Vacuum brazing process of cemented carbide tools
Whether the vacuum brazing process of cemented carbide tools is correct or not plays a vital role in the welding quality. The heating speed has obvious influence on the welding quality. Rapid heating will cause cracks and uneven temperature in cemented carbide sheets, but too slow heating will cause surface oxidation and reduce joint strength.
When vacuum welding cemented carbide tools, uniform heating of tool rod and cemented carbide sheet is one of the basic conditions to ensure the welding quality. If the heating temperature of the cemented carbide sheet is higher than that of the tool rod, the strength of the joint will decrease if the melted filler metal wets the cemented carbide sheet instead of the tool rod. When the alloy sheet is sheared along the welding layer, the solder will not be destroyed, but will be detached with the alloy sheet. The milling marks on the support surface of the tool rod can also be seen on the welding layer. If the heating speed is too fast and the temperature of the tool rod is higher than that of the alloy sheet, the opposite phenomenon will occur.
The placement order and mutual position of flux, filler metal and cemented carbide have direct influence on the brazing quality. The correct placement method is to place the filler metal on the knife groove, sprinkle the filler metal, then put the cemented carbide, and sprinkle a layer of filler metal on the top of the cemented carbide along the side weld. In this way, when brazing, it is easy to control the brazing temperature and reduce the excess solder adhering to the outside of the weld. In the process of vacuum brazing , the vacuum brazing temperature of workpiece should be controlled correctly.
High vacuum brazing temperature will result in oxidation of weld and evaporation of zinc in zinc-containing solder; low vacuum brazing temperature will result in thick weld due to poor fluidity of solder, and a large number of pore and slag in the weld, which is the main reason for de-welding. The vacuum brazing temperature should be 30-50 °C higher than the melting point of the filler metal. At this time, the fluidity and permeability of the filler metal are good, and it is easy to permeate the whole weld.
When brazing in vacuum, the vacuum degree is maintained at 5 X 10-2 Pa. The heating speed of vacuum brazing is an important parameter in the process of vacuum brazing. Overheating speed leads to a sharp decrease in vacuum and easy oxidation of cemented carbide and filler metal. In production, the vacuum brazing process is as follows: from room temperature to 800 °C at the rate of 10 / min, holding for 30 minutes; then to set temperature at the rate of 9 / min, holding for 10 minutes; and then cooling with the furnace.
The heat preservation at 800 °C is to make the base metal heated uniformly; the vacuum will not decrease obviously when the temperature rises to the set temperature at the rate of 9 °C/min; the short-term heat preservation at the set temperature is to allow the filler metal to melt sufficiently and prevent the excessive volatilization of granular magnesium under high vacuum. Although the cooling rate of cemented carbide brazing is required when it is cooled, no cracking due to excessive stress is found in the process of furnace cooling. According to the melting point of the filler metal, the temperature of vacuum brazing is determined to be 1105 – 1120 °C, with a total of 6 brazing temperatures.
After determining the optimum brazing temperature, the EP23 temperature control table is set and the vacuum brazing process is set.