Investigation of the Bonding Mechanism of Al Powder Particles through Pulse Current Sintering Technology

Abstract

Compared with traditional powder metallurgy, pulse current sintering is an advanced powder-forming technology, but its bonding mechanism is still an open topic for debate. In this paper, pulse current sintering is used as the connection technology and millimeter-sized Al particles are used as the research object. In the whole sintering process, no pressure was loaded; the function of the pulse current was the only source of heat with which to achieve the bonding of Al particles. The bonding mechanism of pulse current sintering was investigated from the perspective of material connection behavior. The results show that the pulse current density of the particle surface reaches 3.48 × 10⁵ A/m² instantly, while the current density of the particle center is only 8187 A/m² at the initial stage, which is the main difference between pulse current sintering and traditional powder metallurgy sintering. With the densification process, the current density and temperature distribution in the contact region as well as the center of Al particles contact region tend to be more consistent. Finally, dense interfacial bonding was obtained, and the contact region of Al particles also demonstrated a high hardness value of 0.6385 GPa and yield strength value of 212.83 MPa. The whole process can be considered as a comprehensive action of melting (evaporation), diffusion, and plastic deformation. Based on the above results, a new technology, named high-frequency pulse current sintering, was proposed.

1. Introduction

The pulse current sintering process is a combination of a pulse current, sintering temperature, and loading pressure, which has been used to manufacture various materials, including alloys [1], composite materials [2], biomedical materials [3], and so on; however, the sintering mechanism is very complicated, especially the special role of the pulse current in the sintering process. There is still no unified conclusion. At present, there are many relevant studies on pulse current sintering, but the research on sintering mechanisms is quite limited. In order to clarify the sintering densification mechanism, it is necessary to clarify the flow direction and distribution characteristics of the pulse current.

Some researchers have developed different theories from different angles. Peng Dong [4] analyzed the sintering mechanism in spark plasma sintering (SPS) from the perspective of welding and divided the whole sintering process into a series of connection processes including micro-arc welding, resistance welding, and diffusion welding. Kunlan Huang [5] used SPS to densify the micron-sized copper powder. The whole sintering process was completed in a few seconds, including the combined action of melting and plastic deformation at the particle interface.

Zhang [6] cleverly placed six graphite indenters with different cross-sectional areas and placed a TiB₂ powder layer with an average particle size of 4.5 mm and thickness of 0.3 mm in the middle of the graphite indenter. Finally, the visual discharge phenomenon was directly observed based on direct naked eye observation and a micro-structure analysis. It is proven that the discharge effect can cause local high temperature and plasma in the gap. Tokita [7] also believes that the positive and negative potential difference between powder particles caused by the pulse current induces the generation of plasma, which is generally accepted by researchers at present.

At present, the pulse current sintering process is focused on the distribution of temperature and current density. As an advanced powder metallurgy technology, the quality of powder particle interfacial bonding directly determines the prosperity of the material. In fact, researchers should pay more attention to the bonding mechanism of powder interfaces than to the difficulty of directly investigating whether discharge, plasma, and arcs exist.

In fact, the optimal connection quality between powder particles is mainly required to have the following conditions from the view of material connection: the interface of powder particles presents fine grains and maintains excellent strength as well as toughness, while the inside of powder particles still maintains the original morphology without coarsening characteristics. The high-quality combination of micro-powder particles will ensure the overall excellent performance of materials. Considering that the sintering process is a dynamic process, it is more difficult to directly explore the densification mechanism of pulse current sintering through experiments; therefore, this study explores the bonding mechanism between powder particles by combining designed experiments with finite element simulation.

2. Experimental Procedure

In this study, 7075Al alloy balls with a diameter of 8 mm were selected as the research object, and three Al alloy balls were directly connected by the pulse current produced by SPS (DR. SINTER type SPS331-Lx). The specific chemical composition of the 7075Al alloy is shown in

Table 1. Chemical composition of the 7075Al alloy (wt.%).

Element	Al	Zn	Mg	Cu	Cr	Fe	Si	Mn
Content	89.25	5.53	2.67	1.75	0.26	0.19	0.17	0.082

. The stepped pulse current applied in the Al alloy ball pulse current sintering is shown in Figure 1b, and its waveform was measured by an oscilloscope, as shown in Figure 1c. The frequency and width of the pulse current are 20 Hz and 30 ms, respectively.

The specific test process is as follows: Three Al alloy balls with a diameter of 8 mm are placed into a graphite mold with an inner diameter of 16 mm, and the lower surface of the graphite press head is not loaded with pressure, as shown in Figure 1a. The conventional spark plasma sintering process adopts an automatic mode; that is, the sintering temperature and heating rate are set in advance, and the current values are automatically adjusted by the system. The sintering mode adopted in this paper is to control the current manually through the spin button. In this case, the current loaded during the experiment is completely controllable and fully matched with the finite element simulation process, as shown in Figure 1b.

Micro-structure evaluation was carried out using a JSM-6700F scanning electron microscope (SEM, JEOL Japan Electronics Co., Ltd., Tokyo, Japan) equipped with an electron backscatter diffraction (EBSD, Oxford Instruments, Oxfordshire, UK) system, X-ray diffraction (XRD, Rigaku TTR III, Tokyo, Japan) was tested using monochromatic Cu-Kα radiation with the X-ray wavelength of 0.154 nm at 40 kV and 150 mA. The micro-structure of bonding interface of Al ball was examined using a high-resolution transmission electron microscopy (HRTEM; 2100F JEOL Japan Electronics Co., Ltd., Tokyo, Japan) at an acceleration voltage of 200 kV. The micro-mechanical behavior of the composites was tested using an Agilent-G200 nano-indenter (Agilent Technologies Inc., Santa Clara, CA, USA) with a load of 20 mN.

The current flow direction, current density distribution, and temperature field distribution in the connection process were mainly discussed. Sandpaper from 400# to 2000#, followed by polishing, were employed to ensure that the metallographic specimen was free of scratches. Meanwhile, the micro-structure characteristic of particle interfaces was analyzed by EBSD and the interfacial micro-mechanical properties were tested by nano-indentation. Combined with the simulation, EBSD, and nano-indentation results, the bonding mechanism in the pulse current sintering process was discussed.

3. Results and Discussion

3.1. The Ansys Finite Element Simulation

3.1.1. Finite Element Model

Ansys/Multiphysics, a complete multiphysics coupling analysis platform, was used to perform the three-field sequential coupling of electrical, thermal, and structure analyses. The temperature and current distribution were transiently analyzed for the detection of the densification mechanism during the pulse current sintering process.

Based on the Ansys/Multiphysics simulation platform, this study conducted a three-field sequential coupling of the structural field, electric field, and temperature field in the pulse current sintering process, and carried out a transient analysis on the current density distribution of the Al ball boundary and the inside of an Al ball.

3.1.2. Structure Model

To simplify the calculation, the structural model is assumed to be continuous and isotropic. A simplified elastic constitutive equation is established according to generalized Hooke’s law [8]:

$$\left\{\sigma \right\}=\left[D\right]\left\{\epsilon \right\}$$

(1)

where $\left\{\sigma \right\}$ is the stress array, $\left[D\right]$ is the elastic matrix, and $\left\{\epsilon \right\}$ is the strain array.

The relationship between strain and displacement in the sintering process is as follows:

$${\epsilon }_x={\mathsf{\partial }}_u/{\mathsf{\partial }}_x, {\epsilon }_y={\mathsf{\partial }}_v/{\mathsf{\partial }}_y, {\epsilon }_z={\mathsf{\partial }}_w/{\mathsf{\partial }}_z$$

(2)

It can also be written in tensor form (the relationship between strain and stress):

$${\epsilon }_x=\frac{1}{E}[{\sigma }_x-\mu ({\sigma }_y+{\sigma }_z\left)\right]$$

(3)

$${\epsilon }_y=\frac{1}{E}[{\sigma }_y-\mu ({\sigma }_x+{\sigma }_z\left)\right]$$

(4)

$${\epsilon }_z=\frac{1}{E}[{\sigma }_z-\mu ({\sigma }_x+{\sigma }_y\left)\right]$$

(5)

$${\gamma }_{xy}=\frac{2\left(1+\mu \right)}{E}{\tau }_{xy}$$

(6)

$${\gamma }_{yz}=\frac{2\left(1+\mu \right)}{E}{\tau }_{yz}$$

(7)

$${\gamma }_{xz}=\frac{2\left(1+\mu \right)}{E}{\tau }_{xz}$$

(8)

The relationship between strain and displacement can be expressed by the following equations:

$$\frac{\mathsf{\partial }{\mathsf{\sigma }}_x}{\mathsf{\partial }x}+\frac{\mathsf{\partial }{\tau }_{yx}}{\mathsf{\partial }y}+\frac{\mathsf{\partial }{\tau }_{zx}}{\mathsf{\partial }z}+{\mathrm{f}}_x=0$$

(9)

$$\frac{\mathsf{\partial }{\tau }_{xy}}{\mathsf{\partial }x}+\frac{\mathsf{\partial }{\mathsf{\sigma }}_y}{\mathsf{\partial }y}+\frac{\mathsf{\partial }{\tau }_{zy}}{\mathsf{\partial }z}+{\mathrm{f}}_y=0$$

(10)

$$\frac{\mathsf{\partial }{\tau }_{xz}}{\mathsf{\partial }x}+\frac{\mathsf{\partial }{\tau }_{yz}}{\mathsf{\partial }y}+\frac{\mathsf{\partial }{\mathsf{\sigma }}_z}{\mathsf{\partial }z}+{\mathrm{f}}_z=0$$

(11)

3.1.3. Thermoelectric Coupling Model

In the pulse current sintering process, the heat source is mainly the joule heat generated when the pulse current passes. From Ohm’s law, q_v can be expressed by the following equation [9]:

$$q_{\mathrm{v}}=i_{\mathrm{r}}{\rho }_{\mathrm{r}}+i_z{\rho }_z$$

(12)

where $i_{\mathrm{r}}$ and $i_z$ are the radial and axial current densities, respectively, and ${\rho }_r$ and ${\rho }_z$ are the radial and axial resistivities, respectively.

According to the second law of thermodynamics, a difference in temperature must lead to the transfer of heat. The unstable thermal conductivity of the sintering process makes Fourier’s law inapplicable. Therefore, based on the law of the conservation of energy and Fourier’s law, the thermal conductivity differential equation is established, and the temperature distribution is obtained by combining specific boundary conditions. The established three-dimensional unsteady thermal conductivity differential equation is as follows:

$$\frac{\mathsf{\partial }T}{\mathsf{\partial }t}=\alpha {\nabla }^{2}T+\frac{Q}{\rho {\mathrm{c}}_{\mathrm{P}}}$$

(13)

where $\alpha$ is the thermal diffusivity and $\alpha =\lambda /\rho c_{\mathrm{P}}.$ ${\nabla }^2$ is the Laplace operator and ${\nabla }^2=\frac{{\mathsf{\partial }}^{2}T}{\mathsf{\partial }{x}^2}+\frac{{\mathsf{\partial }}^{2}T}{\mathsf{\partial }{y}^2}+\frac{{\mathsf{\partial }}^{2}T}{\mathsf{\partial }{z}^2}$.

The initial temperature is 22 °C, and the upper and lower copper electrodes are cooled by water cooling, so the surface temperature of the indenter and the electrode contact is kept at 22 °C. The heat loss caused by heat conduction and heat convection is not considered in the case of vacuum sintering; however, the radiant heat flux, $q_{\mathrm{r}}$, must be considered when calculating by the equation, considering that thermal radiation can be carried out without the medium [10,11]:

$$q_{\mathrm{r}}=\epsilon {\sigma }_{\mathrm{b}}(T_1^4-T_2^4)$$

(14)

where ${\sigma }_{\mathrm{b}}$ is the Boltzmann constant and is equal to 5.67 × 10⁻⁸ W/(m²·K⁴). $\epsilon$ is the thermal emissivity and is equal to 0.8. T₁ and T₂ are the source temperature (outer surface of the graphite mold) and the temperature of the body receiving radiant heat (inner wall of the furnace), respectively.

3.1.4. Boundary Conditions and Initial Conditions

(1)

Structural Boundary Conditions

Constraint conditions are applied to limit the degree of freedom of displacement of the model, and a 2 MPa constant pressure is loaded.

(2)

Temperature Boundary Condition

In the sintering process, the initial temperature in the vacuum chamber is set to 300 K, and the initial temperature of the sintering system is also 300 K.

In the simulation process, the graphite indenter and the mold surface as well as contact site are 300 K, there is thermal radiation between the indenter and the mold, the thermal radiation coefficient ε is 0.8, and the Stefan–Boltzmann constant, ${\sigma }_{\mathrm{b}}$, is 5.67 × 10⁻⁸ W/(m²·K⁴).

3.1.5. Simulation Results

The Ansys finite element simulation was used to calculate the current density distribution and temperature distribution of the particle interface under the action of the pulse current. The pulse current loaded during the simulation is consistent with the experiment. As shown in Figure 2h, when the time lasts for 50 s, the current density at the contact point of the Al particles reaches 3.48 × 10⁵ A/m², while the current density at the center of the Al particles is only 8187 A/m², which indicates that the particles preferably flow through the contact point during the sintering process, and only a small part of the current flows through the center of the Al particles. When the time lasts for 100 s, the current density at the contact point of the Al particles increases to 8.77 × 10⁵ A/m², while the current density at the center of the Al particles only increases to 53,065 A/m². At this time, the current density at the contact point reaches its maximum in the sintering process. As the sintering temperature between the Al particles continues to rise, the melting phenomenon will occur. Considering that the evaporation temperature of the Al alloy will be greatly reduced if the vacuum degree is very low, evaporation may occur, which leads to the change in contact mode between particles from point contact to surface contact [12]. At this time, the current density between the Al particles decreases with the increase in contact area during sintering. When the time lasts for 250 s, the current density at the contact point of the Al particles decreases to 1.69 × 10⁵ A/m². As the process goes on, the current density at the contact point of the Al particles increases slowly due to the high current value at the later stage, while the current density at the center of the Al particles increases continually.

The temperature range curve obtained during Ansys finite element simulation is shown in Figure 2h. It can be seen that when the time lasts for 50 s, the temperature of the Al particle contact point reaches 508 °C, and the temperature of the particle center is only 39 °C. When compared with current density curve, it can be concluded that current priority flows through the particle contact point and the temperature will heat up to 508 °C immediately. When the time lasts for 100 s, the temperature of the particle contact point reaches 1278 °C, while the temperature of the particle center is only 79 °C. In this condition, melting (and even evaporation) will occur, considering that the melting point of the Al alloy is 660 ° C. The contact mode changes from point contact to surface contact, resulting in the rapid decrease in current density. In this process, heat dissipation between Al particles and the graphite mold makes the temperature between particles drop to 958 °C temporarily. When the time lasts for 250 s, the main heat resource between particles is converted into joule heat. As the process continues, the joule heat makes the sintering temperature rise slowly, and the inhomogeneity of temperature distribution decreases gradually. Finally, the temperature of the particle contact point is 579 °C, and the temperature of the particle center is 514 °C. By comparing Figure 2g with Figure 2h, it can be seen that a significant difference in the latter half of the curve exists, which can be attributed to the heat dissipation between the Al particles and the graphite mold, and the higher the temperature is, the more obvious the heat dissipation is. In the present study, the sample is an Al ball, which has a higher thermal conductivity (238 Wm⁻¹ K⁻¹) than other alloys (59.3 Wm⁻¹ K⁻¹ for Ni and 80 Wm⁻¹ K⁻¹ for Fe). The difference between the temperature at the bonding interface region and the average temperature of powder particles should be small; however, the temperature difference between the interface and internal particles is large, which can be attributed to the rapid energy concentration of the pulse current.

It is still a controversial topic as to whether there is an arc in pulse current sintering; therefore, we investigated this. From the angle of arc generation, this study has the condition of arc generation. Firstly, the duty cycle of the pulse current is 4:5, and an on–off pulse current is carried out continuously. Secondly, the maximum of the pulse current density is up to 8.77 × 10⁵ A/m², and the current density simulation results confirmed that the pulse current prefers to flow through the particle surface; thus, the particle contact point can generate a micro-arc. The arc phenomena and conclusions were also confirmed by other scholars [13].

3.2. Interfacial Micro-Structure Feature

The macroscopic bonding morphology of Al particles under the action of the pulse current is shown in Figure 3. Obviously, the interfacial micro-structure is characterized by EBSD, and the interface of the Al ball presents high-quality bonding. The grain orientation in the Al matrix and neck region is randomly distributed, and no preferred orientation exists. The grain size in the center area is fine, while some coarse grains exist 50 μm from the particle boundary, which may be due to the heat transfer from the local high temperature at the particle contact interface. In the sintering process, the high-energy pulse current flows preferentially through the particle boundary, and a limited current flows through the particle center, which means that the original structure of the Al particle is saved. The majority of the neck region is fine grain with a high-angle boundary, and the grain size is much smaller than the inner region of the particles; this can be attributed to the fact that the melting or even evaporation caused by an arc can condense finer recrystallized grains. Meanwhile, it can be found that the unconnected areas along the neck (which may result in the generation of an arc) are all finer recrystallized grains. In conclusion, the advantages of pulse current sintering technology ensure a relatively weak area (the particle bonding interface) of the finer micro-structure.

3.3. Nano-Indentation Property

From the perspective of action, hardness is the ability to resist hard objects pressed into the surface. From the perspective of deformation, hardness is the ability to resist residual deformation and failure, and the average stress of the mechanical response of the indenter acting on the material, rather than the basic mechanical parameters of the material; therefore, hardness is not a fixed value under the conditions of different indenter types or indentation depths. Considering the influence of indentation depth on relevant mechanical properties, Nix [14] proposed a hardness model without the influence of indentation depth:

$$\frac{H}{H_0}=\sqrt{1+\frac{h^{\ast }}{h}}$$

(15)

$$H^2=\frac{H_0^2{h}^{\ast }}{h}+H_0^2$$

(16)

where $H_0$ is the hardness without the influence of indentation depth, and $h^{*}$ is a parameter of the length dimension, which is called the characteristic length. The relationship between hardness and depth is shown in Equation (15). After transforming the expression form, it can be seen that the square of hardness ($H^2$) has a linear relationship with the reciprocal of indentation depth (1/H), as shown in Equation (16).

According to the indentation data of 500 nm–2000 nm, the linear fitting of ($H^2$) and 1/h is carried out. The intercept and slope of the fitting line are ($H^2$) and $H_0^2{h}^{*}$, respectively. The corresponding results are obtained, as shown in

Table 2. The modified value of hardness and yield strength of Zone 1, Zone 2, and Zone 3.

Zone 1	${\mathit{H}}_0$/GPa	${\mathit{h}}^{*}$/μm	R2	Yield Strength/MPa
1	0.630	0.511	0.984	210.00
2	0.532	0.954	0.976	177.33
3	0.682	0.426	0.985	227.33
4	0.583	0.653	0.988	194.33
5	0.675	0.546	0.936	225.00
6	0.666	0.311	0.959	222.00
Average	0.6385			212.83
Zone 2	${\mathit{H}}_{\mathbf{0}}$/GPa	${\mathit{h}}^{\mathbf{*}}$/μm	R2	Yield Strength/MPa
1	0.519	0.747	0.999	173.00
3	0.652	0.433	0.942	217.33
5	0.643	0.419	0.952	214.33
6	0.535	0.766	0.954	178.33
7	0.616	0.512	0.964	205.33
8	0.548	0.853	0.995	182.67
Average	0.5855			195.17
Zone 3	${\mathit{H}}_{\mathbf{0}}$/GPa	${\mathit{h}}^{\mathbf{*}}$/μm	R2	Yield Strength/MPa
1	0.617	0.711	0.992	205.67
2	0.536	0.927	0.980	178.67
3	0.576	0.729	0.992	192.00
4	0.653	0.437	0.976	217.67
5	0.590	0.615	0.991	196.67
6	0.599	0.727	0.981	199.67
Average	0.5955			198.50

.

The nano-indentation test results of Zone 1, Zone 2, and Zone 3 are shown in Figure 3c. Under the same load, 20 mN, Zone 1, with finer recrystallized grain, has the highest hardness value, which can be attributed to the local high temperature caused by melting and evaporation. The finite element simulation results also confirmed that the pulse current preferred to flow through the particle boundary, while the current in the center of Al particles was less; therefore, the low hardness values of Zone 2 could be attributed to the grain coarsening caused by the heat conduction induced by local high temperature at the interface, and the hardness values are lower than those of the particle center. Owing to the fact that no current flows, the hardness in Zone 3 reflects the basic hardness of the Al matrix.

Related studies have shown that there is a direct relationship between the hardness and yield strength of alloy materials [15]; therefore, the yield strength of different regions of the particle interface can be calculated from the modified hardness value. Obviously, the yield strength of Zone 1 has the maximum value (212.83 MPa) owing to the finer recrystallized grain, indicating that high-quality interface bonding can be obtained under high-frequency pulse current sintering technology. As a result of comparison, the yield strengths of Zone 2 and Zone 3 are only 195.17 MPa and 198.50 MPa.

Figure 4 shows the XRD results of a raw Al ball and an as-sintered Al ball. The results show that the main phase of a raw Al ball and an as-sintered Al ball is only Al; there is no other new phase, which may be attributed to the low content of precipitated phase. In order to further investigate the grain morphology and precipitated phase distribution, TEM test results were supplemented in Figure 5. Results show that some precipitated phases exist, and the precipitated phases in Z1–Z3 were distributed in the grain and the grain boundary. The average grain size regularities in Z1–Z3 correspond to the EBSD results. Since this paper focuses on the effect of the pulsed current on the powder particle bonding mechanism in the sintering process, the precipitated phase behavior is not extensively discussed here.

Based on the micro-structure analysis and the guidance of finite element simulation, it can be concluded that the local high temperature between particles makes the melting (evaporation) on the surface of particles become the main bonding mechanism in the initial stage. As the process goes on, the neck formation changes the contact mode from point contact to surface contact, and joule heat becomes the main bonding mechanism between particles. If the corresponding pressure is applied in the holding stage, large plastic deformation occurs between numerous powder particles, in which joule heat and plastic deformation together promote bonding between the particles. In summary, the pulse current sintering process can be understood as a combination of melting (evaporation), diffusion, and plastic deformation.

3.4. Development of an Innovative Powder Sintering Technology

Zhenbo Xu [16] investigated the interfacial bonding mechanism of AZ31B magnesium alloy composite plates under the action of a high-frequency pulse current, and it was proven that the good interface bonding quality can be attributed to the skin effect, proximity effect, joule heat effect of a high-frequency pulse current, and the coupling effect of contact resistance, as well as rolling force in the interface micro-region. Based on the investigation of the micro-bonding mechanism in pulse current sintering, at the same time considering the skin effect, proximity effect, and arc discharge effect of a high-frequency pulse current, an innovative technology, named the high-frequency pulse current technique (5 KHz~50 KHz), was put forward. Under this condition, the micro-component edge can be bonded in a shallower depth with the melting status. The whole power-on process can be considered as the continuous self-cycle of “gap arc → heat generation → melting → connection”, until all gaps are closed. If the process parameters are well controlled, the internal temperature of the component is low and there is no great change in the microstructure, so the designed material properties can be obtained. The heating range of the component edge is smaller than low-frequency pulse current sintering, the interface bonding is consistent with the overall prosperity, and the overall performance is more uniform. Figure 6 is the schematic diagram of the high-frequency pulse current sintering technique.

4. Conclusions

(1)

The current distribution in pulse current sintering demonstrates that the pulse current flows preferentially through the particle surface. The pulse current density of the particle surface reaches 3.48 × 10⁵ A/m² instantly, while the current density of the particle center is only 8187 A/m² at the initial stage, which is the main difference between pulse current sintering and traditional powder metallurgy sintering.

(2)

Under the action of pulse current sintering, the gap arc, heat generation, melting, and connection processes occurred in sequence, which can be attributed to the change in the contact mode between Al particles. The micro-structure characteristics of EBSD and TEM also confirm the densification mechanism of the pulse current sintering technology.

(3)

An innovative powder sintering technology, the high-frequency pulse current sintering technique, is proposed based on the development of high-frequency welding.