The main failure forms of YG8 tungsten carbide drill bits includes the broken and shedding of tungsten carbide insert sheet. It is for these reasons that the residual stress, lack of welding and low strength of brazing after the welding. Welding strength is the most direct criterion for testing the quality of welding quality and the main influencing factor is the welding gap. During the welding process, if the welding gap is too small, the parts of tungsten carbide insert sheet will contact with the steel closely, so that the molten solder after soldering can not spread to the entire surface to reduce the joint strength; if the welding gap is too large, after welding the supporting role of the matrix is weakened, solder large gap casting columnar structure, grain is relatively coarse, loose tissue, strength and toughness relative decline, a direct result of the strength of the joint is reduced.

The reasons of residual stress appearing include the difference of the coefficient of thermal expansion and uneven temperature distribution. There is a great difference in the coefficient of thermal expansion between tungsten carbide and steel (the linear expansion coefficient of WC-Co is 5-7×10^-9, the linear expansion coefficient of steel is 12.6×10^-9), so it will produce great internal stress by the large difference of shrinkage after cooling, which is also the main causes of tungsten carbide and welding gap cracking. In addition, the brazing process accompanied by thermal expansion and contraction phenomenon and thermal stresses within the material, with the increasing temperature difference, thermal stress is increasing, tungsten carbide insert sheet will appear cracks easily. There are some solutions to reduce the stress as followed:
1. Control heating and cooling rates to avoid excessive temperature difference, especially the cooling rate after welding;
2. Take the necessary measures to prevent partial hot or cold phenomenon;
3. Uniform heating, minimize solder liquefaction process, to avoid the volatile low-melting phase, resulting in weak tissue and pores and also reduce the phenomenon of the brazing surface oxidation;
4. To enlarge the welding gap appropriately to reduce the additional stress on the cemented carbide piece.

Multiple composite rare earth electrode can not only adapt to different welding current range and different welding materials, but also it has good weldability. Composite tungsten electrode rare earth doped quantity is similar to tungsten electrode doped with single rare earth oxide. Rare earths and thorium has large price differences, but the quantity is small, so raw material cost different between composite electrode and thorium tungsten electrode is small. In addition, during the composite electrode manufacturing, the manufacturers introduced new technology, thus greatly improving the finished product ratio of electrode. And during the preparation, the factory also uses new equipment, which can simplify processes and equipments, improve work efficiency, reduce labor intensity, so that the energy, equipment and labor costs are significantly reduced during production. Meanwhile, manufacturers directly uses APT as raw material, mixed rare earth nitrate, eliminating the APT firing process, greatly reducing the cost of production. The initial temperature of subsequent processing is higher, but its energy consumption is relatively small than APT calcining consumed. Taking all these factors, the production cost of composite electrode is slightly lower than tungsten electrode doped with single rare earth oxide, such as cerium tungsten electrode, thorium tungsten electrode.

Thorium tungsten electrode has excellent welding performance, although thorium tungsten electrode tungsten may be substituted with cerium and lanthanum tungsten electrode, but only a small part, thorium tungsten electrode consumption is still the greatest. In terms of performance composite electrode is greater than thorium tungsten electrode. Further, thoriated tungsten electrode has radioactive contamination, so during production and transportation process should increase the radiation protection fee and composite electrode has obvious cost advantages than it. Composite rare earth tungsten electrode only has the price advantage but excellent performance, so if it having same price with cerium tungsten electrode and thorium tungsten electrode may has bright market prospect. On the other hand, if its price is small higher than cerium tungsten electrode and thorium tungsten electrode, considering its welding performance, than it will accepted.

Multiple composite rare earth electrode no radioactive contamination, is a green electrode. 

During the experiment, the tester uses thyristor controlled DC TIG welding power source and the model is YC-300TSPVTA. Electronic stopwatch model is TREREX. Vernier caliper Model is 025 and No. is 096 583. Electronic balance model is AEL-200 and digital multimeter model is Bestillingsnr.

Arcing performance:           
Operating specifications: tungsten wire diameter 2.4mm, tip taper angle 45 °, the protruded length of the tungsten electrode 8mm, arc current 80A, arc duration 10S, arc length 3mm, repeating 30 times, argon gas flow rate 8L / min, DCSP TIG welding

Testing results: composite rare earth tungsten electrode at 30A, 80A and 150A welding current, arcing repeated 30 times were successful and the success rate was 100%, indicated it has good arcing performance.

Arc static characteristic curve:
Operating specifications: tungsten wire diameter 2.4mm, tip taper angle 45 °, tungsten electrode protruding length 3mm, arc length 3mm, argon gas flow rate 8L / min, DCSP TIG welding, tungsten  wire as cathode and water-cooled copper as anode

Test Results: Form the test result Fig. 4-24 shows that the static characteristic curve of the composite electrode is less than thorium tungsten electrode, indicating that it has low work function and strong electron emission capability, so it has good weldability.

Electrode burning rate:
Operating specifications: electrode diameter 2.4mm, welding current 180A, arc duration 20min, the electrodes extended length 3mm, arc length 3mm, argon gas flow 8L / min, water-cooled as copper anode, DCSP TIG welding

Test Results: From the test result Fig. 4-5 can see the burning property of composite rare earth electrode is better than thorium tungsten electrode.

After commercial welding machine testing found the result is consistent with the component screening test result, indicating optimized manufacturing technology can obtain high performance composite rare earth tungsten electrode, which is better than same specifications thorium tungsten electrode.

YG8 tungsten carbide drill bit is a kind of rock drilling tool that has been widely used in mine drilling, building construction and transport facilities. It is composed of tungsten carbide with high hardness and excellent wear resistance and steel welded. In the process, tungsten carbide drill bits have to endure complex impacting and twisting forces. According to the shapes of the drill bits, YG8 tungsten carbide drill bits can be specifically divided into linear, crossed, X-shaped and button. Linear carbide drill bit is suitable for light internal combustion electric, pneumatic and hydraulic soft rock drilling and convenient for grinding; the crossed and X-shaped drill bit is suitable for greater impact power drills, drilling fractured rock formations and strong abrasive rock formations, such as limestone, granite and marble; button bits usually used with tungsten carbide buttons, in power drills with a large impact, compared with sheet insert drill bits has better performance in applicability, drilling efficiency, service life and the hole quality. In addition, button drill bits can be also divided into coal button, flat button and bullet button according the button profiles.

The main failure mode of tungsten carbide sheet includes breaking and shedding, which welding quality directly affects the service life of drilling tool. The drill bit brazing uses high-frequency induction, the advantage is rapid heating process, the solder liquefaction process short, can effectively reduce the burning of carbide and oxide process, which is beneficial for improving the quality of welding. And add proper amount of Mn element in brazing materials, it can be based on not reduce the plasticity and remarkably improve the strength, elasticity, wettability and corrosion resistance of drill bits. In addition, some of the parameters in brazing process, such as brazing seam gap, heating rate, cooling rate, the induction coil design, solder flux, method of adding and weldments surface roughness should be controlled in a reasonable range, or they will has an bad effect on the quality of welding and increases the residual stress inside of tungsten carbide drill bits.