WC-Co cemented carbide is characterized by high hardness, strong toughness, excellent thermal shock resistance, and oxidation resistance. Milling cutters, CNC inserts, PCB drill bits, and rotary file tools prepared from it are indispensable machining tools in important fields such as automotive, aerospace, national defense and military industry, electronics, and new energy modern manufacturing.

In actual production, the rotary file does not crack after welding, but stress cracking occurs after tooth cutting, but relevant literature reports are few. This paper uses Cu-Ag-based multi-interlayer flux to weld cemented carbide/steel rotary files, analyzes the stress state of the rotary files after welding and the causes of cracking after tooth cutting. Starting from the principle, the process is optimized, and more economical and convenient measures to reduce welding and tooth cutting cracking are proposed.

(a)Original sample after induction welding; (b) Enlarged view of the polished rotary file after welding; (c) Sample after tooth cutting and surface finishing.

Fig.1 ?Macroscopic morphology of rotary file samples after welding, polishing, and tooth-cutting processes

 

Research Direction

Although the thermal conductivity of tungsten carbide is more than twice that of alloy steel, there is a significant difference in the coefficient of thermal expansion between alloy steel and cemented carbide. For example, the linear expansion coefficient of commonly used YG-grade cemented carbide is approximately 5×10??–7×10??/K, while that of commonly used 45 steel is approximately 11×10??–14×10??/K.

Zhang J et al. studied and found that during the cooling process of induction welding, steel shrinks faster, forming tensile stress in the steel shank of the rotary file and compressive stress in the cemented carbide end. Meanwhile, the distribution of residual stress in the above two parts does not change with the decrease in temperature, and the residual stress value reaches the maximum at room temperature. Amelzadeh M et al. found that the residual stress distribution on the welding surface after induction welding is uneven, and the residual stress near the boundary of the welding surface is higher than that inside the weld.

Bang H S et al. further confirmed that the distribution of brazing residual stress has directionality, which can be specifically divided into two directions: parallel to the welding surface and perpendicular to the welding surface. Among them, the peak value of longitudinal residual stress at the welding site is the highest, even reaching the yield strength of the material. The welding failure of cemented carbide/brazing layer/alloy steel mainly occurs near the boundary between cemented carbide and brazing filler metal.

In order to reduce the residual thermal stress in brazed joints, many researchers at home and abroad have carried out a large number of studies, which can be mainly divided into three aspects: pretreatment before welding, treatment during the welding process, and post-weld treatment, to alleviate the residual thermal stress of brazed joints.

(a)Macroscopic photo of the interface; (b) Schematic diagram of residual stress area testing.

Fig.2? ?Macroscopic image of tooth-cutting-induced cracking at the interface and schematic diagram of residual stress zone testing in post-weld rotary files

Research Methods

This paper uses XRD tester, infrared thermometer, metallographic microscope, scanning electron microscope and other equipment to test the micro morphology and residual stress distribution of the welding interface of cemented carbide rotary files, and analyzes the tooth cutting cracking problem of welded parts after induction welding. The results show that the Cu-Ag-based brazing multi-layer structure can release the residual thermal stress at the cemented carbide end near the weld, but the cemented carbide end far away from the weld maintains a high residual stress value, and the rotary file forms a high residual stress gradient along the direction perpendicular to the welding surface. At the same time, the fast heating and cooling rates and short holding time will also cause a large thermal stress gradient from the outside to the inside of the welding surface of the rotary file. When the tooth cutting process introduces processing stress, which destroys the stress balance state of the cemented carbide surface, the compressive stress value on the surface of the cemented carbide near the weld will rapidly increase, easily leading to crack generation. Optimizing the welding temperature, welding time, preheating of welded parts and cooling pressure treatment can all improve the residual stress of the rotary file along the welding surface and vertical direction to a certain extent, optimize the stress gradient distribution, and further reduce the cracking phenomenon of the sample during tooth cutting after welding.

 

Means for Strengthening Welding Performance of Rotary Files

Enhancement of Brazed Joint Strength by Periodic Grooves

Zhang Y et al. achieved the transformation of tensile stress to compressive stress at the interface by carving micron-scale periodic grooves on the ceramic surface, inhibiting crack propagation and enhancing joint bonding strength. Compared with the joint strength of untreated ceramics (24 MPa), the post-welding strength of grooved ceramics reached 66 MPa, increasing by 275%. In addition, the design of special brazing layer structures can also optimize the residual thermal stress in brazed joints. Directly using high-strength brazing filler metal to fill the welding surface is extremely likely to cause stress cracking in the brittle cemented carbide end of the rotary file. Composite solders such as copper-based, silver-based, or nickel-based materials with low yield points, easy deformation, and the ability to reduce shear stress are suitable brazing materials for cemented carbide rotary files. However, pure copper flux has problems such as high melting point and poor performance, while silver-based flux has a high cost. The Cu-Ag-based composite interlayer structure combines the plasticity and low melting point of silver-based solders with the low cost and good compatibility with other metals of copper-based solders, effectively reducing residual stress at the welding site. Such interlayers are mainly composed of an external soft porous metal fiber mesh buffer layer and a middle rigid interlayer. The soft buffer layer can alleviate residual stress through yielding, plastic deformation, and creep, while the internal rigid interlayer transfers the concentrated area of residual thermal stress from the weld side of the cemented carbide to the interlayer, effectively preventing the generation of initial cracks in the cemented carbide. Shirzadi A A et al. used Ag-Cu-based multi-layer brazing for alumina/stainless steel materials, and the bonding strength of the welded parts reached 33 MPa, which can withstand more than 60 cycles of thermal shock in air at 200–600°C.

 

(a)Thermal imaging diagram；(b) Schematic diagram of thermal diffusion

Fig.3 Thermal image of rotary files 2 seconds after welding during cooling and demolding and schematic of heat diffusion at the welding surface

Controlling Temperature to Optimize Brazed Joint Stress

Process control during brazing can also optimize the residual stress of joints to a certain extent. Kar A et al. studied the effects of different welding temperatures on Ag-Cu-Ti-based flux brazing of alumina and stainless steel. During the temperature rise from 900°C to 1100°C, alumina and the brazing layer underwent different diffusion reactions, resulting in significant differences in the composition and structure of the brazing layer. The phase types, micro morphology, and element arrangement in the brazing layer all affect the joint strength and residual stress distribution.

In addition to the brazing layer structure and welding process control, post-weld treatment is also the key to reducing welding cracking of rotary files. Various post-weld heat treatment measures, including cryogenic treatment, tempering, low-temperature and high-temperature pressurization, can reduce the residual stress value of alloy welding components to a certain extent.

Mishra S et al. adopted thermal cycling treatment after welding (200–500°C for 5–10 cycles, followed by cooling to room temperature), and the results showed that post-weld thermal cycling can reduce residual thermal stress caused by the difference in the coefficient of thermal expansion. It was also found that the lower the residual thermal stress value at the joint, the better the relief effect. After cryogenic and tempering treatments on YG8 cemented carbide/A3 steel welded parts, Lu Hangang found that the maximum compressive stress of the cemented carbide was 304 MPa, which was 40% lower than the maximum stress value of conventional welding. Cui Chen et al. found that after cryogenic treatment, the surface residual stress of austenitic stainless steel matrix/ferrite welded parts decreased by 36.8% and 16.3% in the X and Y directions, respectively, while ageing treatment reduced the surface residual stress by 61.2% and 58.8% in the X and Y directions, respectively.

Fig.4? Interface morphology of large-sized (D＞30mm) rotary files fabricated via induction welding

Fig.5? DSC-TG curves of Ag-Cu-based filler metal

Fazit

Induction heating is characterized by short heating time, high temperature, and convenient operation, making it one of the main methods for large-scale industrial and automated welding of rotary files. However, excessively high heating temperature or too short heating time can easily cause high residual stress gradients in the rotary file along the welding surface and the direction perpendicular to the welding surface. Although these high residual stresses do not generate cracks immediately after welding, they often lead to cracking during the tooth cutting process of the rotary file. Based on DSC-TG analysis, combined with thermal imaging, metallographic, and electron microscopy analyses of the macro-micro structure of materials, appropriate welding temperature, moderate welding time, preheating of components (to increase the initial welding temperature of larger-sized rotary files), and cooling pressure treatment can all improve the residual stress gradient distribution of the rotary file in the directions of the welding surface and perpendicular to the welding surface to a certain extent, thereby reducing the occurrence of welding cracking in rotary files.