introduction
Diamond core drills not only have an important position in geological exploration, but also are widely used in marble, granite and reinforced concrete wall because of their unique advantages such as fast drilling speed, low stone loss and no damage to the surrounding environment. hole. A diamond drill bit is a composite sintered body formed by hot pressing of a diamond and a carcass material on a steel substrate, which is often referred to as a cutter head. Domestic drill bits are often connected by brazing. Diamond core drills require high shear strength and high temperature strength due to their extremely fast speed and strong impact and vibration. Brazed bit tips often fall off due to melting of the solder due to high temperature drilling. Therefore, foreign countries have developed laser welding instead of brazing since the late 1980s. Laser welding and brazing have the following advantages: since the base and the cutter head are metallurgically bonded, the strength is high, and the cutter head is not easy to fall off, especially in the case of no water. The weld is a small and uniform columnar crystal structure, which is not easy to crack, ensuring that the drill bit does not crack when it is drilled at high speed; the weld and heat affected zone are extremely narrow, which has no effect on the diamond, ensuring the best product. Performance; easy to automate, high efficiency, stable and reliable quality, and low average product cost.
The structure of the diamond core drill. At present, the laser welding drill bits required by foreign markets are almost equivalent to the laser welding saw blades, and the market potential is large and the profits are relatively high. Since the cutter head of the diamond core drill is powder metallurgy material, there is inevitably a void, and the welding is difficult; the base and the cutter head are welded by different materials, the cutter head is thicker than the base body, and it is difficult to weld on both sides, so compared with the diamond circular saw It is more difficult to weld the film. At present, no reports have been found in China. After hundreds of trials and explorations, we have achieved good results and successfully solved the key technologies. Now we have applied them in actual production. All products are exported and the users have a good response.
1 Test device and method
The test was carried out on a fast-flowing longitudinal laser imported from the United States. The machine is an advanced product of the United States in the 1980s, model EFA-51, the output mode is TEM00 and TEM01, and the maximum output power is 1500W. A f=100mm GaAs focusing lens is used to protect the weld and the lens by coaxially shielding the gas Ar gas. The drill base and the cutter head are clamped to a dedicated fixture. The rotary table can drive the clamp and the drill bit for circular motion, and can adjust the angle within a certain range. The laser beam is slightly biased to the side of the substrate and uses a certain negative defocus. In order to make the base of the cutter head well-matched, the cutter head must be arc-polished and deburred before welding to avoid loss of energy due to light leakage. To reduce welding defects, oil, scale and moisture must be removed from the weld and its vicinity before welding.
The base material is ordinary 45# steel, which is quenched and tempered and has a thickness of 2mm. Since the diamond is easily graphitized at high temperature, voids will appear in the weld seam during laser welding. Therefore, the cutter head needs to design a transition layer of 1.5mm~2mm. Through a large number of experiments, we selected a special Co mixture as the transition layer, and the cutter head is 3mm thick.
After the bit is welded, the appearance of the weld is visually observed and observed with a 10x magnifying glass. The weld is examined for macroscopic defects such as porosity, undercuts, cracks, etc., and then the bending strength of each of the cutter heads is checked by a DSA standard using a special bending strength tester.
2 test results and analysis
2.1 Material Analysis
The base material has good weldability under laser welding conditions. The cutter head is different. Because it is a powder metallurgy material, it is easy to have pores and even voids during welding. When the welding parameters are not correct, cracks will occur, which greatly affects the welding strength. The performance of the cutter head has a great influence on the welding performance. The mechanical properties and the dense cutter head can be obtained under the laser welding conditions. As long as the welding parameters are properly selected, the welds that meet the performance can be obtained, and the bending strength can reach or exceed. 1800N/mm2, broken in the cutter head. When the material is certain, the performance of the cutter head should be excellent. Firstly, the punching density of the cutter head must be increased. The powder with fine particle shape and uniform shape can be selected for pressing. In addition, the sintering temperature and the pressing pressure should be increased as much as possible without affecting the performance of the diamond. And choosing the right holding time is necessary to improve the mechanical properties of the cutter head.
The inevitable porosity exists in the cutter head, and the density of the cutter head cannot reach the theoretical value, but the proper pressure, temperature and holding time, powder mixing time and speed are important guarantees for the compactness and uniformity of the cutter head, thus ensuring good Welding performance. Tests show that when the unit pressure is about 20MPa, the temperature is about 800 °C, and the holding time is 4min, the cutter head that can meet the needs of laser welding without damaging the mold and diamond performance can be obtained. The density of the cutter head has the greatest influence on the weld strength, and the weld strength of the low-density cutter head is very low, and it is almost impossible to achieve the required strength.
2.2 Welding process research
The position of the focus under certain conditions of the optical path system, the position of the focus relative to the surface of the material has a great influence on the penetration depth, the melting width and the melting efficiency. In this test, the use of negative defocus can increase the penetration depth, the defocusing amount is about 0.3mm, in order to reduce the splash during welding, protect the lens, reduce the burning of the alloying elements of the cutter head material, the focus should be biased to the side of the substrate about 0.1mm; In order to obtain the best fillet weld effect, the incident laser should be tilted by about 6°~11°; when welding on the reverse side, we add a mirror to the optical path system, which is placed in the base of the drill bit to reflect the laser beam to the desired position. The problem of unstable strength of the drill bit due to single-sided welding is successfully solved.
Laser power density Laser power density is the main parameter that determines the penetration depth of the weld. In the case where the spot diameter is constant, the power density is proportional to the laser power. The higher the power, the larger the thickness of the plate allowed to be welded, the faster the welding speed and the higher the production efficiency, but the excessive laser power will deteriorate the appearance of the weld. Due to the tumbling of the bath, it is easy to generate a wave of protrusions and The voids seriously reduce the effective bearing area of ​​the weld and reduce the bending strength; even if the required strength can be achieved in the post-weld inspection, the cracks are propagated from the holes during the cutting process, and the fatigue fracture characteristics are obvious; The high welding power will also cause the chemical and physical reaction in the molten pool to be severe, the spatter will increase, and the lens will be contaminated; because the thermal expansion coefficient of the transition layer material and the diamond matrix material is not the same, under the condition of excessive welding power, it is easy to The two layers meet at the junction due to overheating. The double-welded ø82 drill bit can achieve better welding results with a power of 900 W when the spot diameter is 0.4 mm. In the test, we found that welding the drill bit with a small gauge welding method can achieve good welding results. The laser power of the welding bit is relatively higher than that of the diamond circular saw blade because the laser is partially reflected off the round bit base.
The welding speed welding speed affects the heat input of the welding area together with the laser welding power, and thus has a great influence on the shape and size of the weld. As the welding speed increases, the weld width decreases, the penetration depth decreases, and the productivity increases. Excessive welding speed will make the weld unable to penetrate, the strength will decrease, and the harmful gases in the weld such as N2, H2, O2 and CO will not escape, which will increase the weld porosity and affect the bending strength and weld appearance. When the welding speed is too low, the weld is too wide, the heat affected zone is overheated, the grains are coarse, and sometimes cracks are generated, especially in the joint between the two layers, which seriously affects the welding quality and the alloy of the cutter head. The element burned severely, which affected the overall performance of the cutter head. The weld seam appeared one void due to the tumbling of the weld pool. In our test, for the ø82, ø68 two kinds of drills, each with a laser power of 900W and a rotational speed of 8r/min and 9r/min, the bending strength of the welds reached and exceeded 4000N/mm2. In the subsequent cutting process, the phenomenon of the cutter head was not found.
The protective gas flow protection gas has three functions in the laser welding process: the protection welding zone is not oxidized; the focusing lens is protected; and part of the plasma cloud is blown off. If the gas flow rate is too small, it will not function, the weld zone will be oxidized and cause mechanical deterioration, the lens will be easily polluted and damaged, and the cost will increase; excessive gas flow will blow the molten pool, affect weld formation and increase welding defects. At the same time, reduce the penetration. The gas flow rate was chosen to be 0.5 L/min in our welding test.
3 Conclusion
(1) In order to make the laser welding diamond core drill bit successful, in the case of a certain formula of the cutter head, the cutter head must be dense and the mechanical properties are good;
(2) In the process of laser welding diamond core drill bit, the relevant process parameters, such as laser power density, welding speed, focus position and shielding gas flow rate, must be reasonably selected to ensure the welding quality;
(3) The welding process using small specifications is an effective way to weld the drill bit;
(4) The method of adding a mirror in the optical path system can perform the reverse welding of the drill bit, which solves the problem that the bit strength of the drill bit is unstable due to the single-side welding, but this method cannot weld the drill bit with too small diameter;
(5) The method of laser welding diamond core drill bit is not suitable for mass production, and the productivity is not high, which needs further improvement.
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