SURFACE TEXTURE ANALYSIS OF THE X5CRNI18-10 STEEL AFTER CUTTING WITH ABRASIVE WATER JET AND PHOTON BEAM (LASER)

: The article presents an analysis of the geometric structure of the surface of X5CrNi18-10 steel sheet after cutting with a abrasive water-jet (AWJ) and a photon beam (laser). Both methods and the workpiece material were also described. Using the laser triangulation method, the cut surface texture was measured by opto-digital microscopy. Additionally, microscopic images of the cut surface were made using the Dino-Lite Edge AM7915MZT microscope by ANMO Electronics Co. The analysis of changes in the value of the Sa parameter for the surface after laser cutting and abrasive water-jet showed that the thickness of the cut material has the most significant influence on the obtained measurement results.


INTRODUCTION
Cutting is commonly referred to as the process of separating the material along the entire thickness of the material, along the cutting line.A variant of the cutting process is thermal cutting, where the material subjected to this process is liquid due to the supplied thermal energy, then it is blown out using the kinetic energy of the gas stream or reaches a sufficiently high temperature at which it is oxidized in a pure oxygen stream [6].The surface of the material to be processed is shaped by means of a highly concentrated supersonic jet of water with an admixture of abrasive.The process of photon jet cutting is carried out by means of a laser beam.The laser is a device emitting electromagnetic radiation from the visible light, ultraviolet or infrared range, which uses the phenomenon of forced emission.
The above described methods are used in manufacturing companies, however, due to the condition of the surface after cutting, their use involves with quality concessions, which are generated during the process.The elimination of quality problems after cutting is often associated with a longer process time.The photon beam (laser) cutting is faster compared to water-jet cutting, but it causes quality defects of the cut surface, such as, for example, a wider heat affected zone or defects in the cut material.
The aim of the research was to assess the condition of the surface after photon beam (laser) cutting and water-jet cutting, as well as to compare their surface texture at different process parameters.

ABRASIVE WATER-JET CUTTING
The high-pressure water jet is perfectly suited for cutting different harnesses of materials, providing an even and extremely accurate cut, as shown in Figure 1.The scientific achievements of recent years have formed the basis for the dynamic development of new technologies.A special case in this area is the hydro-jet technology, which uses concentrated energy streams in the form of a high-pressure abrasive water-jet containing granular admixtures.The research area includes analyses of elementary phenomena occurring on the surface treated with a high-pressure abrasive water-jet doped with various solid particles such as garnet, quartz sand, silicon carbide or dry ice [1,2].The basic advantages of this technology include following features [10]: versatility of application to various materials such as: steel, glass, ceramics, composites, stone; possibility to cut very thick materials (in case of aluminum alloys up to 300 mm); possibility to cut through multilayer material; possibility to shape three-dimensional elements; no thermal influence on the material to be cut, no warm-body influence zone; no material deformation in the cutting area.An important element of treatment is the selection of the abrasive, which should not have a destructive effect on the head nozzle and at the same time should effectively remove the material.Abrasives with high density, high hardness, isometric shape and a significant number of sharp edges are most valued.The use of abrasive grains with such properties in the machining process favorably influences the machining performance depending on the kinetic energy of the grains.In the abrasive water-jet cutting process, the most commonly used abrasive is garnet [9].

PHOTON BEAM (LASER) CUTTING
Photon beam (laser) cutting is a thermal cutting process whose main source of energy is the energy coming from the laser beam as shown in Figure 2.This beam with continuous or impulse action at the cutting point leads to melting or melting and sublimation of the material.As a result of the erosion process, the kinetic energy of the jet is converted into potential energy deforming the material in the working area.Consequently, microcracks form in the work area and the material loosens, resulting in the separation of material particles from the base mass.In addition, a reactive or inert gas flowing coaxially with the laser beam is used to blow the molten material and its vapors out of the gap [6].
This method can be used for cutting metals, plastics, ceramic materials, cermets and wood, from a cross section equal to 35 mm.The laser cutting method is also used for drilling and punching holes.This operation requires an impulse or continuous supply of processed laser beam energy to the material with a much higher power density than continuous laser cutting.The value of this energy reaches 106-1011 W/mm 2 [4].This method ensures high cutting accuracy and self-cutting edges, where the heat affected zone is very narrow [8].
The advantages of photon jet cutting include [11]: possibility of easy automation and robotization of the cutting process; high cutting speeds; high dimensional accuracy of the cut and smoothness of the cut surfaces, which enables cutting products that do not require further mechanic treatment as opposed to those made by oxygen or plasma cutting methods; lower self-tension and at the same time lower deformation of the cut material; narrow heat affected zone; possibility of conducting several operations during one cycle, e.g.piercing, cutting out holes etc.; minimal rounding of the upper cutting edge and no overhanging of the slag at the lower cutting edge; much lower emission of harmful dusts and fumes.A comparison of both described methods is shown in Table 1.

Methodology of experimental tests
The aim of the research was to determine the most advantageous parameters of the tested samples due to the achieved dimensional and shape accuracy and surface quality after cutting sheet metal made of X5CrNi18-10 steel.To achieve the goal, 54 elements were cut out of which 27 photon beam (laser) on the Kimla FlashCUT LF 1530 6 kW machine with variable adjustable values of the cutting processes listed below: thickness of metal sheet in mm it is described as the thickness of a flat or coiled metallurgical product, much smaller than its length and width, the range used for testing is 6, 8, 10 mm; laser power P in W described as output power, standardized; it is a scalar physical quantity informing about the work done in time, the end point used in the research is 4.0; 5.0; 6.0 kW; axial feed rate of the cutting head vf LASER in mm/s this is the feed rate of the cutting head relative to the workpiece in a unit of time, the range used for testing is 10, 20, 30 mm/s.Similarly, 27 workpieces were cut with an abrasive water-jet on the PTV JETS 3.8/60 Basic machine with the variable adjustable values of the cutting processes under consideration listed below: sheet thickness in mm, range used for testing is 6, 8, 10 mm, axial feed rate of the cutting head vf AWJ in mm/min, range used for testing is 0.80; 1.66; 2.50 mm/s, the process was controlled by adjusting the abrasives output in kg/s, the range used for testing is 0.005; 0.0066; 0.0083 kg/s.

Measuring positions
During the tests, surface microtopographs were acquired after both types of cutting processes.Triangulation measurements were also carried out by Keyence's LK-031 non-contact laser sensor mounted in the Talysurf CLI 2000 measuring system of the British company Taylor-Hobson Ltd. shown in Figure 3  The microscopic images of the cut edge, with a acquired with the Dino-Lite Edge AM7915MZT microscope from ANMO Electronics Co. shown in Figure 4.

ANALYSIS OF RESEARCH RESULTS
An example of acquired microtopographs analyzed with the TalyMap Platinum 4.0 program using Digital Surf's Mountains Techno and 6.They also show the values of the determined texture parameters of the evaluated surface Sa (arithmetic mean deviation of the surface roughness), St (total height of the surface roughness) and Sq (square mean surface roughness deviation).
Further analysis focused on the changes in the value of the parameter Sa, as the most widely used parameter in practice for assessing surface roughness.In Table 5 the values of the parameter Sa measured on all surfaces after cutting were collected according to the adopted research methodology.
Figs. 7-12 show charts illustrating the changes in the value of the parameter Sa depending on the adopted parameters of the photon beam (laser) cutting process (Fig. 7-9) and abrasive water-jet (Fig. 11-12), for three examined sheet thicknesses (6 mm, 8 mm and 10 mm).
The analysis of the changes in the Sa parameter values for the photon beam (laser) cutting surface (Fig. 7-9) showed that the thickness of the cut material has the most significant influence on the obtained measurement results.The change of sheet thickness from 6 mm to 10 mm resulted in approximately doubling the value of the mean arithmetic deviation of surface roughness.The influence of other variable process parameters (feed rate of the vf LASER head and power of the laser source P) were less significant and differed with respect to particular sheet thicknesses.
Generally speaking, it can be stated that for selected combinations of technological conditions of the photon beam (laser) cutting process, increasing the power of the P laser source had a positive effect on the obtained surface roughness after cutting (Fig. 8).On the other hand, the observed trend of changes in Sa values as a function of feed rate of the cutting head vf LASER does not confirm the expected influence of this parameter on the surface quality after cutting.Increasing the feed rate removes larger volumes of material cut in a unit of time, which results in an increase in cutting marks, deformations, outflows and other defects on the analyzed surface.Presented results may therefore result from the size and selection of the area to be measured for microtopography of the surface after cutting.
The influence of the thickness of the cut sheet made of X5CrNi18-10 steel was even more pronounced (than in the case of the surface after photon jet (laser) cutting) on the surface cut by abrasive water-jet (Fig. 10-12).In this case increasing the thickness of the sheet from 6 mm to 10 mm caused about three to five times increase of the value of parameter Sa.It was caused by a significant loss of energy of the abrasive water-jet in the direction of penetration into the material being cut.

Fig. 2 .
Fig. 2. View of workspace in the photon beam (laser) cutting process [13] . It worked with the LK-2001 controller of the same company and allowed to obtain a measurement resolution of 1 Talyscan CLI 2000 version 2.6.1 supplied by the manufacturer.For the analysis and visualization of the acquired measurement data, TalyMap Platinum version 4.0 software from Digital Surf was used.

Fig. 4 .
Fig. 4. Components of the position for acquisition images of the active surface and the surface treated by optodigital microscopy equipped with the Dino-Lite Edge AM7915MZT digital measuring microscope by ANMO Electronics Co.

Fig. 7 .Fig. 8 .Fig. 9 .
Fig. 7. Changes in the value of the Sa parameter of the surface after cutting 6 mm thick sheet steel depending on the variable power P and feed rate of the cutting head v f LASER in the process of photon beam (laser) cutting