Novel Single Crystal Diamonds for Waterjet Cutting Applications

Transcription

1 Novel Single Crystal Diamonds for Waterjet Cutting Applications K.C. Heiniger * University of Applied Sciences, Northwestern Switzerland, CH-5210 Windisch, Switzerland M. Wälti ** MVT Micro-Verschleiss-Technik AG, Ringstr. 3, CH-2560 Nidau, Switzerland In waterjet cutting, orifices from hard materials like sapphires or rubies are normally used to generate small coherent waterjets of very high velocities. The lifetimes of such orifices are strongly limited to only dozens of hours due to the high water pressure environment (typically 350 MPa) and contacts with abrasives through the water hammer effect during cut-offs. Due to its high cost the WJ-market has, until now, only accepted solutions with natural diamonds for special cases. Experiments were performed with newly developed orifices from monocrystalline synthetic diamonds (MCSD) in operation with pure water and with abrasives. Long term experiments of up to 1'000 hours with non-specially filtered water and a 100'000 cycle test with abrasive waterjets are reported. It could be shown that MCSD-orifices in well designed waterjet cutting heads are extremely resistant to abrasives as well as to foreign particles and other impurities in water. As a result, very good waterjet quality and constant volume flow, necessary to insure consistent cutting quality, can be maintained for long operating times without orifice change in the cutting head. Furthermore water produced employing a reduced water treatment is sufficient for WJ-cutting operations. All this considerably reduces cutting costs compared to using cutting heads with sapphire or natural diamond equipped orifices. 1st International Industrial Diamond Conference, Barcelona Oct, 20 th,

2 1. INTRODUCTION When water is pressurized up to 6'000bar and forced through a tiny opening, it can cut a variety of soft materials including food, paper, rubber and foam. When small amounts of abrasive particles, such as garnet, are mixed into the jet stream, the resulting abrasive waterjet can cut virtually any hard material such as metal, composites, stone and glass. High pressure waterjets can also be used to accomplish a variety of industrial surface preparation applications, including cleaning ship hulls or remove hard coatings from surfaces. Waterjet cutting has been a specialty technology used in a wide variety of industries since about Today no other tool cuts or machines a greater variety of materials, ranging from glass and stones to composites and metals. Abrasive waterjet cutting systems cut virtually any shape up to metal thicknesses of 200 mm and even large numbers of different material-layers (Figure 1). In comparison to the laser machining process, waterjet machining is less expensive, and it is functionally superior to conventional metal cutting machines. Additionally, in comparison to laser cutting or wire EDM (electro-discharge machining) the work piece remains cold during the cutting process which is beneficial for many applications. [1 4] Fig.1 Abrasive waterjet cutting According to a report from a market research company, abrasive waterjetting is the fastest growing segment of the machine tool industry. The abrasive waterjet (AWJ) cutting technique is based on accelerating small diameter abrasive particles (~ 0.05 to 0.2 mm in diameter) through a high velocity waterjet to remove material. Water is pumped to high pressures (300 to 400 MPa) and a high-speed waterjet (770 to 880 m/s) is formed by a special sapphire or diamond orifice of diameter of 0.1 to 0.4 mm. Downstream from the orifice, abrasive particles are added in a mixing chamber and accelerated by momentum exchange with the waterjet in a mixing tube (focusing tube). From there they are directed at very high velocities (500 to 700 m/s) to the work piece (Figure 2). 1st International Industrial Diamond Conference, Barcelona Oct, 20 th,

3 Fig 2. Cutting head for abrasive waterjet cutting Besides achieving higher precisions current developments are focused primarily on enhancing the profitability of the cutting-process. The biggest problem during abrasive waterjet cutting is toolwear caused by the high-velocity abrasives reducing drastically the lifetime of the involved components (waterjet orifice, mixing chamber, mixing tube) and thus the profitability of the whole process. 2. ORIFICES FOR WATERJETTING Waterjet orifices have to create coherent waterjets which do not diverge in the cutting head (Figure 3). Such behaviour can only be assured through sharp edged orifices, where the ratio of edge radius to the orifice diameter must remain within in a well specified range during the whole lifetime of the orifice. Total symmetry of the edge also has to be assured (Fig. 4). Fig 3. Waterjet orifice with atomising and coherent jets It can easily be understood from Fig. 2 that not adjusted or diverging waterjets inhibit an effective acceleration of abrasive particles in the mixing tube. Even more, the lifetime of the mixing tubes can thus drastically be reduced. 1st International Industrial Diamond Conference, Barcelona Oct, 20 th,

4 There are only a few suppliers worldwide who possess the skill to manufacture such orifices from hard materials such as sapphires, rubies or diamonds. MVT is one of these having a large world market share. Fig. 4. SEM-pictures of waterjet orifices Closing the high-pressure valve at the inlet of the high-pressure chamber (collimation tube) to stop the cutting produces a strong pressure wave, which travels upstream at the velocity of sound that is able to transport abrasives through the orifice bore into the pressurised inlet chamber. During normal cutting cycles abrasives are accumulated here and tend to destroy the orifice during normal operation. This process seems to be most likely the cause for broken orifices. If an orifice brakes during a cutting operation there is the danger that the work piece could be destroyed. Therefore in many workshops "normal" orifices are replaced following statistical probability (typically after ~50h) before their lifetime is really exceeded. It is obvious that this reduces the profitability of the manufacturing process drastically. 3. ORIFICE MATERIALS To ensure constant jet conditions during the whole lifetime hard materials like sapphire, rubies or diamonds have been used for a number of years. The majority of workshops use sapphires/rubies because they are cheap and - for the specialized suppliers - relatively easy to craft. But they suffer from short lifetimes, although the materials are normally selected with the best crystalline orientation. Many hundred thousands of such waterjet orifices are manufactured worldwide. For waterjetting in the food processing or paper industry (mostly pure waterjetting) and the automotive industry e.g. where waterjetting machining centres are working around-the-clock, there is a strong need to use materials with higher lifetimes. Natural diamonds (industrial diamonds) have been processed to waterjet orifices by a few suppliers for some time. The prices are extremely high compared with sapphires (typically factor > 10). Lifetimes are usually high but they cannot be predicted because of randomly distributed cracks which are inherent for natural (industrial) diamonds. Besides it is very difficult to select the best crystalline orientation for the specific stress conditions of waterjet orifices. For normal operations the risk is too large in relation to the high prices. Therefore, there is no acceptance in the waterjetting industry and the majority of the waterjet machines worldwide are not equipped with such diamond orifices. Recently some orifice suppliers started to sell orifices from polycrystalline diamonds (PCD) which are moderate in price with a relatively long lifetime. But they suffer from the same problem as natural diamonds: Because of imperfect material structure caused by the sintering process 1st International Industrial Diamond Conference, Barcelona Oct, 20 th,

5 (Figure 5) the spread of possible lifetimes is (too) large as well. There is no optimal orientation of the crystalline structure, which is responsible for the reduced physical properties in comparison with monocrystalline diamond. Thus, these orifices are not suited for the special conditions in abrasive waterjetting. Fig.5 Possible PCD-structures (Element Six) 4. NOVEL SOLUTION: MONOCRYSTALLINE SYNTHETIC DIAMONDS (MCSD) Two years ago Element Six BV, Cuijk (NL) contacted the LTFE and MVT with the offer to co- operate in the development of new waterjet orifices using monocrystalline synthetic diamond. (MCSD). MCSD from Element Six has the excellent feature of very high purity, which inhibits crack initiation in the material and leads to very high transverse rupture strengths (Table 1, Figure 6) which are of high relevance for the lifetime of orifices in harsh waterjetting environments. Of course, the crystal orientation can be optimally selected for the stresses in the waterjet orifices. This results in a number of important advantages in comparison with waterjet orifices from sapphires and from natural diamond. Table 1. Physical properties sapphire - MCSD Sapphire MCSD Density [10 3 kg/m 3 ] Hardness [GPa] Fracture toughness [GPa] Young's modulus [GPa] 440 ~1050 Poisson's ratio [-] Tensile strength [GPa] (orientation dependent) Transverse rupture strength [GPa] small 2.9 Compressive strength [GPa] Thermal conductivity [W/mK] 40 ~2' 000 Thermal exp. coeff. [µm/m] st International Industrial Diamond Conference, Barcelona Oct, 20 th,

6 Advantages of Synthetic diamond over natural diamond Exceeds performance of natural diamond in lifetime Contains no cracks and contamination and therefore is showing a reliable/predictable behavior from a lifetime point of view Reproducible material quality is secured within the production process Secured availability due to the industrialized process of synthetic diamond production (PVD) Prices of synthetic material will decrease over time when volumes increase (natural diamond prices are only going up) Fig. 6 Advantages of MCSD over natural diamond 5. TEST RESULTS Today, we can present the result of this development after intensive testing in the laboratory as well as in a normal industrial environment: 1'000h, 15'000 cycle pure water long-term test at the LTFE (Figure 7a), using non-specially filtered water 100'000 cycle test with abrasives at the LTFE (Figure 7b) ~250 orifices in (normal) industrial waterjetting environment each running without problems during many hundred to even thousands of hours Figures 7a/7b SEM-photographs after laboratory long term tests The available test results show that the new waterjet orifices are extremely resistant to abrasives as well as to foreign particles and other impurities in water. Even small metal chips could not destroy the orifice. 1st International Industrial Diamond Conference, Barcelona Oct, 20 th,

7 As a result, very good waterjet quality and constant volume flow, necessary to insure consistent cutting quality, can be maintained for long operating times without orifice change in the cutting head. Additionally it can be observed that the lifetime of mixing tubes which are exposed to strong erosion through the abrasives is enhanced by about 20 to 30% using MCSD-waterjet orifices because the waterjets are better aligned during the whole life of the focusing tubes. Up to now it has been shown that a real breakthrough could be achieved using MCSD orifices in waterjetting applications. The lifetimes could be improved by factors between 15 and 20 (up to now) compared with sapphire orifices normally used in the waterjetting environment. Up to now not a single orifice has been destroyed during waterjetting operation (pure water and AWJ)! 6. CONCLUSION By tests in the laboratory and in a normal industrial environment it could be shown that monocrystalline synthetic diamond orifices in well designed waterjet cutting heads are extremely resistant to abrasives as well as to foreign particles and other impurities in water. As a result, very good and constant waterjet quality as well as constant volume flow, necessary to insure consistent cutting quality, can be maintained for very long operating times without orifice change in the cutting head. Furthermore water produced employing a reduced water treatment is sufficient for WJ-cutting operations. All this considerably reduces cutting costs compared to using cutting heads with sapphire, natural diamond or polycrystalline diamond equipped orifices. It can be stated that the orifice problem could be solved with novel MCSD-waterjet orifices. REFERENCES [1] D.A. Summers, Waterjetting Technology, Spon, 1995, ISBN: [2] C. Olsen, Waterjet Web reference, [3] Omax Corp., A Comprehensive Overview of Abrasive Jet Technology, [4] Flow Corp., How Waterjet Cutting Works, ACKNOWLEDGEMENTS The authors would like to acknowledge the assistance of Element Six which originally proposed to start the development of the novel waterjet orifice described in this paper using their MCSDmaterials. The authors would also like to thank the Innovation Promotion Agency (CTI) of the Swiss Federal Office for Professional Education and Technology for supporting this project (No ) PERSONAL INFORMATION * K.C. Heiniger, Prof. Dr. sc. techn.; head of Laboratory for Thermal- and Fluid Engineering and of Swiss Competence Centre for Waterjet Technology, University of Applied Sciences, North western Switzerland, CH-5210 Windisch, Switzerland, kurt.heiniger@fh-aargau.ch ** M. Wälti, CEO MVT Micro-Verschleiss-Technik AG, CH-2560 Nidau, Switzerland, martin.waelti@mvt.ch 1st International Industrial Diamond Conference, Barcelona Oct, 20 th,

8 1st International Industrial Diamond Conference, Barcelona Oct, 20 th,