The technology of SPS applied to material joining
Spark plasma sintering (SPS) is a powder metallurgy technology with high efficiency, low cost, and low energy consumption. In the past three decades, this technology has been widely used in preparing materials such as high-temperature alloys, ceramics, cemented carbides, high entropy alloys, and refractory metals. The research on spark plasma sintering SPS applied to material joining is also very extensive.
Spark plasma sintering (SPS) is also known as field assisted sintering technology (FAST), direct current sintering (DCS), or fast hot pressing sintering (FHP). As a multi-energy field assisted sintering technology, SPS applies a controllable DC pulse current to the sample while pressurizing the powder to induce unique effects such as local Joule heating and plasma activation. Thus, fast, low-temperature, and efficient sintering of materials can be achieved.
At the microscopic level, the essence of material sintering and joining is the same. Both are processes of eliminating the “solid-gas” interface between materials, achieving “solid-solid” interface substitution, and forming metallurgical bonding. Dong et al. analyzed the microscopic mechanism of SPS from the perspective of material joining. It is believed that the sintering process includes the basic connection principles of resistance welding, micro-arc welding, diffusion welding, etc. Therefore, it is theoretically feasible to apply SPS technology to material joining.
In recent years, SPS technology has been successfully applied to the joining of different materials and has shown satisfactory results. Compared with traditional diffusion welding processes, SPS diffusion welding often has outstanding advantages such as low joining temperature, short welding time, interface mass transfer, fast bridging, and high welding precision.
SPS joining technology classification
SPS joining is a high current and low voltage joining method. During the joining process, axial pressure and controllable pulse current are applied to the sample. The sample is heated by the Joule heat generated by the pulse current. Under the action of pressure and high temperature, the joined surfaces approach each other and undergo local plastic deformation. The mutual diffusion of interface elements causes the interface to recrystallize or (and) produce a diffusion layer, eventually forming a reliable metallurgical connection. Therefore, SPS joining directly uses the Joule heat of current as the welding heat source without the need for an external heating device. The characteristics of “high current and low voltage” and “no external heat source” are its biggest differences from traditional electrically assisted joining technologies such as anodic bonding.
Classification according to joining interface structure
SPS joining technology is classified into direct joining and indirect joining. The joining process of direct joining does not add any intermediate layer material. It is often used to join the same material or dissimilar materials with similar properties. It can avoid the adverse effects of complex interface reactions and interface corrosion caused by added materials.
Indirect joining uses powder or foil as the intermediate layer material for connection. It is generally suitable for the joining of thermal expansion mismatch materials and difficult-to-weld materials. It can effectively relieve joint stress and promote interface diffusion to obtain reliable joints. Compared with direct joining, indirect joining has lower requirements for joint shape and surface flatness. And is currently the most commonly used joining method.
Classification by heating mode
SPS joining technology is classified into temperature, current, voltage, and other control modes.
Temperature control mode, that is, the pulse current size is adjusted through the temperature feedback of thermocouples or infrared thermometers. Joining is carried out according to the set temperature schedule. This is the current mainstream mode.
Current control mode, that is, a set current (or current density) is applied to the sample for heating and joining.
Voltage/power control mode: that is, the sample is heated and joined according to the set voltage/power.
In addition, molds can be used or not in the connection process. When the conductivity of the base material is poor or a powder intermediate layer is used, a mold is usually used for the joining. At this time, the current passes through the external mold and the sample, and the sample is heated by both internal and external heat sources. When the conductivity of the joining material is good, the sample can be directly heated by the current. At this time, the heating can be faster, and the sample will also cool quickly after the power is off. Studies have shown that the effects of local Joule heating, electrothermal dispersion, and electroplasticity brought by pulse current are more obvious when there is no mold.
Basic principles and features of SPS joining technology
SPS joining technology is generally solid-phase diffusion joining. Traditional diffusion joining can be divided into three stages: physical contact, activation of the contact surface, diffusion, and joint formation. Pulse current has an important influence at different stages.
In the physical contact stage, due to the large impedance of the contact interface, the pulse current will generate more Joule heat at the interface and cause a local high temperature. Electroplasticity can reduce the yield strength of the parent material, promote rapid deformation of the joining interface and the disappearance of holes, and enhance the interface contact.
In the activation stage of the contact surface, the high-frequency pulse electric field will induce spark discharge at the joining interface. The principle is shown in Figure 1. This phenomenon is conducive to removing the oxide film and adsorbed gas on the surface to be joined, and has a surface activation effect. At the same time, spark discharge causes the grains to melt and evaporate, triggering the evaporation-solidification transfer of the material, which can promote the formation of the interface.
In the diffusion and joint formation stage, the local high temperature of the interface (including grain boundaries) promotes the bulk diffusion and grain boundary diffusion of atoms. The “electron wind” and other effects increase the equilibrium defect concentration of defects and reduce the diffusion activation energy, thereby promoting element diffusion. These factors are conducive to micropore bridging and interface recrystallization.
The features of SPS joining technology
Therefore, the high-frequency pulse current brings about effects such as Joule heat, discharge plasma, electron diffusion, and plasticity. Compared with traditional diffusion joining, SPS joining often has the characteristics of low joining temperature, short welding time, fast interface bridging, and high welding precision. However, which effect is dominant in the connection process may be related to factors such as the physical and chemical properties of the material, the heating mode, and the assembly form of the sample. The relevant mechanism needs to be studied.
Current status of SPS applied to Material joining
SPS technology is widely used in metal joining. Reported research involves copper alloys, aluminum alloys, TiAl compounds, high-temperature alloys, refractory metals, stainless steel, and other material systems.
SPS applied to metal material joining
TiAI series metal joining
TiAI intermetallic compounds have the characteristics of low density, high melting point, and excellent mechanical properties. They have broad application prospects in the fields of aerospace engines. Due to high brittleness and strong crack sensitivity, this type of material is difficult to join by methods such as fusion welding. The use of traditional diffusion welding to prepare related components is not only time-consuming and energy-consuming but also has problems such as large residual stress and many brittle compounds. SPS technology can achieve direct joining of TiAI compounds. The connection temperature and time can be further reduced through a suitable intermediate layer, and a connection joint with excellent mechanical properties can be obtained. Therefore, SPS technology provides a new idea for the joining of this type of material.
Traditional diffusion welding usually requires the entire workpiece to be exposed to a high-temperature environment for a long time. It is easy to cause problems such as phase change, grain growth, and composition segregation of the parent material. SPS joining can better deal with such problems. He et al. found that when SPS joined Ti-6AI-4V, the joint temperature decreased from the interface to both sides, showing a gradient distribution. This not only promotes the diffusion of elements at the interface but also avoids the degradation of the parent material properties due to heat.
Joining with powder intermediate layer
Yang et al. used a powder interlayer to further amplify the local high-temperature phenomenon at the interface. Due to the small contact area between powder particles and the presence of an oxide film on the surface, the impedance of the powder interlayer is much higher than that of the parent material. When powered on, the heating rate can reach 100 times that of the parent material. Therefore, by reasonably adjusting parameters such as power and heating time, the joining of 316L stainless steel was finally achieved within a few seconds. The heat-affected zone was minimized. Fu et al. pointed out that the use of SPS technology can achieve the joining of oxide dispersion-strengthened stainless steel (ODS steel) without obvious grain growth and oxide agglomeration.
Joining dissimilar materials
For dissimilar material systems with poor metallurgical compatibility such as titanium/steel, SPS joining inhibits the growth of brittle compounds to a certain extent. It is expected to replace common methods such as explosive welding to prepare heterogeneous joints with excellent performance.
Miriyev et al. used SPS technology to prepare Ti-6AI-4V/carbon steel joints without intermetallic compounds. The tensile strength can reach 250 MPa. Ananthakumar et al. used SPS technology to achieve rapid welding of titanium and 304L stainless steel under the condition of 5 minutes of heat preservation at 650℃. The growth of Fe-Ti brittle compounds was inhibited by low-temperature short-time connection. The shear strength of the obtained joint reached 429MPa±18MPa.
In addition, by utilizing the advantages of SPS in sintering and joining. The process flow can be shortened and the cost can be reduced. Huang Hao et al. used SPS technology to simultaneously realize the sintering of TZM powder and the connection with the tungsten block. And the grains of the joint were refined by rapid cooling by power off. Thus, the one-step preparation of tungsten-molybdenum composite targets with high density and strong bonding force was achieved. Yang et al. used SPS to realize the welding of TZM alloy and WRe alloy. No intermetallic compounds were generated at the joint interface. There were no micropores and microcracks, and the mechanical properties of the weld were excellent. And it still maintained high strength after 1500 thermal shocks.
SPS applied to ceramic material joining
SPS is also widely used in ceramic material joining. However, related research mainly focuses on SiC and its composite materials, and ultra-high temperature ceramics. The material system needs to be expanded.
Diffusion bonding of ceramics using an intermediate layer SPS
To achieve rapid joining of ceramics, an intermediate layer material is needed to promote interface contact and diffusion during joining. Li et al. used Ta5W intermediate layer SPS diffusion joining SiC ceramics. It was found that the element diffusion coefficient in SPS mode is two orders of magnitude higher than that in traditional diffusion joining. In the study, it was found that a good joining can be achieved by keeping it at 1600℃ for 5 minutes. And the shear strength of the joint can reach 122MPa±15MPa.
Rizzo et al. pointed out when joining B-SiC that when there is no intermediate layer. The surface of the sample needs to be polished. When an intermediate layer is used, the surface requirements can be reduced. Zhao et al. used two composite intermediate layers of Mo/W/Mo and Ti/Nb/Ti to connect Cf/SiC materials. It was found that the optimal connection temperatures were 1600℃ and 1200℃, respectively. It shows that a reasonable intermediate layer design can significantly reduce the connection temperature.
Compared with metal intermediate layer materials, the thermal expansion of ceramic intermediate layers and substrates is generally more matched. It can reduce the residual stress of the connection joint. In response to the application needs in the nuclear energy field. Zhou et al. designed a series of ceramic intermediate layers including Ti3SiC2 adhesive tape, SiC/Al4SiC4, and SiCw/Ti3SiC2 powder for SPS joining of SiC ceramics. All of them achieved good results, with the highest shear strength reaching 250MPa.
Problems with SPS applied to Material joining
As a new type of welding technology, SPS connection has shown good application prospects in a variety of material systems. However, with the promotion of this technology and the continuous emergence of new material systems, there are still many problems to be solved.
- The relevant mechanism has not been clarified. At present, the research on SPS joining is mainly focused on the process and feasibility level. In terms of the joining mechanism, the relevant theories of SPS sintering are simply applied. And there is a lack of guiding theories.
- The joining materials and forms need to be expanded. In terms of joining form, the current SPS joining mainly adopts two forms: sintering welding and diffusion welding. Whether the forms such as “SPS+brazing”, “SPS+transient liquid phase diffusion welding” and “SPS resistance welding” are feasible remains to be studied. Whether SPS joining is suitable for systems such as ceramics/metals in terms of joining materials remains to be studied.
- The “equipment-process-organization-performance” chain has not yet been opened. SPS joining is a non-steady-state process, and the joint organization and performance depend on the joining process. The joining process depends on the equipment (such as pulse waveform, etc.).
Summarize
SPS joining has many advantages such as low welding temperature, short connection time, high efficiency, good precision, wide material adaptability, etc. It has received great attention from domestic and foreign academic and engineering circles. And is an important new direction of connection technology in the future.
Establishment of SPS joining technology theoretical model.
Deeply reveal the pulse electric field interface activation mechanism. The microscopic mechanism of electron diffusion welding, the interface mass transfer under multi-field coupling, and the dynamic recrystallization behavior. Establish the thermodynamic and kinetic laws and theoretical models of mass transfer under a pulse electric field. Finally, establish a welding theory that helps improve the quality of SPS joining and process.
SPS joining technology equipment and process database development.
Combined with the characteristics of SPS joining, develop domestic connection-specific equipment with strong practicality and wide applicability. Combined with the specific needs of the manufacturing industry, establish a connection process database to accelerate the industrial application of SPS connection.
The part of this article is excerpted from：
Current status of application of spark plasma sintering technology in the field of material connection
Lin Panpan, Harbin Institute of Technology
