Abrasive waterjet factors sensitivity identification.

Hloch, S. ; Fabian, S.

Abstract: The paper deals with experimental research and evaluation of abrasive waterjet cutting process by evaluation of process factors, which influence the micro geometry feature Ra of 10 mm thick stainless steel AISI 304, surface in depth 1, 5, 9 mm through design of experiments. Significance of four independent variables (traverse rate, abrasive mass flow rate, pressure and J/T abbreviation that influence on the surface quality has been evaluated by factors analysis type [2.sup.4]. The regression equations obtained from ANOVA and multiple linear regressions give the level quality Ra as a function of the treatment factors. Different factor significance has been found, which generated surface profile under defined conditions by abrasive waterjet. Study has confirmed some research works about influence of feeding direction and traverse direction witch probably causes roughness asymmetry with the change of 180[degrees] feeding direction that is connected also with wavy particle distribution.

Keywords: abrasive waterjet, factor analysis, Ra, DoE

1 INTRODUCTION

The work presented in that study investigates a micro-geometrical aspect of the cutting quality of the average roughness. At the top of the surface, erosion at shallow impact angles controls the abrasive wear mode, while deformation wears by impacts at large angles takes place further down the kerf (Hashish, 1987). Abrasive water jet (awj) machined surfaces exhibit the texture typical of machining with high energy density beam processing It has a superior surface quality in the upper region and rough surface in the lower zone with pronounced texture marks called striations. The nature of the mechanisms involved in the domain of awj machining is still not well understood but is essential for awj control improvement. In spite of great research effort and good knowledge in the field of abrasive waterjet cutting, there are number unexplained facts. One of them is influence of process factors on workpiece surface quality. (Hloch & Blagodarny, 2001)

2 RELATED WORKS

Most scientific papers concerning to the evaluation of microgeometrical features of abrasive waterjet cutting are available (Annoni, Monno, 2001); (Annoni et al., 2001). The object is to determine the final shape of the surface quality, which is a function of the geometric characteristics of the abrasive waterjet tool and its awj factors that are divided into two basic groups (fig.2); direct and indirect. Factors of indirect group influenced quality of the created tool where hydrodynamic factors, mixing factors and abrasive factors belong. These factors influence the qualitative characteristics of the tool, the speed, diameter kinetic energy of the stream. Generated tool through these factors enters to the cutting technology process at material at the large number locality, by means of direct factors. There belongs traverse rate; stand off distance, impact angle and number of passes. Through cutting factors, created tool hits the workpiece the at upper erosion base (fig.1), where erosion process begins. These factors create surface as area of trajectory working movement of abrasive waterjet. It is specific way of material machining because there are used particles with more edges; that are random oriented in the liquid phase waterjet. This random position and different shape of abrasive particles causes irregular removal mechanism of material. Specific of abrasive waterjet technology is that consist of three phases (liquid, solid and fluid) (Lebar & Junkar, 2004).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

3. EXPERIMENTAL DESIGN

In order to investigate the influence of abrasive waterjet process factors on average roughness Ra, [2.sup.4] design for four independent variables has been designed. Full factorial analysis has been used to obtain the combination of values that can optimize the response. Factors submitted for the analysis in the design of each constituent at levels [-1; +1] are listed in the table 1.

The behavior of the present system is described by equation (1), which includes all interaction terms regardless of their significance (Blagodarny et al., 2002)

y = [b.sub.0][x.sub.0] + [b.sub.1][x.sub.1] + [b.sub.2][x.sub.2] + [b.sub.3][x.sub.3] + [b.sub.4][x.sub.4] + ... + [b.sub.1234][x.sub.1][x.sub.2][x.sub.3][x.sub.4] (1)

According (fig. 3), each cut has been replicated three times, yielding total of 48 cuts. From the experimental material has been made two specimens series A and B, both with number of 8 specimens groups. The set A has been made with the configuration of J/T abbreviation [D.sub.v]/[D.sub.A] = 0,14/1,2. The set B has been made with J/T configuration of Dv/[D.sub.A] = 0,1/1. The properties of each sample are: length 35 mm, width 8 mm, and height 10 mm. The abrasive used in this experiment is recycled Barton garnet, Mesh 80.

[FIGURE 3 OMITTED]

4 RESULTS AND DISCUSSION

The regression equations below express mathematical dependence average roughness behavior Ra at factors combinations on sample depths [h.sub.1]=1mm, [h.sub.5]=5mm. [h.sub.9]=9mm. The models, expressed by equations, were generated by linear multiple regression of the data and is a function of the more significant variables [y.sub.Ra1], [y.sub.Ra5] and [y.sub.Ra9]:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

All regression models contain four linear terms. The regression coefficients and equations (2, 3, 4) obtained after analysis of variance give the level of significance of variable parameters tested according Student's t-test. The graph (fig. 4) shows linear fits of investigated factors influenced average roughness Ra in depth [h.sub.9] = 9 mm with the optimization center, that have been obtained according equation (4).

[FIGURE 4 OMITTED]

It has been found (fig. 4) that traverse rate is more sensitive factor as abrasive mass flow rate. With an increasing speed of abrasive cutting head the average roughness increase. This observation agrees with results on aluminum, ceramics, metal-matrix composites, glass, and fiber-reinforced composites. An increase of the pressure, in general, improves surface quality. With pressure increase the abrasive increases water jet kinetic energy. The increase in the number of impacting particles at lower traverse rates contributes to the improved surface finish. The additional particles serve to smooth the surface that forgoing particles generated. Material being machined by factor [x.sub.1] at level -1, samples set B, is characterized by lower surface roughness. In fact [x.sub.1] is J/T abbreviation, is change diameters of focus tube and diameter of water orifice. But it is assumed that the small diameter water orifice causes the speed water jet and that causes the higher velocity of formed cutting tool. Thus abrasive water jet disposes higher energy concentrated to the smallest area of the work piece. Out of measured data result those roughness average values an asymmetry of the roughness values because the samples created for this purpose have been cut in two directions +180[degrees] and -180[degrees]. That phenomenon is may be caused probably due to feeding direction of solid phase and consequently distribution of abrasive particle in the waterjet (fig. 5) (Chen & Siores, 2001).

[FIGURE 5 OMITTED]

5 CONCLUSION

The problem analyzed is the study of awj cutting in terms microcutting quality. The quality parameter average roughness has been measured. This analysis has pointed out that variable independent factors influence the morphology of cutting surface. It has been found that influence of process factors are variable related to different depth. Evaluation has been carried out according to DoE. Full factorial design has been used to study effects of selected process factors. The selected awj factors, has been evaluated their significance and their impact to the Ra as a dependent variable. Obtained regression equations after analysis of variance give the level quality as a function of the process factors. It has been found that pressure, and traverse rate are important with the depth. It has been observed that dominant factors influencing quality are pressure and traverse rate. Experiment also proved an asymmetry of the roughness values due to the feeding direction of solid phase--abrasive at the entry of the mixing chamber. According theses results new experimental scheme will be created to continue of evaluation abrasive water jet machining. That phenomenon will be studied by special prepared experiment (DoE [2.sup.5]) because the feeding direction of the abrasive to the mixing chamber was not perpendicular to the feed rate direction.

6. REFERENCES

Annoni, M., Monno, M. A lower limit for the feed rate in AWJ precision machining, BHR Group 2000 Jetting Technology, p. 285-295, ISBN 1 86058 253 2 archives/1297/1297wj.html, 2001,

Annoni, M., Monno, M., Vergari A.: The macrogeometrical quality of the kerf in the awj process parameters selection, Politecnico di Milano--Dipartimento di Meccanica, Milano, Italy,

Blagodarny, V., Hloch, S., Kmec J. Planovanie experimentov vplyvu technologickych parametrov vodneho luea na kvalitu obrobenej plochy. In. Vedecka konferencia s medzinarodnou uca-stou: Informatika a algoritmy 2002, ISBN 80-88941-21-0,

Chen, F. L., Siores, E. The effect of cutting jet variation on striation formation in abrasive water jet cutting, International Journal of Machine Tools & Manufacture 41 (2001) 1479-1486,

Fabian, S., Hloch, S. Prispevok k technickym moznostiam zvysovania kvality produktov technologie VHAP. In: AT&P journal. 1335-2237: XII, 2005.

Hashish, M.: An Improved Model for Erosion by Solid Particle Impact. In: Proc. Of the 7th International Conference on Erosion by Liquid and Solid Impact, ELSI VII, Cambridge, England, Sept. 1987, 66/1-66/9,

Hloch, S., Blagodarny, V. Mechanizmus tvorenia VHAP. In: Seminar doktorandov. Trenein 2003. ISBN 88 914-80-9.

Hloch, S., Radvanska, A. Application of Doe in manufacturing engineering. In: Management of manufacturing systems; Presov : 2004. s. 100-105. ISBN 80-8073-209-4.

Lebar, A.; Junkar, M.: Simulation of abrasive water jet cutting process. Modelling Simul. Mater. Sci. Eng. 12 (2004) 1159-1170.

Tab. 1 Coded factors at various levels (Blagodarny et al., 2002)

 Factors Factor level
N V. Terminology -1 +1

1 [x.sub.1] J/T abbreviation [mm] 0,1/1 0,14/1,2
2 [x.sub.2] Abrasive mass flow rate [g 300 500
 x [min.sup.-1]]
3 [x.sub.3] Pressure [MPa] 200 350
4 [x.sub.4] Traverse rate 70 120
 [[mm.min.sup.-1]]