The semisolid metal joining (SSMJ) process or thixojoining process has recently been developed based on the principles of SSM processing, which is a technology that involves the formation of metal alloys between solidus and liquidus temperatures. Thixojoining has many potential benefits, which has encouraged researchers to carry out feasibility studies on various materials that could be utilized in this process and which could transform the production of metal components. This paper reviews the findings in the literature to date in this evolving field, specifically, the experimental details, technology considerations for industrialization, and advantages and disadvantages of the various types of SSMJ methods that have been proposed. It also presents details of the range of materials that have been joined by using the SSMJ process. Furthermore, it highlights the huge potential of this process and future directions for further research.
The rigid perpetual joining of materials is one of the main activities in the manufacturing and assembly process. Fusion welding is one of the most commonly used conventional joining techniques. Unfortunately, fusion welding is characterized by high temperature gradients that lead to high thermal stress and rapid solidification, which gives rise to the occurrence of segregation phenomenon. Also, the morphology of the welded interface is typically dendritic and the natural progression of solidification frequently leads to internal structural defects, such as shrinkage porosities and loss of alloying elements, resulting in a nonhomogenous microstructure [
Following the work of Spencer et al. (1972), which identified the essential thixotropic properties [
However, one of the most important and useful characteristics of semisolid alloys is their so-called “joinability.” As is well known, semisolid alloys consist of solid and liquid components. The liquid component is usually very active during diffusion compared with the solid component and because of the active diffusion effect of the liquid component, semisolid alloys have very good joinability with other metallic and nonmetallic materials [
During the last 30 years, a number of process variants have been developed under the heading SSMJ. Several authors have shown that the thixojoining process has the potential to be used for a wide range of processes that are either already patented or under investigation within research and development centres worldwide. In spite of some technical and technological differences between the available semisolid joining processes, they can be categorized into the following types: (1) addition of functional features, (2) joining metals by using semisolid slurries, (3) semisolid stir joining, and (4) semisolid diffusion joining, which are discussed below.
Thixojoining has many potential benefits, which has encouraged researchers to carry out feasibility studies on various materials that could be utilized in this process. The process of thixojoining has been shown to increase the functionality and complexity of components by allowing the inclusion of additional inserts in the semisolid matrix during forming. One special approach that takes advantage of the material’s high flowability is the addition of functional features to a forged part. At present, there are three possible ways to obtain this additional functionality: (1) special contours, such as a screw thread, that can be thixoforged [
Other researchers have investigated the use of thixojoining to incorporate metal inlays into a workpiece and have found that it is possible to integrate hollow and two open-ended inserted parts into a workpiece [
Female thread inserted in brass [
By using the semisolid joining technique with constant pressure, it is possible to join metal/metal composites. Liu et al. [
An intensive investigation into combining aluminium alloys with aluminium and steel alloys was conducted by Kiuchi et al. [
Previous semisolid joining undertaken by Sugiyama et al. [
Base blank material (semisolid state) | Joined material (solid state) | Joined form | |
---|---|---|---|
Case 1 | Aluminum alloy (A2011); cast iron (FC, FCD); stainless steel (SUS304, SUS316) | Short cut steel fiber; steel ball; ceramics particle; stainless steel wire; craft glass bar, plate, bead, and frit; tile; stone |
|
|
|||
Case 2 | Aluminum alloy (A2011) | Aluminum alloy wire, fin, and sheet; copper alloy wire; stainless steel wire, fin |
|
|
|||
Case 3 | Aluminum alloy | Aluminum alloy bar, tube, and plate |
|
In case
Further investigations by Kopp and colleagues in the field of semisolid joining revealed the possibility of producing prototype components by combining the forming operation with the simultaneous insertion of additional components [
Lower die (a) and produced part with imagery of microstructure at interface (b) [
Further research was conducted to investigate the possibility of joining lower melting point materials such as copper-based alloys [
Thixojoined demonstrator part of X210CrW12 with integrated functional elements (shaft: X5CrNi18-10, bearing bush: copper-beryllium alloy) [
The research effort has concentrated on the modelling of the process in order to achieve a better estimation of the actual temperature distribution and to provide an initial temperature field for the simulation of the forming operation and on developing a model that can be used for process optimization. For instance, Baadjou et al. [
Simulated temperature gradient in (a) matrix of formed X210CrW12 and (b) tools and inserts after joining [
Some researchers have attempted to incorporate the semisolid joining principle into the welding process as a method to join materials together. Thus far efforts in this direction have been restricted to Sn-Pb alloys. For instance, Mendez and Brown [
Joining metals using semisolid slurries [
Semisolid stir joining (SSSJ) is a new joining process that was developed to improve upon existing bonding technologies. Specifically, this process was patented to overcome the deficiencies of the then-current joining techniques which were based on friction stir welding. A technique to achieve a globular weld structure by stirring the localized semisolid zone during the butt-joining of Pb-15 wt%Sn, zinc AG40A die cast alloy, and A356 has been reported by Amirkhiz, Narimannezhad, and Alvani, respectively [
Schematic of semisolid stir welding process [
The critical parameters for obtaining a desirable joint in this method are gas flow, temperature control, stirrer rotation speed, and welding speed. For the rather short samples to be welded successfully, this process requires that the weld pool temperature is extremely accurate. The results showed that an increase in temperature improves the final welding properties. At liquid fractions of less than 50%, joining is not fully practicable; the best mechanical properties are achieved with a liquid fraction of about 70%.
Recently, there has been increasing interest in developing a vacuum-free SSSJ process, which allows a low joining temperature when applied to a semisolid base metal or semisolid filler metal. Xu et al. have carried out extensive experiments by using the SSSJ technique and have shown that the joining of SiCP/A356 composites with similar material or different materials such as 2024-aluminium alloy can be successfully achieved by using a Zn-Al eutectic filler metal in air with the aid of mechanical stirring [
Schematic of semisolid stirring brazing [
The experimental results showed that the tensile strength of the joints increases with increasing stirring rate. This suggests that increasing the stirring rate will promote the disruption of oxide film on the surface of the composites and thereby enhance the metallurgical bonds at the joint interface. In addition, increasing the stirring rate will also promote the formation of a fine brittle g-Zn phase and its uniform distribution in bond. When this occurs, the solid fraction of the filler alloy is about 60%. However, with increasing temperature the solid fraction of the filler alloy decreases quite significantly. In this process, the joining temperature is critical to achieving an ideal shape and a good microstructural interface at the joint [
Similarly, Hosseini et al. [
More recently, an attempt was made by Petkhwan et al. [
Semisolid state joining equipment [
Kiuchi and Kopp [
Joining of semisolid metals [
Intensive investigations to achieve a high-quality globular join structure were undertaken by Mohammed et al. [
Schematic of furnace setup for remelting experiment [
This process has four distinct characteristics: (1) adhesion, (2) wetting, (3) spreading, and (4) diffusion attraction, whereby the joining or uniting of an assembly of two or more parts into one structure is achieved; the assembled parts are directly heated to a temperature high enough so that they are between solidus and liquidus; the liquid phase spreads into the joint and wets the metal surfaces; the parts are cooled to freeze the liquid phase of the metal, which is held in the joint by capillary attraction and anchors the part together.
Based on the results of the experiments conducted in this study of D2-D2 joining, the use of this technique can produce homogeneous properties with high surface quality. Furthermore, a smooth transition from one part to the other is obtained with no evidence of microcracking or porosity. As for the interfacial reactions that occurred during the heating process, these provided elemental diffusion across the interface and the eutectic liquid spread between the joined surfaces to achieve what appears to be good join at the interfaces of both parts of the steel [
On the other hand, a study of D2-304 joining revealed that a new type of nonequilibrium diffusion interfacial structure was created at the interface of the two different types of steel [
Produced part (a) with microstructure at interface (b) [
This study, which simply joins two parts, shows that SSMJ is a promising joining technique that enables face-to-face joining rather than line-to-line joining, which occurs in conventional welding. Therefore, this method not only prevents corrosion at the interface of the components but also confers a better shape than welding. The high diffusion ability, low viscosity, and good flow behaviour make it possible to obtain this type of join. The work of [
Similarly, Kalaki et al. [
The SSMJ process can be utilized for manufacturing various near-net-shape products with complex shapes and geometries where brazing, welding, and diffusion brazing cannot be used for various reasons (e.g., low melting temperature joint, insufficient resulting mechanical properties, and unacceptable plastic deformation). In addition, this process can contribute to the development of new types of products as well as materials. Various applications have been considered for this new joining process such as shock absorbers for cars [
Types of materials joined by SSMJ.
Substrate | Interlayer combination(s) | Reference(s) | |
---|---|---|---|
a | b | ||
CuZn40Al2 | Tool steel | [ | |
|
|||
Stainless steel | Aluminum alloy | [ | |
|
|||
Aluminum alloy, cast iron, or stainless steel | Particles, balls, short fibers of metallic or nonmetallic materials, such as ceramic, glass, tile, or stone, aluminum alloy, copper alloy, or stainless steel | [ | |
|
|||
Stainless steel | M2 tool steel | [ | |
|
|||
X210CrW12, 100Cr6, and X5CrNil18-10 | 9SMn28, CuCo2Be, CuSn12, and GBZ12 | [ | |
|
|||
Sn-15 wt%Pb | Sn-15 wt%Pb | Sn-5 wt%Pb | [ |
|
|||
Pb-15 wt%Sn | Pb-15 wt%Sn | [ | |
|
|||
Zinc AG40A die cast alloy | Zinc AG40A die cast alloy | [ | |
|
|||
A356 | A356 | [ | |
|
|||
SiCP/A356 composites | 2024 aluminum alloy | Zn-Al | [ |
|
|||
SiCP/A356 composites | SiCP/A356 composites | Zn-Al | [ |
|
|||
AZ91 alloy | AZ91 alloy | Mg-25 wt%Zn | [ |
|
|||
A356 aluminum alloy | A356 aluminum alloy | [ | |
|
|||
D2 tool steel | D2 tool steel | [ | |
|
|||
D2 tool steel | 304 stainless steel | [ | |
|
|||
D2 tool steel | M2 tool steel | [ |
As stated earlier, the many potential benefits of thixojoining have encouraged different researchers to carry out feasibility studies on various components. Furthermore, the SSMJ of dissimilar alloys/metals has attracted extensive research interest due to potential engineering benefits and SSMJ’s ability to overcome the problems associated with conventional welding. Indeed, for some material systems, joint properties and performance capabilities that are difficult or impractical to achieve using conventional joining methods are possible with SSMJ. However, on the other hand, in the conventional welding process, which involves fusion of a component with the base metal, the liquid metal solidifies as heat is extracted via weld pool walls. Also, the morphology of the growing solid-liquid interface is typically dendritic and the natural progression of solidification often leads to internal structural defects, such as entrained oxides or shrinkage porosities, which combine to yield a weld metal of relatively poor mechanical properties that is inferior to the microstructures that can be achieved through casting.
The semisolid joining of alloys has the potential to avoid many of the problems mentioned above because the solidification and heat transfer processes are basically different than those of typical welding. Therefore, it is important not only to show the feasibility of SSMJ but also to delineate its advantages over other techniques. The main advantages of SSMJ are outlined below: SSMJ, by combining the forming operation with the simultaneous insertion of additional components in the semisolid matrix, has the potential to produce a range of new components with greater functionality and complexity from different composites. Moreover, with this technology it is also possible to shorten conventional process chains when producing such components [ SSMJ can be used to successfully embed simple geometric forms such as thin sheets, small pins, and fine wires into a base material because the low viscosity and good flow behaviour that is achieved through this technique makes it possible for the inserts to be fully covered by the semisolid base material. In addition, this joining process allows surface-to-surface joining rather than the line-to-line joining of conventional welding. Therefore, it prevents corrosion at the interface of the components and confers a better shape than welding. Furthermore, SSMJ can be used to embed metallic or ceramic materials for use in reinforcement or as bearing systems [ The most distinctive advantage of the SSMJ process is that the operating temperatures and thermal gradients are smaller than in arc welding and this opens up the possibility of combining semisolid materials with different solidus temperatures. For instance, parts made of copper-based alloys, which have a lower solidus, can be inserted into a higher-solidus base material such as steel alloys. This advantage is especially important for temperature-sensitive materials whose microstructures can be damaged by too much thermal energy input and therefore need to be joined at lower temperatures [ Some welding problems such as an inhomogeneous weld microstructure, sputtering, and high process temperature have been solved and satisfactory mechanical properties have been gained when certain alloys have been joined in a semisolid state [ SSMJ results in a globular microstructure, which offers the possibility of joining rather thick plates autogenously. The SSMJ process also dramatically decreases welding distortions because the welding temperature is below the liquidus temperature and the temperature difference between the weld pool and the bulk substrate is smaller than in commonly employed fusion welding methods. Moreover, the relatively lower welding temperature in SSMJ reduces the heat-affected zone. Another advantage of this process is the absence of fumes and spatters [ For some material systems, the required bond properties and performance capabilities are difficult or impractical to achieve using conventional joining methods but are possible with SSMJ, which is often used in high-stress, high-temperature applications where brazing, welding, and diffusion brazing cannot be used for various reasons (e.g., low melting temperature joining, insufficient resulting mechanical properties, and unacceptable plastic deformation) [ Another advantage is that the SSMJ process often has microstructural and therefore mechanical benefits in that the properties of microstructure of the produced join are often similar to those of the base material. In fact, in some cases the joint area becomes indistinguishable from other grain boundaries due to significant diffusion at high temperature. Such joints are often as strong as the bulk substrate material, or stronger, causing the joined assembly to fail in the substrate material rather than in the joint [
The advantages described above are not exhaustive by any means and it is conceivable that other advantages will become apparent when the full potential of SSMJ processing is taken into account from the first steps of a component design.
Despite the clear benefits that can be gained by using the SSMJ process, it has some limitations in its current form, like the difficulty to control the temperature and solid fraction during the joining process [
However, most of the disadvantages of the SSMJ process can be overcome by developing an appropriate methodology, which includes finding a way to optimize the joining parameters and enlarging the size of the process window so that SSMJ can be applied to a wider range of suitable materials. Achieving process optimization often requires much experimentation and there is therefore still a need for further investigations in this area.
Generally speaking, previous studies have demonstrated the feasibility of using the SSMJ technique to join materials that have both simple and complex geometries. However, analyses of the quality of the produced components have shown that the technique is still limited in terms of the range of materials that can be utilized and the diversity of the geometries of the inserts that can be fused or embedded in this way. One of the important issues that has to be clarified in further investigations is the influence of the thermal transient, which determines the mechanical properties of the component and its inserts, and thus affects the quality of the join at the contact areas between base material and insert [
A tight mechanical closure between the joining elements and the matrix has been achieved in most of the material and parameter combinations in the reported studies [
Semisolid fillers have the unique advantage of enabling a controlled flow during deposition, even for deposition rates that cannot be contained by capillary forces. In addition, operating temperatures and thermal gradients are smaller than in arc welding. The other main advantages of this process are that it allows the creation of a joint with an equiaxed microstructure, it allows the deposition of large amounts of filler metal in one pass, and there is absence of fumes and spatter [
Future efforts in this direction could include the application of this technology to alloys of commercial interest with higher liquidus temperatures and better structural qualities, such as ferrous alloys or aluminium alloys that have been studied theoretically in [
Semisolid stir joining is newer development in semisolid joining technology. In the semisolid joining process, a semisolid substrate can be joined under constant pressure. Usually, a join with a linear interface appears as a result of using traditional techniques, whereas a joint with a nonlinear interface can be realized by SSSJ, which enhances the strength of the joint interface. In the vibration brazing process [
In semisolid stir welding [
While there are several benefits of using variations of SSSJ as outlined above, the process also has limitations in its current form. First, the optimum parameters have to be changed according to the specimen size. It is not possible to maintain the same parameters for a long distance of welding as the temperature of the whole specimen increases; therefore, it is necessary to develop a more sophisticated process monitoring system which changes the parameters with time if SSSJ is going to be used successfully on an industrial scale. Moreover, the tools used in this process need to be modified to provide a better and smoother mixing of the substrates and prevent the formation of pores. Friction stir welding tools could be considered as a starting point to design suitable tools for SSSJ from. Another limitation is that the heating medium that has been used in studies thus far [
In order to enhance the potential of the SSMJ process and also to overcome the problems associated with conventional welding methods, a method to join two metals in a thixotropic state (SSDJ) was developed [
Future efforts in this direction could include the application of this technology to alloys of commercial interest with higher liquidus temperatures and better structural qualities, such as ferrous alloys or aluminium alloys including those studied theoretically in [
The semisolid metal joining (SSMJ) process or thixojoining process is a relatively new joining process that has been developed based on the principles of the semisolid metal (SSM) processing to improve on traditional joining processes. The semisolid joining processes can be categorized into the following types: (a) addition of functional features, (b) joining metals by using semisolid Slurries, (c) semisolid stir joining, and (d) semisolid diffusion joining. This paper has reviewed the findings in the literature to date in this evolving field, specifically the experimental details, technology considerations for industrialization, and advantages and disadvantages of the various types of SSMJ methods that have been proposed thus far. It has also presented details of the range of materials that have been joined by using SSMJ. Furthermore, it has highlighted the huge potential of this process and future directions for further research. Based on the present review, the following conclusions can be drawn: The potential of SSMJ is vast and its advantages include the capability to process specially designed alloys and composites, to combine the forming and joining processes, and to reduce production costs and energy consumption. In addition, some welding problems such as inhomogeneous weld microstructure, sputtering, and high process temperature have been solved by employing this technique and satisfactory mechanical properties have been achieved when alloys have been joined in a semisolid state. All the works reviewed have improved our knowledge of the SSMJ process from an experimental point of view. They have also provided essential data that can be used to model the behaviour of semisolid steel. Indeed, the early results of analyses of the mechanical properties of thixoformed steel products give us confidence that it should be possible to produce parts of satisfactory quality. A rigid perpetual joining between the inserting elements and the matrix has been achieved in most of the material and parameter combinations in the reported studies. This shows that various kinds of joining elements made from different materials (e.g., higher melting steel, nonferrous metals) could be combined to produce a variety of products suitable for different purposes such as a screw thread or a nut for assembling, a gliding functional element or a reinforcing element. This thixojoining technique opens up the possibility of designing a range of new parts and also has an additional advantage in that it can shorten processing time. The prototypes from different projects demonstrate that the production of complex parts and the joining of different parts via SSMJ are generally possible, but improvements are still needed to enhance the properties of the components produced. To be able to transfer semisolid joining technologies to the industrial scale further detailed analysis (including, e.g., dynamic fatigue tests, bending tests, and pull-out tests) has to be carried out to quantify the quality of the connection achieved between materials. It is also necessary to pay some attention to finding ways to apply SSMJ to stone/metal composites not least because the properties of stone change with temperature. For example, some stone materials change colour, others become brittle, and there are those that break apart into pieces. While SSMJ is a specialized joining process that requires more resources to implement compared to typical joining processes, in some cases an SSMJ technique is the best or only way to join certain materials for specialized applications, so it is worth pursuing this line of inquiry. Finally, efforts are still needed to produce defect-free components in serial production at a competitive cost. In this field, there is still a great need for research to set a complete processing map that will lead to a better understanding of all the SSMJ process parameters and their possible impact on a part’s microstructure and thus its mechanical properties. At present, the modelling of the SSMJ is not well developed because of the difficulties in obtaining experimental parameters, but the principles should be similar to those for modelling semisolid die forming in general. The advantages of SSMJ that have been highlighted in this paper are not exhaustive, so it is conceivable that other advantages will become apparent as research in this field evolves.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to thank Universiti Kebangsaan Malaysia (UKM) and the Ministry of Education (MOE), Malaysia, for the financial support under Research Grants GUP-2012-040 and AP-2012-014.