(Machinery of Taiyuan University of Technology, Taiyuan, Shanxi, 030024) Inspect the vacuum diffusion welding process parameters to determine the quality of welding products and reduce production costs.
Characteristics of PDC bits PDC bits, which have a higher hardness and wear resistance than carbide roller bits, have a high cutting speed, and have no moving parts, greatly reduce costs and increase efficiency. The structure eliminates hidden accidents. It can significantly improve the drilling speed and overall progress, has obvious comprehensive economic and social benefits, and has been widely used in the machining, wood processing and drilling industries. However, the working conditions of the PDC bit are very harsh. Drill bit not only bears huge pressure, but also bears huge impact force. It not only bears the erosion of mud, but also bears the thermal effect of friction and impact during the cutting process, especially due to the local high temperature of cutting teeth. Material thermochemistry occurs. Therefore, PDC drills, especially diamond compacts (PDCs) as cutting teeth, require both high strength and hardness as well as sufficient toughness; they must have good thermal shock resistance and require certain corrosion resistance. . In the manufacturing process of the PDC bit, there is a key procedure, that is, the PDC composite sheet welded on the arbor, often using Ag-Cu-Zn-Cd solder brazing process, but due to the brazed PDC bit shear strength Not high enough, usually about 200MPa, PDC composite flakes often occur in practical use, which greatly reduces the cutting efficiency and service life of the drill. The application on the site shows that the main reason for bit failure is due to the friction and impact of the composite sheet and the rock, which causes the polycrystalline diamond layer to break or fall off and lose its cutting ability. Therefore, improving the quality of the composite sheet and increasing the service life of the drill bit is an urgent problem to be solved.
The use of vacuum diffusion welding process to manufacture artificial polycrystalline diamond compact drill bit (PDC drill bit) is one of the most highly-regarded high-tech. The vacuum diffusion welding process is considered to be able to replace the currently widely used brazing process due to its outstanding advantages. It can overcome the sharp decrease of the brazing filler metal strength caused by the brazing temperature during the drilling process of the PDC bit. The composite sheet easily falls off and other serious problems. However, during the process of vacuum diffusion welding of composite sheets and cemented carbides for PDC bits, the wear resistance of the composite sheet is significantly reduced when the welding temperature exceeds 700*C, and the welding pressure and welding time are not possible due to objective conditions. Infinitely increased, therefore, the process of vacuum diffusion welding is particularly important.
PDC's vacuum diffusion welding method and equipment pressure, so that parts in the vacuum clean surface close to each other, in a relatively small distance within the atom diffusion to connect the two parts of the welding method. Vacuum diffusion welding is generally carried out at a temperature of 60% of the melting temperature (absolute temperature) of the material to be welded, and therefore for materials with large differences in coefficients of expansion (for example, cemented carbide substrates of PCD composite sheets and 45 steel arbors ) This method is very effective. In the case of diffusion welding, the heating of the parts in the vacuum chamber is performed under the condition that the air is continuously evacuated, so that the adsorbed gas and the oxide film on the surface of the parts can be removed. In addition, vacuum diffusion welding maintains the geometry and shape accuracy of the workpiece, resulting in a vacuum-sealed, thermally stable, shock-resistant joint. Its application can guarantee the quality of the drill, increase the welding strength, and increase the drill depth of the drill. Such as the United States Sandia laboratory in the welding surface of nickel plating, plating thickness 25wn-50wm, and then under 650°C withstand 214162MPa pressure for 4h, vacuum diffusion welding, the shear strength of 413 vacuum diffusion welding for PCD composite When the sheet is welded, the welding process is complicated, the welding time is long, special equipment is needed, and the one-time investment is large. At present, vacuum diffusion welding is generally used only for the welding of drills with high welding strength requirements and large vibration during use.
Vacuum diffusion welding machine generally consists of a vacuum chamber, a vacuum pumping system, an electric heating system, a hydraulic system, an operation control system, a water cooling system, an air cooling system, and the like. In the welding process, the machined, flattened welding surface and the grease-removed welded component are placed in a vacuum-protected environment, and the welded component is operated according to a preset temperature (T), pressure (P), and time 0 parameters. The atoms diffuse each other to obtain a solid joint.
3 welding process research and experiments The main factors affecting vacuum diffusion welding process and joint strength are: process parameters (temperature, pressure and time) after welding process processing; selection and thickness of the middle layer material; heating and cooling speed and stability.
3.1 Determination of process parameters (temperature, pressure and time) After the pre-processed workpiece enters the furnace, the mechanical pump is first started to vacuum the vacuum chamber. When the vacuum chamber pressure B is below 6.7 > 40-1 Pa, the oil diffusion pump is started. Pump high vacuum. After the oil diffusion pump is operated, when the pressure in the vacuum chamber B is lower than 1.33>40-2Pa, the heating system is started to supply power for heating. In order to avoid local overheating, melting of the workpiece and thermal stress due to the difference in thermal expansion coefficient, we determined the heating rate and holding time according to the process experiment to ensure the temperature uniformity during welding. Determine the rapid temperature rise at the beginning, when the workpiece temperature rose to 500C, Chen Hong PDC bit vacuum diffusion welding process study table 1 shear experimental factors and horizontal level factor dehydration method cooling initial temperature / C cooling time / h1 bulk acetone 60032 ethanol 30053 Bottled Acetone 40074 No Dehydration 5001 Table 2 Experimental Results Keep warm for 5 minutes, then reduce the heating rate. At this time, pressure is applied to the workpiece by the hydraulic system so that the components to be welded can be brought into closer contact. When the temperature rises to 695*C, it is kept for 6 hours, then the workpiece is cooled with the furnace under vacuum. When the workpiece temperature drops to about 100*C, the workpiece is taken out and the welding process is completed. Due to repeated research and experiments, reasonable process parameters were determined. The quality of the drills mass-produced by vacuum diffusion welding was stable, and the fluctuations of instantaneous tensile strength, impact toughness, vacuum compactness, and other technical indicators did not exceed 2%%. No joints such as incomplete penetration, pores, oxides, and other welding defects will occur at the place where the fatigue of the joint is good.
3.2 Pre-welding process after welding PDC bit pre-welding dehydration method, the initial temperature and cooling time in the furnace after cooling the reasonable choice of process parameters is the basis for improving the PDC bit welding strength. Through experiments, we studied the relationship between the above process parameters and the shear strength of the PDC welding surface, and determined the optimum value of each process parameter.
The experimental selection factors were the dehydration method, the initial cooling temperature and the cooling time in the furnace after welding. The levels of each factor were shown in Table 1. The higher the shear strength of the bit weld, the longer the service life of the drill bit, so the experiment The test index is defined as the shear strength T experimental results shown in Table 2. After the system experimental study (analysis chart omitted), the following conclusions are drawn: the dehydration method, the initial temperature and the cooling time in the furnace after welding have different degrees of PDC welding The influence order of influence is: cooling initial temperature> cooling time> dehydration method.
The optimized process parameters were as follows: initial cooling temperature 695C, heat preservation 6h, dehydrating agent used cheap bulk acetone.
3.3 Selection and Thickness of Intermediate Layer Materials During the process of vacuum diffusion welding of composite sheets and hard alloys for PDC bits, the wear resistance of the composite sheet is significantly reduced due to the welding temperature exceeding 700C, and the welding pressure and welding time are affected by the objective conditions. The limit is also impossible to increase indefinitely, so it is particularly important to study the influence of the material, thickness, and diffusion depth of the alloy elements on the strength of the weld seam.
The influence of intermediate process sandwich material on weld quality was studied under the conditions of welding temperature of 695C, welding pressure of 178 MPa, and welding time of 6 h. The influence of Cu, Ni intermediate process sandwich on the quality of the weld was found and no intermediate process was found. When the sandwich plate is directly welded to the hard alloy, the shear strength of the weld is very low, only about 56 MPa, which is far from the requirement of the general PDC bit manufacturer's requirements for shear strength greater than or equal to 400 MPa. After the cold-zipped Cu foil or Ni foil is used as the intermediate process interlayer, the shear strength of the weld seam has been greatly improved. For example, when a 45-μm-thick Cu foil or Ni foil is used as an intermediate process interlayer, the shear strength is improved. It is 6.3 times and 8.7 times without intermediate process sandwich. This shows that the addition of intermediate process can significantly increase the weld strength, the main reasons are: First, CuNi has a good plasticity, easier to produce plastic deformation after heating, which increases the effective contact area of ​​the welding surface, the welding rate is higher; It is Cu, Ni and Co can form infinite solid solution, accelerate the diffusion of atoms and activate the diffusion process; Third, CuNi is solid solution strengthened by alloying elements.
The influence of interlayer thickness on the quality of the weld seam in the intermediate process is due to the fact that under the same conditions, the strength of the weld seam with the Ni intermediate process interlayer is significantly higher than that with the Cu intermediate process interlayer, so the Ni intermediate process was mainly studied under the above conditions. The relationship between interlayer thickness h and weld shear strength t found that h has a significant effect on the weld strength t, and there is a critical value of h = 45um. When h is 45um, the welding rate of the weld has reached 100%, And the contact enhancement effect is also greater, the result of the combined effect of both makes t reach the maximum value, namely W=485MPa; when h is less than 45um, t decreases with the decrease of h, the welding rate decreases; when h is greater than At 45um, t decreases with increasing h, while the weld rate remains at about 100%. This phenomenon can be explained by the degree of contact strengthening and the formation of physical contact conditions. In the process of shear deformation of the diffusion weld with an intermediate process sandwich, the plastic deformation of the intermediate process is constrained by the hard alloy material, so that the intermediate process sandwich is in a three-way stress state, and thus is strengthened, and its degree of strengthening With the intermediate process, the thickness of the sandwich increases. From the contact conditions, the physical contact form of the surface to be welded depends mainly on the creep speed of the intermediate process interlayer, which in turn is determined by the thickness of the intermediate process interlayer. When the intermediate process interlayer is thinned, the plastic deformation conditions at the joint surface of the weld are deteriorated during the welding process, thereby preventing the development of the plastic deformation of the intermediate process interlayer, reducing the creep speed, and causing the physical contact time of the entire joint surface to become very long. Long, may cause incomplete penetration. This tendency increases as the thickness of the intermediate process interlayer decreases. It can be seen that, under certain welding conditions, there is an optimum value for the interlayer thickness of the Ni intermediate process. Under the conditions of this test, the value is 45 μm. 4 Conclusion Through the past few years, the PDC bit has been vacuum-diffused. We have thoroughly studied repeated experiments. Determined reasonable process parameters and process routes. In the PDC bit manufacturing, stabilizing and improving the quality of the PDC bit welding, expanding the PDC bit to adapt to the scope of the strata, and increasing the drilling efficiency has done a lot of work, reduced production costs, accelerated the promotion of the PDC bit, created a certain economy and Social benefits.
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