Abrasive water jet machining is a process involving the material removal by the mechanical energy of water and abrasive particles, has a large number of advantages over other abrasive processes. It is possible to cut any type of material, minimise the structural damage of the work piece and no thermal distortion on the work piece with AWJM. Like other jet cutting technologies, AWJM also produce kerfs which have some distinct features and the quality of the kerf determines the quality of the work. The quality and precision of the kerf is determined by different factors like kerf side surface texture, roughness and waviness, kerf width, taper, burr, kerf entrance radius, jet impinging surface roughening, kerf side micro hardness. The effect of the above parameters on quality of the kerf in AWJM for different materials is discussed in this report.
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2.0 General Kerf Characteristics in Abrasive Water Jet Machining
Generally a kerf is produced with a wider at the entrance of the jet and reduces at the bottom which is called a kerf taper. Due to the bombardment of the jet, rounding off the top edges of the kerf occurs due to the plastic deformation of the workpiece. The amount of round corners depends on the time. They are more visible in ductile materials than brittle materials. Even burrs may be formed at the bottom of the kerf as removed material may roll over the surface.
Two zones are generally present on the surfaces produced in machining. The upper zone is smooth in nature. This depth of cut is called while the bottom zone is called striation zone which has waviness as the predominant feature. The waviness is dragged in the backward direction due to the lag produced in the jet. This backward drag angle depends on the velocity used in cutting the work piece. When the abrasive jet cuts on the same jet for more number of times then it contributes for the depth of cut. When the pressure of the jet is unable to cut the material then a large pocket is formed at the bottom of the kerf. The cutting process in the first zone takes place by cutting wear, in second zone by deformation wear and in the pockets by erosive wear and upward deflection of the jet.
The depth of cut, kerf taper and surface roughness can be controlled by changing the pressure of water, mass flow rate of the abrasive particles, jet traverse speed and distance between the nozzle and the work surface. Generally among all the above parameters water pressure and jet traverse speed majorly control the depth of cut and surface roughness.
Kerf profiles for the Through and Non-Through cuts.
3.0 Case study on Quality issues in Metallic Coated Sheet Steels
This case study is on the kerf characteristics in the AWJM of the metallic coated sheet steels. The experiments were conducted on Zincalume G300 which is the structural steel with a spangled surface coated with hot dipped zinc or aluminium alloy.
Four major parameters were considered for the study of the effect on the quality of the sheet steels. These parameters include jet transverse speed, water pressure, abrasive flow rate and stand-off distance between the nozzle and the work piece, where at predetermined maximum stand-off distance, minimum abrasive flow rate and minimum water pressure, the traverse speed was adjusted at different levels. Other parameters like nozzle diameter, orifice diameter, nozzle length and the size of the abrasive are kept constant. The parameters were selected such that a through cut is obtained in all samples which are easy for comparison. In sheet steels the main factors which effect the kerf properties are Kerf width, kerf taper and kerf surface roughness and also burrs which form at the jet exit portion.
3.1 Experimental Results
After performing the AWJM on the metallic sheet steels, general kerf characteristics explained above were observed. From SEM study two types of burrs like hard burrs and loose hair line burrs. Hard burrs occurred at the jet exit portion of the kerf where the material was firmly attached, which were removed through a secondary material removal process.
Loose hair line burrs were observed at the both the edges of the kerf which occurred due to the removal of the zinc/aluminium coatings. Only few sites were found with striations and no traces of micro-cracks and heat affected zones were observed.
At the entrance portion of the jet a small portion of the material was damaged due to the bombardment of the abrasive particles, below which the cutting marks formed due to two erosion stages were observed. Striations were observed only in some samples were the water pressures were low and the jet traverse speeds were higher. In the jet exit region the cut surfaces were more irregular as the material was hugely deformed which lead to the formation of the burrs at the bottom.
The sheet steels are cut by the impingement of individual abrasive particles where micromachining and plastic deformation are the main reasons for material removal.The top region is brittle material erosion where as at the lower region the material removal is by ductile erosion.
SEM graphs of the kerf profiles
3.2 Effect of Process Parameters on Different Kerf Characteristics
3.2.1 Kerf Geometry
Kerf geometry plays an important in determining the work quality as the kerf produced has some taper angle, which is wider at the top and narrow at the bottom. Kerf taper is represented by the relation Ï´=tan((Wt-Wb)/h), where Wt is the top kerf width and Wb is the bottom width of the kerf and h is the depth of the cut.
The experimental results were plotted on graphs which revealed the relation between the kerf geometry and the process parameters. Both the top and bottom kerf widths increased with increase in the water pressure as the higher kinetic energy would have cut a wider slot. The impact of water pressure reduced on the kerf top width with its increase from 290 MPa to 340 MPa which implies that the abrasive water jets become less effective when the cross a threshold value. The threshold value depends on the other process parameters. A result similar to above case was seen in the kerf taper angle i.e the taper angle reduced with an increase in water pressure as the top kerf width became almost constant and bottom kerf width increased steadily with an increase in water pressure.
Graphs showing the effect of water pressure on kerf geometry
An increase standoff distance between the jet nozzle and the work piece resulted in an increase in the kerf top and bottom widths but, the top width increased rapidly compared to the bottom width. The cause of the above phenomenon might be due to the diversification of the jet with an increase in the stand off distance. As the top width increased at a faster rate compared to the bottom width, the kerf taper also increased with an increase in the stand-off distance.
Graphs showing the effect of stand-off distance on the kerf geometry
The experimental results on the effect of jet traverse speed on the kerf geometry revealed that , it showed a negative impact on the kerf widths where as it acted directly proportional to the kerf taper angle. The negative impact was observed due to the fact that the faster traverse speeds allow less particles to cut the material resulting in a narrow kerf and the kerf taper increases as the bottom portion is exposed for a very less time compared to the top portion.
Graphs showing effect of jet traverse speed on the kerf geometry
From all the above plots, the effect of abrasive mass flowrate had no proper effect on the kerf geometry within the selected ranges of flowrates.
3.2.2 Surface Roughness
Surface roughness is an important factor in determining the quality of the kerf. Here in our case as the thickeness of the sheet steels are very less the chances of formation of surface striations are very less, so only surface roughness was studied in this case.
Due to the increase in the traverse speed the surface roughness increased as the surface roughness as the increase in traverse speed leads decreases the number of particles impinging the material. This lead to improper cutting of the workpiece , which resulted in the increase of surface roughness.
It was observed that an increase in water pressure upto a certain value resulted in the smoother surfaces. But a further increase in the water pressure increased the surface roughness. This phenomenon is due to the concept of strength zones in the water jet. When the speed of the jet is increased upto a certain extent, the effective zone becomes wider resulting more effective cut of the material which leads to smooth surfaces. If the water jet still increases then the outer portion of the water jet attains good strength resulting in irregular surfaces. An increase in the abrasive particle flow rate resulted in the decrease of the surface roughness, as the number of particles impinging the kerf surface increases. The surface roughness was observed to be increasing with the increase in the stand-off distance as the water jet diverges cutting work piece irregularly.
3.2.3 Burr Formation
As discussed earlier mainly hard burrs and loose hair line burrs were found were observed on the jet exit protions of the kerf. Number of burrs were high at low water pressures probably due to the roll over of the chips at the bottom portion of the kerf. Also low jet traverse speed resulted in increase of the burr height as the slow traverse speed allows the thorough cutting of the particles and the increase in the standoff distance caused an increase in the burr height due to the reduction in the power of the jet.
4.0 Case Study on Quality Issues in Alumina Ceramics
Alumina ceramics are being used in many industrial applications. As they are brittle the AWJM of the alumina ceramics is by the crack propagation and then by
In ceramic materials the kerf taper is wide at the entry zone of the jet, while the width decreases with the increase in the thickness of the workpiece due to the decrease in the cutting efficiency of the jet resulting from the particle fragmentation and the energy absorption by the material. The entrance width can be mentioned as the inverse of the traverse speed. If the abrasive flow rate is increased then it can be observed that the kerf width increases.
Experimental results showed that kerf taper angle is proportional to the traverse speed, inversely proportional to pressure and abrasive flow rate. With the decrease in cutting efficiency of the fluid, it cannot cut the material completely which leads to the formation of the through hole. Hence it can be concluded that rate of decrease in depth of cut decreases with the increasing traverse speed.
