EXPERIMENTAL ANALYSIS OF THE INFLUENCE OF FEED RATE ON QUALITY OF CUTS PERFORMED BY AWJ

Abstract: Abrasive waterjet (AWJ) cutting technology is one of the most recent developed non-traditional processes used in industry for the processing of difficult-to-cut materials which usually lead to a pronounced wear of conventional tools and generation of damaged surfaces. The efficiency and the quality of AWJ depend on many parameters, some of them precisely controllable (e.g. water jet pressure, feed rate) while others uncontrollable (e.g. focusing tube size, waterjet orifice size). The aim of current study was to analyze the influence of feed rate at low working pressures on the quality of cuts, quantified by the parameters specified in the ISO/WD/TC44 N 1770 standard.


INTRODUCTION
Water jet cutting is an unconventional technology used to process a wide range of materials from food to all kinds of difficult-to-cut materials (hard materials -e.g.ceramics; structurally heterogeneous -e.g.composites; heat sensitive -e.g.shape memory alloys; higher strength at elevated temperatures -e.g.aerospace alloys), the later requiring the entrainment and mixing of dry abrasives with a high-speed water jet (abrasive water jet cutting) [1,2].
Presently, the most common application of the abrasive water jet technology is cutting, due to the distinct advantages over other cutting processes as: it does not induce thermal distortions in the workpiece that may alter its mechanical properties or visual appearance; it has the ability to cut virtually any material, in any direction, without excessive forces; no burrs or rough edges are formed that may require subsequent removal; it is environmentally friendly [6,7].
All these advantages can be achieved if the proper combination of process parameters (hydraulic, abrasive, work material and cutting parameters -Figure 1) is chosen.According to the quality requirements, part of these parameters especially the cutting feed rate, water pressure and the abrasive flow rate are precisely controllable and is being adapted [8,9].
The aim of current study is to analyze the influence of feed rate at low working pressure on the quality of cuts performed by AWJ, quantified by the following parameters: width of the processed surface at the jet inlet (Li)/outlet (Lo), deviation from perpendicularity (u), inclination angle (α) and surface roughness (Ra).

EXPERIMENTAL SETUP
The experiments were run on common industrial used carbon steel, namely OL 37 (STAS 500), with the chemical composition and mechanical properties presented in Table 1 and Table 2, respectively.Fig. 1.Parameters influencing the AWJ cutting process [8].
The thickness of plates was of 6.5 mm.AWJ cutting tests were conducted on a Hydro-Jet Eco 0615 machine equipped with a piston pump, capable of providing a maximum pressure of 150MPa and a Roctec 76 nozzle.The most extensively used abrasive, GMA 80 garnet, was chosen for this investigation.Three values for the feed rate were tested: 50 mm/min, 100 mm/min, 150 mm/min.The constant process parameters are presented in Table 3. [mm] 1.02 The quality of cut surfaces was quantified in terms of parameters specified by the ISO/WD/TC 44 N 1770 standard, namely: width of the processed surface at the jet inlet (Li), width of the processed surface at the jet outlet (Lo), deviation from perpendicularity (u), inclination angle (α) and roughness (Ra).The accuracy parameters were measured with a Leica MZ75 microscope with PC interface.
The surface roughness was performed at 10% of the material thickness, in three points on the cut section using a Mitutoyo SJ 201 digital roughness device.At each location three measurements were recorded, their average being taken into account to analyze the dependence of roughness on the feed rate.

Dimensional accuracy
The variation of parameters that quantify the quality of cuts as a function of feed rate is presented in the graphs bellow.The influence of feed rate on the width of processed surface at the jet inlet (Li) is presented in Figure 2.
The experimental results show that the higher the feed rate, the smaller the value of Li parameter.The explanation is related to the amount of abrasive grains that come in contact with the material to be processed, which is smaller for higher feed rates.
The variation of the width of processed surface at the jet outlet (Lo) with feed rate is presented in Figure 3.It is observed that the increase of feed rate leads to the decrease of Lo parameter, too.
However, it must be noticed that if too high values of feed rate are used, it is possible to compromise the cut since the fluid jet (mix of water and abrasives) does not penetrate the entire thickness of material.On the contrary, at low feed rates the processing productivity decreases, fact that should be avoided.
1.17 at the jet outlet with feed rate.
The variation of deviation from perpendicularity (u) as a function of feed rate is presented in Figure 4.The obtained data highlights that deviation is getting higher as the value of feed rate increases.
This variation is explained by the fact that the fluid jet does not erode correctly the material at high feed rate, since the shape of the fluid jet is conic and it has not the necessary power at the edges to erode the workpiece material.The same influence of feed rate is manifested on the inclination angle (α) as showed in Figure 5.It is observed that higher feed rates lead to higher values of inclination angle.

Surface roughness
The surface roughness obtained by abrasive water jet cutting as a function of feed rate is presented in Figure 6.It is observed that the surface roughness is getting coarser as the feed rate increases.

Fig. 2 .
Fig. 2. Variation of width of the processed surface Fig. 3. Variation of width of the processed surface at the jet inlet with feed rate.at the jet outlet with feed rate.

Fig. 4 .
Fig. 4. Variation of the deviation from Fig. 5. Variation of the inclination angle with feed rate.perpendicularity with feed rate.

Table 1 .
Chemical composition of the used material OL 37 (S 235).

Table 2 .
Mechanical characteristics of the OL 37 carbon steel (S 235).

Table 3 .
The AWJ working parameters.