Preparation and Research of Gradient Cemented Carbide Based on Cobalt and Carbon-Based Migration Dynamics

Preparation and Research of Gradient Cemented Carbide Based on Cobalt and Carbon-Based Migration Dynamics

• Admin
• 2024/12/13

Compared with WC-Co, functionally graded cemented carbide (FG WC-Co) can provide excellentcombination of hardness and fracture toughness. Based on the driving force of cobalt and carbon that liquid-Co migrates fromhigher Co content to lower one, from higher C content to lower one, bilayered samples of WC-15%Co with carbon-rich andhigh-cobalt layer and WC-6%Co with carbon-depleted low-cobalt layer were designed. The microstructure and mechanicalproperties of the bilayered cemented carbide were studied by means of scanning electron microscopy (SEM) and X-raydiffraction (XRD).

The results show that the Co phase in the bilayered samples migrates from a carbon-rich and high-cobaltlayer to a carbon-depleted and low-cobalt layer to form a gradient structure, in addition, as the difference in C content betweenthe two layers decreases, the gradient of Co concentration becomes larger; the WC grain size in the high cobalt layer is largerthan that in the low cobalt layer, besides, as the difference in C content between the two layers decreases, the difference inWC grain size becomes larger; as the difference in C content between the two layers decreases, the magnetic coercivity (Hc)becomes smaller and the cobalt magnetic (Com) becomes larger; the hardness gradually increases from 1 260 HV to 1 408 HV, the TRS of 2 612 MPa measured from WC-15%Co is higher than that of 2 388 MPa measured from WC-6%Co.

Cemented carbide (WC-Co) has become the most widely used industrial tool material due to its high hardness, good strength, heat resistance, wear resistance, and corrosion resistance. However, the hardness of WC-Co is inversely related to fracture toughness. Wear resistance is usually improved at the expense of fracture toughness, which in turn limits the life of many WC-Co tools. The mechanical properties of WC-Co mainly depend on the Co content and WC grain size. With the decrease of Co content and the refinement of WC grains, WC-Co has higher hardness and lower toughness. By designing different Co content or WC grain size, from the surface of WC-Co to the inside, a functionally graded cemented carbide (FG WC-Co) with a gradient of mechanical properties (such as hardness and toughness) is formed. Compared with the traditional uniform WC-Co, FG WC-Co can provide an excellent combination of hardness and fracture toughness, thus having excellent engineering properties.

At present, gradient cemented carbide has been extensively studied: DP (Dual property) series of carbide drill tools developed by Sandvik Company and the first reported surface-rich cemented carbide phase and beta-phase coated cemented carbide blades reported by Suzuki and others in Japan The matrix is ​​currently the most in-depth research and the most mature application of two types of functional gradient cemented carbide; another well-known process is Zhigang Zak Fang and colleagues through the design of different cobalt content or different WC grain size, studied the gradient cemented carbide . Although the gradient cemented carbide has been successfully prepared, the mechanical properties of FG WC-Co with gradient structure are still not comprehensive. Therefore, this experiment designed WC-6%Co with the same grain size and different carbon content (the content is mass fraction unless otherwise stated) and WC-15%Co double-layer cemented carbide to study the gradient cemented carbide Formation and its mechanical properties.

1 Experimental Materials and Methods

High-purity WC with the same powder particle size of 1 μm and different carbon content (carbon content 6.28%, 5.92%, 5.70%, 5.49%) and Co powder were selected as raw materials. The XRD patterns of the four kinds of WC powders with different carbon contents are shown in Figure 1(a). The WC powder with a carbon content of 6.28% has only the WC phase. As the carbon content of the WC powder decreases, the W2C0.85 phase and WC appear. The peak intensity of the phase also decreased, indicating that the WC phase content has decreased. A kind of carbon-rich WC-15%Co and three kinds of carbon-poor WC-6%Co mixtures were prepared by traditional wet grinding process. The nominal of WC-15%Co and WC-6%Co mixtures The composition and particle size of WC are shown in Table 1. Two-layer samples of WC-15%Co and WC-6%Co were prepared by two-shot molding method. The upper layer of the sample was WC-15%Co (layer A) and the lower layer was WC-6%Co (layer B), as shown in Figure 2 As shown. And prepared a single layer sample for comparison. For convenience, the WC-15%Co and three WC-6%Co single-layer samples are denoted No1, No2, No3, and No4 in sequence, and the double-layer samples are denoted No1+2, No1+3, No1+4. The sample was prepared by low-pressure sintering at 1440°C.

Fig.1 XRD patterns of WC powder (a) and monolayer and bilayered samples (b)

The microhardness of the sample was measured using a HV-Vicker hardness tester under a load of 98 N, and the coercive force (Hc) and the coercive force (Hc) of the sample were measured using a SKY-III coercive force meter and an ACoMT automatic cobalt magnetometer Cobalt magnet (Com). Scanning electron microscope (SEM, QuantaFEG 250) and energy spectrometer (EDX) were used to analyze the microstructure, and X-ray diffractometer (XRD, Rigaku D/MAX-2250, Japan) was used to analyze the WC phase and the binder phase. Characterized. According to ISO 3327:2009, the size of the flexural strength sample is 20 mm × 6.5 mm × 5.25 mm, and the TRS value is calculated according to formula (1) using the three-point bending test of type B specimen:

• R is TRS;
• F is the force required to break the sample
• l for two supports distance between points
• b is the width of the sample
• h is the height of the sample

Fig.2 Geometry of bilayered cemented carbide samples

2 Research Results and Discussion

2.1 Formation Mechanism of Double-Layer Gradient Structure and Co Gradient

At the liquid phase sintering temperature, WC-Co is a solid-liquid two-phase system, and the matrix of WC particles is filled with liquid cobalt phase. In order to form a gradient non-uniform structure in the FGWC-Co process, it is necessary to redistribute the liquid binder phase in the uniform structure. Lisovsky described the redistribution of the liquid phase as liquid phase migration (LPM). Liquid phase migration reduces the total energy of the system, so the WC-Co system has a spontaneous tendency to reduce the total energy, resulting in liquid phase migration. As shown in Figure 3, there are three factors that affect the liquid Co migration process: liquid Co migrates from high Co content to low Co content, from coarse WC grain to fine WC grain, and from high C content to low C content . Therefore, in this experiment, the WC crystal grain size is the same, the carbon content and cobalt content are different, and the WC-6%Co and WC-15%Co double-layer cemented carbides are designed. Layer A is rich in carbon and layer B is poor in carbon. Co expands from layer A with high cobalt rich carbon to layer B with low cobalt poor carbon to form a gradient structure.

Fig.3 Three factors affecting the migration direction of Co:volume fraction of liquid Co; WC grain size and C content in liquid Co phase

The three kinds of double-layer cemented carbide samples after sintering were used to detect the change of Co content in the height direction by EDS spectroscopy. Fig. 4(a) is a point diagram of the energy spectrum of the two-layer sample after sintering. Five points are taken from layer A to layer B in order to measure the change of Co content. Fig. 4(b) is the variation curve of the Co content in the height direction of the double-layer sample. The Co content of the three kinds of double-layer samples gradually decreases from the A layer to the B layer, and the Co content in the A layer is less than 15%, and the Co content in the B layer Both are higher than 6%, indicating that Co migrated from the high-cobalt carbon-rich A layer to the low-cobalt carbon-depleted B layer, forming a certain gradient structure. And the difference between the maximum and minimum values ​​of Co content δCo is 5.72% (No1+2)>4.68% (No1+3)>3.99% (No1+4). Because the difference in the C content between No1, No2, No3, and No4 gradually increases and the diffusion driving force gradually increases, the diffusion of Co gradually becomes larger, the Co content between the two layers becomes more uniform, and the δ value becomes smaller. At the same time, as shown in Figure 4(c), the line scan in the height direction of the double-layer sample detected that the C content in the double-layer sample changed little from the A layer to the B layer, and the W content fluctuated to some extent. There is no carbon-rich or carbon-lean double-layer sample. During the liquid phase sintering process, C also migrated, diffusing from the carbon-rich A layer to the carbon-poor B layer, and after 1 h of heat preservation, the double-layer sample The carbon in WC is saturated.

Fig.4 (a) Point taking diagram of EDS in height direction of bilayered samples; (b) cobalt content profiles after liquid-phasesintering of bilayered samples; (c) line scan figure in height direction of bilayered samples

2.2 Microstructure

Figure 1(b) shows the XRD patterns of single-layer and double-layer samples. It can be seen from the figure that the No1 and No2 single-layer samples are composed of WC and Co phases, and the No3 and No4 single-layer samples have a low C content, and part of Co reacts with WC and W to form the η phase (Co6W6C). No1 + 2, No1+3, No1+4 The three double-layer samples consist of WC and Co phases, and there is no graphite phase or η phase. This is because C in the carbon-rich A layer migrates to the carbon-lean B layer during the liquid phase sintering process, and the η phase in the B layer reacts with C to regenerate WC and Co. Moreover, because Co diffuses from the A layer to the B layer, the Co content in the carbon-lean B layer increases, and for the YG cemented carbide, the increase in Co content will cause the upper and lower limits of the carbon content in the two-phase region to expand In other words, the increased Co content makes the B layer less prone to η phase.

Fig. 5 is the microstructure of No1~No4 single layer sample. It can be seen from the figure that the No1 and No2 samples are composed of gray matrix WC and black bonding phase Co. The Co phase evenly wraps the matrix WC. The shape of WC is irregular rectangle or triangle, and there is no graphite phase or η phase. As the C content of the sample decreases, the dark gray η phase appears in the No3 and No4 samples, the color is between WC and Co, and the η phase content in the No4 sample increases as the degree of carbon depletion increases.

Fig.5 Microstructures of monolayer samples: (a) No1; (b) No2; (c) No3; (d) No4

Figure 6 shows the microstructure at the interface of No1+2, No1+3, and No1+4 double-layer samples. As can be seen from the figure, the three types of double-layer samples are composed of gray matrix WC and black binder phase Co. Partly composed. Since C in the carbon-rich A layer migrates to the carbon-lean B layer, C and η should be reversed to regenerate WC and Co, and the η phase disappears. Moreover, the two-layer sample binds well at the interface without delamination or uneven composition. It can also be clearly seen that the WC grains of layer A are larger than those of layer B, and the difference in grain size between the two layers of No1+2 is the most obvious. This is because the Co content of layer A is higher, and the dissolution of WC is stronger during the dissolution-reprecipitation process, resulting in the growth of WC grains more significantly than that of layer B. And the difference of No1+2 Co content δCo is the largest, so the dissolution-reprecipitation process difference between the two layers is the most obvious, and the grain size difference is the largest.

Fig.6 Microstructures of bilayered samples: (a) No1+2: (b) No1+3; (c) No1+4

2.3 Magnetic Properties

Table 2 shows the cobalt magnetic (Com) and coercive magnetic force (Hc) of single-layer and double-layer samples. The Com and Hc of the single-layer and double-layer samples are shown in Fig. 7. The Hc of No1, No1+2, No1+3, No1+4, No2, No3, and No4 gradually increase, and Com gradually decreases.

Samples No.Cobalt magnetic /%Coercivity Hc /(kA ·m-1)

No1	14.9±0.02	9.9±0.04
No2	5.4±0.02	14±0.04
No3	4.2±0.03	16.1±0.02
No4	2.8±0.02	16±0.03
No1+2	10.6±0.03	11.1±0.02
No1+3	9.6±0.02	11.7±0.03
No1+4	8.4±0.02	12.6±0.04

Table 2 Magnetic properties of monolayer and bilayered samples

The coercive force of WC-Co cemented carbide is mainly affected by the content of cobalt phase in the alloy and the grain size of WC. Under the same WC grain size, the coercive force of the sample can reflect the content of Co phase in the alloy. The coercive force of hard alloy is basically inversely proportional to the thickness of the Co layer. For the single-layer sample, the Co mass fraction of No1 is 15%, and the Co mass fraction of No2, No3, and No4 is 6%, so the Hc of No1 is the smallest. And because the carbon depletion degree of No2~No4 increases sequentially, the amount of η phase formed increases sequentially, resulting in the decrease of magnetic cobalt, the thickness of the cobalt layer decreases sequentially, and the Hc increases. For the double-layer sample, the Co content is between 6% and 15%. Due to the diffusion of carbon, the η phase reacts with C to regenerate the Co phase, the thickness of the cobalt layer increases, and the Hc decreases compared to the single-layer sample. And Hc decreases as the grain size increases. As shown in Figure 6, the smaller the difference in C content between the two layers, the larger the difference in grain size between the A and B layers, and the larger the coarse grain content in No1+2. The average grain size is the largest and Hc is the smallest, so the Hc of No1+2, No1+3, and No1+4 gradually increases.

Fig.7 Cobalt magnetic (Com) and magnetic coercivity (Hc) of monolayer and bilayered samples

In WC-Co cemented carbide, the cobalt metal is a magnetic material. Cobalt magnetism indicates that the alloy can be magnetized in the magnetic field. Cobalt accounts for the mass fraction of the alloy under test. It is an indirect assessment of the magnetic cobalt scale of the alloy. A parameter of carbon content. Generally speaking, under other conditions being the same, the greater the cobalt content in the alloy, the greater the Com, and as the carbon content increases, the Com increases. For the carbon-depleted No2~No4, part of Co generated η phase, and η phase is non-magnetic, resulting in a decrease in Com. Therefore, Com and Hc showed opposite trends.

2.4 Mechanical Properties

From the top to the bottom, 5 points were selected at equal distances in the height direction in order to measure the hardness value (for double-layer samples, layer A was defined as the upper layer and layer B as the lower layer). The selection of hardness points is shown in Figure 2. Figure 8 shows the top-down hardness of single-layer and double-layer samples. As shown in Fig. 8(a), the hardness value of the single-layer sample: No4>No3>No2>No1, and the hardness of each position of the same sample is basically the same. The hardness of cemented carbide depends on the two factors of grain size and cobalt content. The sample WC particle size is the same, so the main reason for determining the hardness is the difference in cobalt content. No1 has a maximum cobalt mass fraction of 15%. The higher the cobalt content, the lower the hardness, so No1 has the lowest hardness.

No2~No4 The cobalt content is the same but the carbon-depleted amount increases in sequence, and the content of the formed η phase increases in sequence. The η phase is characterized by being hard and brittle. Therefore, as the amount of carbon-lean increases, the hardness increases. As shown in Fig. 8(b), the general trend of hardness from layer A to layer B gradually increases, forming a gradient structure. The difference δHV between the maximum and minimum hardness of different double-layer samples is 148.42 (No1 + 2)> 114.16 (No1+3)>64.79 (No1+4). For the double-layer sample, according to the liquid phase migration (LPM) theory, when the WC grain size is the same and the carbon content and cobalt content are different, Co will diffuse from the A layer rich in carbon and high cobalt to the B layer poor in carbon and low cobalt To reduce the total energy of the system. As shown in Fig. 4(b), the Co content gradually decreases from the A layer to the B layer, forming a gradient structure. As the cobalt content decreases, the hardness increases, so the hardness generally shows an upward trend. The δCo between different bilayer samples is No1 + 2>No1+3>No1+4, the greater the difference in Co, the greater the difference in hardness HV and the greater the hardness gradient.

Fig.9 TRS of monolayer and bilayered samples

Figure 9 shows the flexural strength of single-layer and double-layer samples. The same sample is pressurized from the top and bottom directions (for double-layer samples, layer A is defined as the top and layer B is the bottom). Bending strength value. It can be seen that the flexural strength values ​​of the single-layer samples: No1>No2>No3>No4, the flexural strength values ​​measured under different pressures are basically the same. At the same grain size, the flexural strength is proportional to the cobalt content, the carbon-lean sample forms the η phase, and the flexural strength decreases. And each component of the single layer sample is uniform, and the flexural strength measured in different areas is the same. For double-layer samples, the flexural strength values ​​measured under pressure on side A are higher than the flexural strength values ​​measured under pressure on side B, and the flexural strength measured on the same side for different samples is similar. Because the cobalt content of layer A is higher than that of layer B, cobalt is a tough phase that can prevent the propagation of cracks, so the bending strength measured from layer A of high cobalt is higher. Moreover, the distribution of cobalt content between different double-layer samples is basically similar, so the flexural strength of different double-layer samples measured from the same surface under pressure is similar. The bending strength of the two-layer sample is between WC-15%Co and WC-6%Co single-layer sample.

3 Conclusion

• After sintering, the carbon concentration gradient between the layers in the FG WC-Co double-layer sample is eliminated, and the sample has no graphite phase or η phase; under the premise of the same powder particle size and the difference in cobalt content between the two layers, the FG WC-Co double The smaller the difference in carbon content in the layer samples, the more obvious the cobalt concentration gradient formed, indicating that the driving force for cobalt phase migration is weaker, which directly verifies the relationship between the formation of cobalt gradient and carbon migration during the sintering of gradient alloys.
• In the FG WC-Co double-layer sample, the WC grain size in the high cobalt layer is larger than that in the low cobalt layer, and the smaller the difference in C content between the two layers, the larger the WC grain size difference.
• The hardness of the FG WC-Co double-layer sample gradually increases from the high cobalt layer to the low cobalt layer, showing a gradient structure. And the smaller the difference in C content between the two layers, the more obvious the hardness gradient is, the maximum hardness gradient is from 1 260 HV to 1 408 HV. The bending strength of the FG WC-Co double-layer sample measured from the high cobalt surface WC-15%Co is 2 612 MPa higher than that measured from the low cobalt surface WC-6%Co 2 388 MPa. The strength is between a single layer of high cobalt and low cobalt.

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