Abstract
A sliding-pressure additive manufacturing technique with low cost and high accuracy based on Joule heat (SP-JHAM) was developed for the small metal parts. The temperature field and thermal history of the system are important for the experiment analysis. In this research, a thermal-electrical-structural coupling finite element simulation model for three-dimensional SP-JHAM process was established. The temperature field variation law during manufacturing, the temperature distributions inside the wire and substrate, and the shape of isothermal surfaces were analyzed. Results show that the Joule heat is generated between the wire and roller, and the internal temperature of wire rises to 2700 °C within 0.1 s. The position of the maximum temperature is moved with the roller moving. The temperature gradient inside the wire presents the arching shape, and that inside the substrate presents the semi-ellipsoidal shape. The simulated cross-section melting regions are in good agreement with the experimental ones. Thus, the established finite element model can accurately simulate the temperature field of SP-JHAM process, which is of great significance for the guidance of mechanism investigation and actual production.
Additive manufacturing (AM) is an advanced manufacturing technique, which has been rapidly developed in the past 30 years. Due to its low cost, short duration, and high personalized degree, AM has been widely used in various manufacturing industrie
Chen et a
However, it is difficult to accurately obtain the detailed temperature fields and thermal history during experiments. In recent years, numerical simulation has become a critical method to study the temperature field in metal AM processe
Therefore, in this research, a thermal-electric-structural coupling finite element model was established to simulate the temperature field and thermal history of SP-JHAM process. The accuracy of finite element model was verified through experiments. This research provided guidance for the finite element simulation method of SP-JHAM process.
The schematic diagram of SP-JHAM process is shown in

Fig.1 Schematic diagram of SP-JHAM process
The equipment of SP-JHAM experiment system is shown in

Fig.2 Equipment set of SP-JHAM process
The 304 stainless steel wire with cross section of 0.2 mm×0.2 mm was used as the feedstock because it has excellent corrosion resistance, high strength, and high plasticity, which is widely used in the power industry, steam turbine manufacturing industry, and medicin
Material | Cr | Mn | Mo | Ni | Si | C | P | S | Fe |
---|---|---|---|---|---|---|---|---|---|
316L | 16.38 | 0.86 | 2.11 | 10.11 | 0.44 | 0.018 | 0.026 | 0.007 | 68.33 |
304 | 18.01 | 0.77 | 0.03 | 8.03 | 0.47 | 0.052 | 0.027 | 0.003 | 69.34 |
Temperature/°C | Stainless steel | Copper electrode (CuCrZr) | ||||
---|---|---|---|---|---|---|
Specific heat/ J·k | Heat conductivity/ W· | Specific resistance/ ×1 | Specific heat/ J·k | Heat conductivity/ W· | Specific resistance/ ×1 | |
21 | 412 | 20.0 | 72 | 397.8 | 390.0 | 2.64 |
93 | 445 | 20.1 | 77 | 401.9 | 380.6 | 3.00 |
204 | 502 | 20.2 | 85 | 418.7 | 370.0 | 4.00 |
316 | 551 | 20.4 | 93 | 431.2 | 355.0 | 5.05 |
427 | 622 | 20.6 | 101 | 439.6 | 345.4 | 5.63 |
538 | 858 | 20.7 | 107 | 452.2 | 334.7 | 6.06 |
649 | 876 | 21.0 | 113 | 464.7 | 320.0 | 8.31 |
760 | 889 | 21.5 | 119 | 471.5 | 315.5 | 19.14 |
871 | 657 | 22.0 | 124 | 477.3 | 310.3 | 22.03 |
982 | 643 | 21.0 | 135 | 485.4 | 305.0 | 26.03 |
1093 | 690 | 20.0 | 150 | 497.8 | 300.1 | 31.69 |
1200 | 711 | 19.7 | 164 | 502.4 | - | 34.87 |
1500 | 1082 | 38.19 | 197 | - | - | 38.24 |
To simplify the calculations and to apply the boundary conditions, a finite element model of SP-JHAM process was simply established. The substrate size was reduced to 10 mm×10 mm×3 mm, and the corresponding copper ring size was also reduced to 10 mm×10 mm×1 mm. Meanwhile, the wall thickness of the copper ring was 1 mm. The upper half of the roller was cut away for convenient loading of model. The dimension of the metal wire in the model was 0.2 mm× 0.2 mm×8 mm. After the assembly was completed, the roller was placed directly above the metal wire, and the roller bottom was tangent to the upper surface of the metal wire. The model was divided into the grids with overall mesh size of 0.2 mm. The mesh of the temperature concentration region was subdivided with the minimum mesh size of 0.01 mm. The schematic diagrams of finite element model are shown in

Fig.3 Schematic diagrams of finite element model of SP-JHAM process
The SP-JHAM process is a complex process involving the multiple physical fields and multiple scales. During the process, the motion of the substrate-copper ring and the pre-loading of wire by roller should be investigated by transient structural analysis. The energization process should be investigated by transient electrical analysis. The generation of Joule heat after electrification as well as the heat conduction between different parts should be investigated by transient thermal analysis. Based on these information, a transient fully-coupled thermal-electrical-structural model was used to simulate the SP-JHAM process in this research. A fully-coupled thermal-electrical-structural analysis consisted of a coupled thermal-displacement analysis and a coupled thermal-electrical analysis.
The heat generation in SP-JHAM process is attributed to the Joule heat. Joule heat is governed by the Joule's law, and it describes the power dissipation rate Pec of electric current flowing (J) through a conductor, as expressed by
(1) |
where
The heat Q can be calculated by Joule's law, which is gov-erned by
(2) |
where is the current value in the system; is the total resistance value in the system; is the power-on time.
The SP-JHAM process should be investigated by transient thermal analysis. The fundamental equation of non-steady-state heat conduction obeys the Fourier's law, as follows:
(3) |
where T is the temperature function related to coordinates x, y, and z as well as time t; is the specific heat capacity of the material; is the material density; is the thermal conductivity of the material; is the generated heat per unit volume.
The thermal balance formula is expressed by
(4) |
where is the total heat; is the heat required for metallurgical bonding between the metal wire and the substrate; is the heat conduction; is the heat loss due to convection and radiation. In this research, since the process was conducted under the vacuum environment, only the radiation heat loss was considered, as expressed by
(5) |
where is the heat flux per unit area; is the surface temperature of parts; is the ambient temperature; is the absolute zero temperature; is the surface emissivity; is the Boltzmann constant.
The thermal-displacement coupling is mainly reflected by the thermal expansion of the material during the metal heating and the significant change in the thermal physical properties of the materials due to the rapid temperature evolution. In this research, the part movement and the change of Joule heat generation region during the printing process can also reflect the thermal-displacement coupling.
The Von Mises yield criterion was used. The plastic behavior of the material satisfies the plastic theory. The stress and strain satisfy the instantaneous linear relationship. According to the incremental theory, the governing equation for stress and strain increments is expressed by
(6) |
where is the stress increment; is the elastic-plastic stress-strain matrix; is the total strain increment.
The elastic-plastic stress-strain matrix can be obtained by simple calculation of the elastic and plastic matrices, as follows:
(7) |
where De is the elastic stress-strain matrix; Dp is the plastic stress-strain matrix.
The total strain increment consists of three parts: elastic strain increment , plastic strain increment , and thermal strain increment . Thus, the governing equation can be expressed by
(8) |
The governing equation of thermal strain increment is shown in
(9) |
where is the thermal expansion coefficient of the material.
The Newton method can be used to realize the asymmetric Jacobian matrix. The strong coupling governing equation of the incremental displacement and temperature is expressed by
(10) |
where and are the corrections to the incremental displacement and temperature, respectively; is a submatrix of the fully coupled Jacobian matrix; and are the mechanical and thermal residual vectors, respectively.
Due to the complexity of SP-JHAM process, a multi-analysis step was proposed for the simulation. To improve the convergence of the model calculations, the motion of substrate-copper ring was converted to the motion of roller. Therefore, the analysis was divided into seven steps, including the force loading step, force holding step, current loading step, roller moving step, current unloading step, cooling step, and force unloading step.

Fig.4 Timeline of analysis steps
In different analysis steps, the system is subjected to different boundary conditions and loads. In the initial analysis step, the surface of each component was set with its corresponding surface emissivity, and the absolute zero temperature (-273.15 °C) and ambient temperature (20 °C) were defined. The specific boundary conditions and loads were described as follows.
(1) All degrees of freedom for transition/rotation of the entire roller, except for the pressure direction, were constrained to 0.
(2) The bottom of the copper ring was fixed, and this surface was defined as a zero potential plane.
(3) In the roller moving step, the degree of freedom for the transition of the roller motion direction was constrained to L with L=vt, where v is the speed process parameter and t is the duration of the roller moving step.
(4) During the force loading step, a linearly increasing force was applied to the roller upper surface and held until the force unloading step started.
(5) During the current loading step, a linearly increasing current was applied to the roller upper surface and held until current unloading step started.
One of the major challenges in the numerical simulation of SP-JHAM process is the interface treatment, especially the contact part between the roller and the wire. Since the contact pair needs to transmit the force, electricity, and temperature parameters during the roller moving step, the conduction coefficients of force, electricity, and temperature between the contact pairs should be set. In this case, the electrical conduc-tion requires the electrical conductivity between two inter-faces, which involves the contact resistance. The total resis-tance value in the system is composed of four parts: the body resistance of the roller and copper ring (Re), the body resistance of the wire and substrate (Rw), the contact resistance between the roller and the wire (Rew), and the contact resistance between the wire and the substrate (Rc). Therefore, , and the schematic diagram of the components of total resistance is shown in

Fig.5 Schematic diagram of components of total resistance
The study of contact resistance can be traced back to the one-dimensional contact resistance finite element model, where the contact resistance is a function of temperature. Currently, the contact resistance is usually considered as a function of contact pressur
Temperature/°C | Thermal conductance/ mW·m | Electrical conductance/ m |
---|---|---|
20 | 140 | 6.4 |
205 | 150 | 6.4 |
425 | - | 6.5 |
650 | 160 | 6.7 |
870 | 160 | 8.5 |
1093 | - | 14.3 |
Temperature/°C | Thermal conductance/ mW·m | Electrical conductance/ m |
---|---|---|
20 | 560 | 12.7 |
205 | 640 | 14.9 |
425 | - | 20.4 |
650 | 1300 | 53.2 |
870 | 4800 | - |
1093 | - | - |
According to the analysis steps in
The primary goal of the 1st and 2nd analysis steps is to establish a complete contact for the entire system, ensuring the current flow in subsequent analyses.
In the third analysis step, the model is sliced along the X-axis in the direction of roller motion to analyze the changes in the 3D temperature field of the system. To provide more intuitive observation of the temperature field of the wire, the roller is set at the semi-transparent state. As shown in

Fig.6 Temperature field variations in the third analysis step: (a) t=0.02 s; (b) t=0.04 s; (c) t=0.06 s; (d) t=0.08 s; (e) t=0.10 s
During the fourth analysis step, the roller slides along the positive direction of X-axis, which leads to the change in the contact area between the roller and the wire. As a result, the region of Joule heat generation also changes. Path 1 is established by taking equidistant nodes along the middle layer of wire in the motion direction, as shown in

Fig.7 Schematic diagram of temperature field along Path 1 (a); temperature variations in the fourth analysis step (b); temperature fields at different times in the fourth analysis step (c)
During the fifth analysis step, the input current decreases from Imax to 0 within 0.01 s. However, the temperature field of the system does not reduce significantly during this step, and the highest temperature only decreases by 10–20 °C. Addi-tionally, in the sixth analysis step, the system temperature rapidly decreases: within the initial 0.1 s, the highest temper-ature decreases by nearly 2500 °C; in the next 0.4 s, the temperature decreases from 180 °C to 77 °C. In conclusion, after pre-loading, the Joule heat is generated between the roller and the wire with increasing the current. In the roller moving step, the region of Joule heat generation moves along with the roller. After the roller stops moving and the current is unloaded, the system temperature drops to about 70 °C within 0.5 s. After cooling, the highest temperature region is located in the front half of the wire due to its larger thermal accumulation, compared with that of the back part. The temperature decreases more slowly when it gets closer to the position directly below the roller. The temperature field variations of cooling process (the sixth analysis step) are shown in

Fig.8 Temperature field variations during the sixth analysis step
The temperature field at t=0.3 s during the fourth analysis step is shown in

Fig.9 Schematic diagram of Path 2 (a) and temperature field distribution along Path 2 (b) at t=0.3 s of the fourth analysis step
As shown in
The isothermal surfaces of the wire and substrate are shown in

Fig.10 Different isothermal surfaces at t=0.3 s: (a) system surface; (b) wire-substrate surface; (c) substrate surface; (d) wire surface
To verify the reliability of SP-JHAM finite element model based on the thermal-electric-structural coupling method, experiments were conducted with 304 stainless steel wire. The as-printed parts were cut, sampled, ground, and polished along the direction perpendicular to the heating direction. The simulated and experimental cross section morphologies of experiment object are shown in

Fig.11 Simulated and experimental cross section morphologies of experiment object (a); appearance and sampling position of experiment object (b)
1) The finite element model of the sliding-pressure additive manufacturing technique based on Joule heat shows high accuracy under the premise of ignoring the metal melt flow.
2) After the pre-loading, the Joule heat is generated between the roller and the wire with increasing the current. In the roller moving step, the region of Joule heat generation moves along with the roller. The area with the temperature above metal melting point includes some areas of wire and substrate. After the roller stops moving and the current is unloaded, the system temperature drops to about 70 °C within 0.5 s.
3) When the roller moves, the isothermal surfaces inside the wire form an arch-bridge shape, and the isothermal surfaces of the substrate has the semi-ellipsoidal shape. With increasing the distance between the roller and the position below the roller center, the height and span of the arch shape and the semi-axis length of the semi-ellipsoidal shape are increased.
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