Preparation of WC-TiC-Ni₃Al-CaF₂ functionally graded self-lubricating tool material by microwave sintering and its cutting performance

2.1 Gradient design

Self-lubricating gradient materials composed of solid lubricants CaF₂ and WC-TiC-Ni₃Al matrix materials are designed. Symmetrical exponential distribution function was used to design material gradients in this paper. The composition of the material changes symmetrically along the thickness direction, and it ignores the effects of stomata. Firstly, the WC-TiC-Ni₃Al cemented carbide matrix is regarded as a component m, and the gradient volume fraction distribution function of CaF₂ along the thickness direction of the material is expressed by the following forms:

Where x is the ratio of the distance of the component points from the surface of the gradient material to the total thickness of the tool material; V0CaF2andV1CaF2are the volume fraction of CaF₂ in the middle layer and surface layers respectively. n₁ is the gradient distribution exponent of CaF₂ in the tool material.

The volume fraction distribution function of the matrix element m is following:

In the matrix materialm, TiC-WC is set as a component p, the volume fraction of Ni₃Al obeys the gradient distribution exponent n₂, expression as following:

Where V0Ni3AlandV1Ni3Alare the volume fraction of Ni₃Al in the middle layer and surface layers respectively. n₂ is the gradient distribution exponent of Ni₃Al in the tool material.

Therefore, the volume fraction of TiC-WC composite phase components is:

In p, the WC as a free phase, then the volume fraction of WC obeys the function gradient compositional distribution exponent n₃, the expression is:

Where V₀^WC and V₁^WC are the volume fraction of WC in the middle layer and surface layers respectively, n₃ is the gradient distribution exponent of WC in the tool material.

In the multicomponent self-lubricating functionally graded cemented carbide material, the volume fraction distribution function of TiC is expressed by the following forms:

In order to ensure the overall strength and hardness of the tool material, the bottom matrix layer does not contain the solid lubricant CaF₂, and the surface volume of CaF₂ does not exceed 15%. For the Ni₃Al-TiC-WC-CaF₂ self-lubricating gradient cemented carbide tool material, the thermal expansion coefficient of TiC/WC is smaller than that of Ni₃Al. In order to gradually reduce the thermal expansion coefficient of the functionally graded tool material from the bottom to the surface layer, the content of TiC-WC in the matrix material Ni₃Al-TiC-WC should be increased from the middle layer to the two surface layers. Therefore, Ni₃Al and WC of the intermediate layer were set to 10% and 40%, respectively, and Ni₃Al and WC of the surface layer were set to 45% and 25%, respectively. So this article take V0CaF2=0,V1CaF2=15%,V0Ni3Al=40%,V1Ni3Al=10%,V0WC=45%,V1WC=25%,and the balance is TiC. Due to the symmetrical structure of the designed material and the particularity of the tool structure, the upper part of the designed tool was selected as the research object in this experiment. The bottom layer was made of Ni₃Al-TiC-WC and the surface layer was Ni₃Al-TiC-WC-CaF₂, and there are 4 layers of gradient self-lubricating tool materials between the bottom and the surface layer. This study mainly investigates the influence of CaF₂ distribution on material properties. Therefore, the gradient distribution index takes n₁=0.5, 1, 2, 4, n₂=n₃=1.

2.2 Material preparation

The raw materials were mixed by a planetary ball mill with a rotation speed of 340rpm for 2h with anhydrous ethanol and wax, and then dry at a temperature of 80^∘C. The dried powder was pressed by layer method according to the gradient design. The green compacts were sintered in a vacuum furnace heated to 400^∘C for 1 hour to remove the wax.

Finally, the pre-sintered sample was placed in a vacuum microwave oven with a frequency of 2.45 GHz heated to the temperature of 1400 to 1550^∘C for 30 minutes. Schematics of the microwave sintering furnace therein are shown in Refer [12, 13]. There was no auxiliary heating unit in the heating structure. To accelerate the rate of heating at low temperatures, a nitrogen pressure of 0.2 bar was achieved at 573K to induce microwave plasma heating; the nitrogen pressure reached 0.8 bar by soaking time. The temperature were measured by a pyrometer (Raytek Corp., America).

2.3 Mechanical properties and microstructure

The strength of the tool materials was evaluated by three-point bending test. The fracture morphology was observed by a scanning electron microscopy (SEM, JOEL, Japanese). The microstructure of the cross section of the polished sample was observed by a scanning electron microscope (SEM, Philips Corp., The Netherlands) in backscattered electron mode (BSE).

2.4 Cutting experiment

Orthogonal cutting experiments were carried out on a CNC lathe (LBR-370, Okuma Corp., Japanese). The cutting parameters were as follows: v_c=100 to 250 m/min, a_p=1mm and f = 0.1 mm/r, and the workpieces material was an AISI1045 steel sample with a hardness of 190 HB. Cutting force is measured by a three-way piezoelectric dynamometer and a charge amplifier, and the cutting forces in the three directions of X, Y and Z are collected by a data collector and data acquisition software (Kistler Corp., Switzerland). Friction coefficient between tool and chips was calculated by the following equation:

There, γ0is rake angle, F_x and F_y are main cutting force and radial force, respectively. In this paper, γ0is zero.

3.1 Process of microwave sintering

Figure 1 shows a typical temperature curve of Ni₃Al-TiC-WC-CaF₂ tool materials heating by microwave sintering. The microwave heating process is relatively easy to reproduce. The temperature error is negligible, especially in the high temperature range. The thermal runaway and sudden temperature changes described in the literature [10, 20] have not occurred. The materials can be heated from room temperature to 1500^∘C within 100 min with a microwave power of about 1KW. With this process, gradient TiC-WC-Ni₃Al-CaF₂ tool materials with regular shape can be obtained, as shown in Figure 2.

Figure 1

Temperature and power curve of microwave sintering process

Figure 2

Macro-picture of gradient tool material prepared by microwave sintering

3.2 Microstructure of gradient materials

Figure 3 shows the morphology (SEM, BSE) of the distribution of gradient layers for distribution index n₁ = 1, 2, and 4, respectively. It can be seen from the figure that when n₁ = 1, the gradient materials have good bonding between the gradient layers, but there are some uneven distributed components in the middle layer. When n₁ = 2, the gradient material is well bonded and there is no significant macroscopic non-uniform distribution. When n₁ = 4, there is no significant macroscopic non-uniform distribution between the gradient layers, but a large macroscopic crack, which could be caused by the stress mismatch original from the large difference in physical properties between the layers. The above phenomenon indicates that the distribution index has a significant impact on the mass transfer and stress generation.

Figure 3

Distribution of gradient layer (a-n₁ = 1, b-n₂ = 2, c-n₃ = 4)

Figure 4 shows the distribution of the cross-section elements of the Ni₃Al-TiC-WC-CaF₂ gradient tool material with distribution index n₁ = 2. The Ti, W, Ni and Al elements are evenly distributed in the tool section, indicating that the element diffusion is sufficient during the sintering process. Ca element has a significant transition between the surface layer and the subsurface layer, indicating that the goal of the gradient design is achieved.

Figure 4

Element distribution of Ni₃Al-TiC-WC-CaF₂ gradient material (n₁ = 2)

The surface layer’s microstructures of the TiC-WC-Ni₃Al based composites after microwave sintering at different gradient distribution are shown in Figure 5. The micrographs clearly illustrate the role of the addition of CaF₂. Homogeneous TiC-WC-Ni₃Al material has no distinct corering structure and exhibits a less pronounced black, gray, and white phase structure. When adding of CaF₂, the gradient material surface shows a distinct black, gray and white phase structure, and the white matter exhibits a certain agglomeration. In addition, the particles of all the TiC-WC-Ni₃Al-CaF₂ gradient materials are more refined. When the gradient index n₁ = 2 (Figure 5 d), the gradient material has a relatively obvious core-ring structure, with black core-grey ring as the main. The results of EDS show that black, gray, and white are WC-based, Ni₃Al-based, and TiC-based materials, respectively. Similar phenomena are found in transition layer. The above results show that the addition of CaF₂ refines the grains and promotes the formation of the core-ring structure.

Figure 5

Surface layer’s microstructure of TiC-WC-Ni₃Al based composites (a-homogeneous, b-n₁ = 0.5, c-n₁ = 1, d-n₁ = 2, e-n₁ = 4)

3.3 Mechanical properties

Figure 6 shows the transverse rupture strength of sintered functionally graded materials WC-TiC-Ni₃Al-CaF₂ and homogeneous WC-TiC-Ni₃Al. It can be seen from the figure that the strengths of WC-TiC-Ni₃Al and WC-TiC-Ni₃Al-CaF₂ functionally graded materials first increase and then decrease with increasing temperature, reaching maximum at 1500^∘C. WC-TiC-Ni₃Al-CaF₂ graded materials show a slightly higher strength than homogeneous WC-TiC-Ni₃Al. In addition, the gradient material with the distribution index n₁ = 2 has a higher intensity than that of the others. WC-TiC-Ni₃Al-CaF₂ with distribution index n₁ = 2 have almost the same strength when sintering temperature at 1450^∘C and 1500^∘C.

Figure 6

Effect of Temperature on the transverse rupture strength of sintered material

Figure 7 shows the fractured morphology of homogeneous WC-TiC-Ni₃Al and WC-TiC-Ni₃Al-CaF₂ gradient materials. Homogeneous WC-TiC-Ni₃Al material (Figure 7a) has larger particles and the fracture morphology shows obvious trans-granular fracture characteristics. WC-TiC-Ni₃Al-CaF₂ gradient materials (Figure 7b-e) showed inter-granular fracture and with fine particles, and this is consistent with the picture in Figure 4. When n₁ = 0.5, 1, 4, there are significant voids and micro-cracks appear at the fracture of the material. When n₁ = 2, the fracture of the gradient material shows a distinct dimples, which will help it achieve higher mechanical properties.

Figure 7

Fracture morphology of WC-TiC-Ni₃Al based materials (a-homogeneous, b-n₁ = 0.5, c-n₁ = 1, d-n₁ = 2, e-n₁ = 4)

Generally, CaF₂ reduces the mechanical properties of WC-Ni₃Al based materials. Deng et al. [16, 21] show that adding of CaF₂ reduces the mechanical properties of Al₂O₃-TiC ceramic materials by about 26%~47%. However, several researches show that CaF₂ can lower the sintering temperature [22, 23] and hence improving the density of sintering materials [24]. The main mechanism is the low melting point of CaF₂, and it served as a sintering aid in the sintering process, and also it has a refinement effect on the grains of the gradient materials, which contributes to the improvement of the mechanical properties of the material.

The mechanical properties of the functional gradient material will not decrease but will increase with a reasonable gradient design.WC-TiC-Ni₃Al-CaF₂ with distribution index n₁ = 2 have the highest strength when sintering temperature at 1450^∘C and 1500^∘C. However, WC-TiC-Ni₃Al-CaF₂ with other distribution index have no similar phenomenon, this indicate the gradient design have an important effect on the sintering process, and further effect the microstructure and mechanical properties of the gradient materials. Considering the effect of CaF₂ on the degradation of mechanical properties, the gradient design greatly improves the mechanical properties of WC-TiC-Ni₃Al-CaF₂ material. The main reason of the mechanical properties improving is release of the residual thermal stress resulting from fabricating process [25]. Our previous finite element analysis shows that WC-TiC-Ni₃Al-CaF₂ gradient materials with a distribution index n₁ = 2 has a low residual stresses [26], which is beneficial to obtain higher mechanical properties.

3.4 Cutting performance

Figure 8 shows the cutting force and the coefficient of friction between the chip and the tool when the WC-TiC-Ni₃Al and WC-TiC-Ni₃Al-CaF₂ gradient self-lubricating tools (n1 = 2) are used to cut 45 steel. As can be seen from the figure, the gradient tool has lower cutting force and chip-tool friction coefficient. The friction coefficient of homogeneous WC-TiC-Ni₃Al increase with the increase of the cutting speed, however, that of WC-TiC-Ni₃Al-CaF₂ gradient tool decreases with the increase of the cutting speed. Furthermore, the friction coefficient of gradient tools is reduced by 12.9-42.6% compare to homogeneous tool, showing that adding of CaF₂ can effectively reduce the cutting force and friction between tool and chips.

Figure 8

Cutting force and friction between tool and chips

Generally, cutting temperature rise as the cutting speed increasing. CaF₂ has good lubricating properties at high temperature because it have low shear strength a stable thermo-physical and thermochemical properties at elevated temperatures [22]. Therefore, CaF₂ plays a key role in reducing the coefficient of friction between the chip and tool, especially at higher cutting speed.