are associated to some widely followed-up industrial applications such as: the lack of the work-through forces, a constant stability, a proper availability etc.

Among the main limits of the work-through applicability, one can counter both the insufficient knowledge level of the roughness function obtained when different composite material are worked through and the insufficient knowledge level of roughness "addiction" to the process factors (Iliescu et al., 2007). In order to solve the problem, one must find out both the roughness determination and the roughness dependence of the electrical and technological parameters.

The authors' new proposed and designed solution consists of the experimental modelling of the process by finding out the "roughness-of-cutted-surface" process function determination and of its interaction with the main electrical and technological parameters (namely the laser beam power, the pattern of the worked-through material) due to the experimentation of three materials and of their thickness.

Taking into consideration the results provided by the authors, there is hope to be a future opportunity of going on with the research to elaborate and experiment a mathematical approach to optimising the process parameters such as the defining of the goal function and of the restrictive functions, functions among which "roughness-of-cutted-surface" function to become one of the restrictions of the model.

2. METHODOLOGY AND MEANS OF RESEARCH

The variables and the process functions. In the instance of laser cutting we appreciate that the independent global variables are, basically speaking, the processed material and the work regime. As regarding the processed material, for the specific case of the laser cutting of the polymeric composite materials, a series of parameters connected to the material are particularly important, like the thickness of the material, the nature of the material of the basic matrix, the type of fibbers, the arrangement of fibbers, the fibber glass concentration, etc.

The variables associated with the pattern of the material have to be taken into consideration, so we have designed three representative composite polymeric materials, having as base material the polyesteric resin and armed with glass fibbers.

[FIGURE 1 OMITTED]

The name of these representative materials have been chosen depending on the shape, the dimensions and the fibbers arrangement, as follows: STRATIFICATION, symbolised in the paper as ST, for which the fibbers are long, and arranged in layers (Figure 1a), CHOPPED FIBBER, symbolised FT, for which the glass fibbers are shorter than 10 mm and randomly arranged (Figure 1b) and FABRIC, symbolised TS, for which the glass fibbers are long and resemble a woven cloth, arranged in several layers (Figure 1c).

The base matrix is an orthoftaltic polyesteric resin with moderate reactivity (Hadar, 2002). The main characteristics of the glass fibbers are: fibber diameter--13[micro]m, density--2,54 g/[cm.sup.3], hardness--6,5 MOHS and tensile breaking strength 1,47 GPa. The fibber glass contents are 25-30% for FT, 30-35% for ST and 35-40% for TS.

As concerning the work regime, based on the preliminary research we have established the following main parameters: the power of the laser PL (W), the thickness of the material g (mm) and the cutting speed v (m/min). The pressure of the processing gas--nitrogen, was maintained at the constant value of 6 bar.

This paper presents the regarding "The roughness of the cut surface" process function determination for those three under research materials, namely Yi = Rai = Rai ([P.sub.L], v, g) [[micro]m], under a general form (Gheorghe et al., 1985):

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

For those three materials above mentioned, i = FT, ST and TS, we have established the absolute process indexes [I.sub.aFT], considered to be a referential one, [I.sub.aST] and [I.sub.aTS], that will be calculated for the central values of the independent variables, respectively [P.sub.L] = 1330 W, v = 2,6 m/min and g = 3,5 mm. Also, we established the relative process indexes [I.sub.rFT] = [I.sub.aFT]/[I.sub.aFT] = 1, [I.sub.rST] = [I.sub.aST]/[I.sub.aFT] and [I.sub.rTS] = [I.sub.aTS]/[I.sub.aFT].

Characteristics of the means of inspection and the measuring mode. Roughness has been measured as follows (Amza, 2007): for materials with thickness less or equal to 2 mm--in the middle of the thickness of the material--and for materials thicker than 2 mm--at 2/3 of the thickness of the material from the side where the laser goes in.

Experimental programme and variation intervals for the processing regime parameters. A programme consisting of 12 experiments has been undergone in order to find out the roughness process function determination for those three under research materials, a programme in which each of the three under variation parameters has got three variables, variables determined due to some preliminary attempts such as: [P.sub.L] = 570; 1330 and 3100 W; v = 1,5; 2,6 and 4,5 m/min; g = 2; 3,5 and 6 mm.

[FIGURE 2 OMITTED]

3. EXPERIMENTAL RESULTS

After conducting the experiments (Popescu, 2004) and mathematically processing the data, we have determined the following process functions:

[Ra.sub.ST] = [e.sup.2,160] x [P.sup.-0,148.sub.L] x [v.sup.0,314] x [g.sup.0,048] (2)

[Ra.sub.FT] = [e.sup.1,730] x [P.sup.-0,057.sub.L] x [v.sup.0,010] x [g.sup.0,049] (3)

[Ra.sub.TS] = [e.sup.1,794] x [P.sup.-0,062.sub.L] x [v.sup.0,011] x [g.sup.0,057] (4)

Absolute micro-geometrical precision indicators have been determined, based on (2), (3), (4) functions as follows: [I.sub.aST] = 4,287; [I.sub.aFT] = 4,018 and [I.sub.aTS] = 4,178 and relative micro-geometrical precision indexes: [I.sub.rST]=1,066; [I.sub.rFT]=1,00 and [I.sub.rTS]=1,039.

4. THE INFLUENCE OF THE PARAMETERS OF THE WORK REGIME ON ROUGHNESS

The influence of power on roughness. Based on these functions, we can establish that for the three materials being studied, the roughness lowers when the power increases, according to figure 2, designed for ST material. This can be accounted for by the fact that with the increase of the laser power, the amount of energy transferred to the material increases and, as a consequence, the amount of vaporised material increases. We notice that we get similar values of roughness for the three materials and that the variation of power within the limits 200-3500 W does not lead to a significant reduction of roughness. Also, we can appreciate that, for most of the values of power, the highest values of roughness are those for the material ST, followed by the materials TS and FT.

The influence of speed on roughness. We've found a slight increase in the roughness when the cutting speed increases within the limit 0,5, ..., 5 m/min (Figure 3).

This can be accounted for by the fact that with the increase of the cutting speed, the contact time between the laser beam and a certain area of the material decreases and, as a consequence, the amount of heat transferred to the material decreases and thus the mass of vaporised material decreases. We notice that, for most cutting speed values, the roughness has its highest values for the material ST, followed by the materials TS and FT respectively.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

The influence of thickness on roughness. We notice that, the roughness slightly increases with the increase of the thickness of the material (Figure 4). This can be accounted for by the fact that, with the increase of the thickness, the amount of material that needs to be vaporised, at the same power level, increases, which determines a slight melting of the fibber glass.

We can establish that the smallest variation of the roughness when thickness varies, is that of the FT material. The smallest roughness being that of the material FT followed by that of the material TS and that of the material ST. This observation leads to the conclusion that the roughness depends on the fibber glass percentage but also on its distribution.

5. CONCLUSIONS

1. The roughness increases when the g thickness and the v speed are increased and decreases when the [P.sub.L] power increases.

2. The strongest influence on the roughness is that of the speed v, for ST, of the thickness of the material g, for TS and the power [P.sub.L] for FT.

3. For most of the regimes, the smallest values of roughness are obtained for FT, followed in order by TS and ST.

4. For all three studied materials, we find a slight reduction of roughness with the power increase within the interval 200, ..., 3500 W, a relatively small increase in the roughness with the increase of the thickness g within the interval 0,5, ..., 6 mm and a slight increase in the roughness with the increase of the cutting speed v within the limit 0,5, ..., 5 m/min.

5. The relatively close values of the roughness for the three materials, lead to the idea of extrapolating the results for other types of materials with similar structures and properties.

6. The limitations of our research are given by the difficulty of results' extrapolation beyond the studied area of values.

7. The results of our research will be used to elaborate a mathematical model to optimise the process parameters from the micro-geometrical precision point of view.

6. REFERENCES

Amza, C. (2007). Intelligent X-Ray Imaging Inspections System for Composite Materials with Polymeric Matrix, Composites--Statistic Models of Surface Roughness, Plastic Materials, No. 4, MPLAAM 44(4)2007, pp. 326-331, ISSN 0025/5289

Gheorghe, M. et al. (1985). Algorithm for regression functions, Scientific Bulletin of POLITEHNICA University of Bucharest, Series D, ISSN, 1220-3041, tom XLVI-XLVII, pp. 176-189

Hadar, A. (2002). Stratified Composite Structures, Romanian Academy Publ., ISBN 973-27-0961-8, Bucharest

Iliescu, M., Spanu P., Costoiu, S., (2007). Glass Fibres Reinforced Polymeric Composites--Statistic Models of Surface Roughness, Plastic Materials, No. 4, MPLAAM 44(4)2007, pp. 365-369, ISSN 0025/5289

Popescu, D. (2004). Contributions in determination of technological features of laser beam machining, Ph.D. Thesis, POLITEHNICA University of Bucharest