Carbide crack initiation mechanism
Different from general machining, the wire cutting wire doesn’t contact the work piece directly, but gradually erodes the metal material by the continuous pulse spark discharge generated between them. In WEDM, the temperature of the machined surface changes rapidly and lead to uneven stress on work piece. This situation is particularly at ease to occur in the processing of hard and brittle materials, such as cemented carbide, cermets, etc. In past empirical research ,When the electrical parameters are not set up properly, crack initiation will appear on the surface of the work piece once the thermal stress exceeds the strength limit.
According to the thermal stress model, the amount of heat entering the material is proportional to the peak stress. This heat is directly related to the input electric pulse energy under the same other conditions, which is that the greater the input power, the more heat the material absorbs and the greater the stress. Under the condition of neglecting the energy loss, the energy acting on the workpiece during processing can be simplified as below,
In this formula, W is the pulse energy (J); U is the intermittent instantaneous discharge voltage (V); I is intermittent instantaneous discharge current (A); T is the time (s); Tk is the discharge duration (pulse width, s).
In this formula, W is the pulse energy (J).U is the intermittent instantaneous discharge voltage (V).I is intermittent instantaneous discharge current (A).T is the time (s). Tk is the discharge duration (pulse width, s).
It can be seen from the formula that when the discharge duration (pulse width) is determined, increasing the discharge voltage and current will aggravate the crack initiation. When the discharge voltage and current are determined, increasing the discharge duration (pulse width) will result in the same outcome.
Experiment plant of micro crack initiation on carbide
A bunch of cemented carbide bushings (see Figure 2) ,made of YG6 cemented carbide (Chinese carbide grade), will be processed for the experiment. The bushing’s in height of 30mm, with key slots in width of 1.6mm and depth of 1mm. After being processed, the bushings will be made into metallography sample for SEM inspection to observe the crack initiation and propagation on the machined surface of cemented carbide。
In the test, the medium wire cutting equipment CTP350 is used. The cutting fluid is emulsion with a concentration of 8%, and the cutting wire is φ 0.18mm molybdenum wire, one piece is clamped at a time for processing, and the processing parameters of wire cutting are shown in Table 1.
Other main test condition:
edm machine: CTP350
cooling fluid: 8% emulsion
cutting wire：φ0.18 molybdenum wire
processing times limit: 1 piece a time
and WEDM processing set-up parameters are shown in the following table 1.
The test result about WEDM carbide crack initiation involving 4 factors
Electricity pulse width
Metallographic photos of 1 #~4 # products are shown in Fig. 3~Fig. 6. It can be seen from the figure that with the decrease of pulse width, the microcracks on the alloy surface gradually become smaller. When the pulse width is 40ms, the depth of microcrack reaches mmmm; When the pulse width is 12 ms, there is basically no crack initiation.
Electricity current value
The metallographic photos of 5 #~8 # products are shown in Fig. 7~Fig. 10. It can be seen that when the processing current is 3.5A, the crack depth is more than 30mm; When the processing current is 2.8A, the crack depth is 30mm; When the processing current is 2.4A, the crack depth is 20mm; While the processing current is 2.0A, the crack depth is 10mm. All in all, the greater the machining current, the greater the crack depth.
The metallographic photos of product 9# ~ 12# are shown in Figure 11 ~ 14. It can be seen that when the current is 2A, the pulse width is 20μm, and the pulse interval is 8 times the pulse width, the processing voltage is 70 ~ 120V, and no alloy microcracks are found in the cutting section. In other words, the influence of voltage on the carbide crack initiation is not obvious when the current and pulse width are constant.
The metallographic photos of 13 #~15 # products are shown in Figures 15~17. It can be seen that through multiple cutting processes, the surface quality of the product become better and micro-crack’s depth gradually reduced as cutting times grow. The depth of the micro-crack of the product is within 15mm after two times of WEDM processing; The depth of micro-cracks in the product is within 10mm after three times of WEDM processing. Thus, through two times of cutting processing, it can meet the requirement that the micro-crack depth of carbide bushing should be less than 20mm.
The experimental summary of cracks in cemented carbide produced by WEDM
When WEDM is used to process brittle materials such as cemented carbide, the pulse width and processing current have obvious effects on the microcracks on the surface of the alloy, which are shown in the following aspects: the greater the pulse width and current, the deeper the crack. However, the effect of voltage on the surface microcracks of the alloy is not obvious. No microcracks were found when the pulse width was 12 μm.
When the working current is above 2A, the workpiece will have cracks to a certain extent. Therefore, when selecting electrical parameters, avoid selecting the processing current above 2A. The superposition effect of temperature field should be fully used to reduce the size of thermal stress and its impact property, so that the workpiece material can be thrown out in gas phase to avoid overheating of the workpiece surface.
In addition, multiple cutting is a very effective method to reduce and remove surface microcracks.