The result of this study confirms that the EAPJ concept is effectively applicable for solid state joining of different material combination. The result of the present study confirms that the EAPJ concept is applicable for solid joining of different material combinations.
Some metal alloys, such as aluminum and copper alloys (Ozel et al., 2005), can be successfully joined at ambient temperature. For example, Ng et al. 2015) successfully joined aluminum and copper sheets using an electrically assisted roller bonding process, and Xu et al.
Microstructural analysis was performed on the cross-section of the joint perpendicular to the width. The microstructure of the joint was further observed using a field emission gun scanning electron microscope (FE-SEM, SU70, Hitachi, Japan) equipped with an electron backscatter diffraction system (EBSD, EDAX/TSL, Hikari, USA).
Results and discussion
In the base metal (Figure 2.6(a)), the microstructure observed in EBSD showed the same trend as OM. As shown in Figure 2.6 (b), very fine grains along the shear band are observed due to recrystallization in joint 1.4kA1.3d. At the current intensity of 1.6 kA, the shear strength of the joint was greatly improved and only the tensile fracture mode was evident, as shown in Figure 2.10.
Note that in the tensile failure mode region, the failure load also increased with increasing current intensity for values of similar thickness reduction, as marked by a dotted box in Figure 2.10. The trend line of maximum breaking load for each current intensity can be approximated as shown in Figure 2.10. Based on the results shown in Figure 2.10, the optimal thickness reduction, which corresponds to the maximum fracture load for a given current intensity, can be plotted as a function of current intensity.
Kim MJ, Lee K, Oh KH, Choi IS, Yu HH, Hong ST, Han HN (2014) Electric current-induced annealing during uniaxial tension of aluminum alloy. Kim MJ, Lee MG, Hariharan K, Hong ST, Choi IS, Kim D, Oh KH, Han HN (2017) Electric current-assisted deformation behavior of Al-Mg-Si alloy under uniaxial tension. Ozel K, Sahin M, Akdogan A (2005) Mechanical and metallurgical properties of aluminum and copper sheets joined by cold pressure welding.
Park JW, Jeong HJ, Jin SW, Kim MJ, Lee K, Kim JJ, Hong ST, Han HN (2017) Effect of electric current on recrystallization kinetics in interstitial free steel and magnesium alloy AZ31. Peng L, Xu Z, Lai X (2014) An investigation of electric-assisted solid-state welding/bonding process for thin metal sheets: experiments and modeling. A solid-state pressure joint using resistance heating as a heat source and adopting an additive-manufactured porous metal inner layer between the joint samples is demonstrated.
This produces solid-state assemblies of metals by frictional heat obtained from a mechanically induced relative sliding motion (generally by rotation) between the surfaces of two items in contact under pressure (Kimura et al., 2017; Sarsilmaza et al., 2017). In particular, a particularly complex braking system is needed, as the rotation of a workpiece must be stopped completely and quickly for successful joining (Kimura et al., 2017). Also, a large amount of flash (or burr) is depleted from the joint interface by the final axial compression during friction joining and therefore must be removed after the process (Louw et al., 1974).
To overcome or reduce the disadvantages of conventional pressure joining and friction welding, electrically assisted pressure joining (EAPJ) using resistance heating as a heat source is possible (Peng et al., 2014; Harada et al., 2016). Furthermore, when present, the electroplastic effect of electric current increases the mobility and diffusion of interface atoms (Roh et al., 2014), which can improve the mechanical/material properties of the solid junction. For example, Ng et al. 2015) performed electrically assisted bonding of aluminum and copper plates.
In this study, process efficiency improvement for EAPJ of cylindrical bulk components is demonstrated by using an additively manufactured metal porous interlayer. Cylindrical porous samples (8 and 12 vol.% porosities) with a diameter of 10 mm and a height of 15 mm were also fabricated by the same additive manufacturing process to measure the baseline compressive properties. The chemical compositions of solid and powder SUS316L are quite similar, as listed in Table 3.1.
As depicted in Figure 3.4 and listed in Table 3.2, to heat the resistor during compression, a continuous electric current was first applied to the sample mount to rapidly increase the temperature of the sample mount. For the microstructural analysis, the cross-section of the joint was prepared along the compression direction (height direction). The microstructure of the joint was further observed using a field emission gun scanning electron microscope (FE-SEM, SU70, Hitachi, Japan) equipped with an electron backscatter diffraction system (EBSD, EDAX/TSL, Hikari, USA) and an energy dispersive spectrometer (EDS: X-Max50, Japan).
Results and discussion
First, the bulk resistivity of the porous interlayer is higher than that of the solid sample due to its porosity (Ibrahim et al., 2016). This difference is due to the effect of different elevated bond temperatures and the effect of the relative thickness of the porous interlayer in the sample assembly. No macroscopic defects were observed by OM at the articular interfaces for any of the EAPJ joints with or without the porous interlayer (Figure 3.9).
The IPF map of the cross-section along the height at the joint interface of the S+S joint (Figure 3.10(c)) shows finer grains (average grain size of 29 μm) compared to the solid sample before EAPJ. The IPF maps of the cross-section along the height at the joint interface (Figure 3.10(d)) and in the middle of the porous interlayer (Figure 3.10(e)) of the S/3D12% joint also confirm that there was no discontinuity in the microstructure visible across the joint interface (approx. The result of the EDS line scan across the interface (marked with a micro-indent) of the S/3D12% joint shows that the distribution of the main alloying elements (Mo, Ni, Fe, Cr) in the interface and interlayer is almost identical to that in the solid sample, as shown in Figure 3.11.
Dewidar MM, Khalil KA, Lim JK (2007) Machining and mechanical properties of porous 316L stainless steel for biomedical applications. Harada Y, Sada Y, Kumai S (2016) Joining of steel studs and steel plates by solid state stud welding and temperature evaluation near the joint interface. Kimura M, Suzuki K, Kusaka M, Kaizu K (2017) Effect of friction welding conditions on joining phenomena and mechanical properties of a friction welded joint between aluminum alloy 6063 and AISI 304 stainless steel.
Meshram MP, Kodli BK, Dey SR (2014) Mechanical Properties and Microstructural Characterization of Friction Stir Welded AISI 316 Austenitic Stainless Steel. Electrically assisted solid joint (or electrically assisted compression joint, EAPJ) of dissimilar alloys of stainless steel 316L (SUS316L) and Inconel 718 (IN718) is under experimental investigation. The results of the present study confirm that the concept of EAPJ is applicable to solid-state compounds of dissimilar material combinations.
In addition, metallurgical problems related to melting and solidification may occur, such as elemental segregation, Laves phase, solidification cracks, and liquidation cracks (Cortes et al., 2018; Lin et al., 2014; Iturbe et al., 2016). Adequate filler material is required in conventional fusion joining of dissimilar metal alloys (Ramkumar et al., 2014). As an alternative to fusion splicing, solid state splicing is becoming attractive to avoid joint failures caused by melting and solidification in fusion splicing.
To overcome or minimize the disadvantages of friction joining and conventional pressure joining, electrically assisted pressure joining (EAPJ) has been proposed as a new solid state joining technique (Peng et al., 2014). EAPJ has several technical advantages over conventional hot press joining, such as fast and local heating, simple and cost-effective process facilities, and much shorter process time (few seconds) (Hong et al., 2018). Finally, EAPJ using resistance heating as a heat source can result in the electroplastic effect (Roh et al., 2014; Kim et al., 2014; Kim et al., 2017), which improves the mobility and diffusion of atoms and improves the mechanical/material properties of the joints.
Subsequently, axial compression and an electric current were simultaneously applied to the sample assembly, as schematically described in Figure 4.2. The electrical current parameters for the combination of a continuous electrical current and a pulsed electrical current in Figure 4.2 are listed in Table 4.2. The continuous electric current initially applied was used to rapidly raise the temperature of the samples.
A pulsed electrical current was then applied to the sample assembly to increase atomic diffusion at the interface by maintaining the elevated temperature without overheating the samples. In this study, two different electric current holding times, 0.5 sec and 36.5 sec, were used. Due to the limited capacity of the electrical current generator used in this study, the size of the assembly was not large enough to produce a standard tensile test.
Results and discussion
Compared to the SUS316L BM, much smaller grains were observed in the OM of the IN718 BM. Compared to the SUS316L BM, much smaller grains were observed in the IPF maps of the IN718 BM. As shown in the GOS map of Figure 4.11(c), the fraction of recrystallized region on the IN718 side is much higher than that of the SUS316L side.
Also note that for both EAPJ joints, the IN718 side of the joint exhibits higher hardness than the BM. In contrast, the increased hardness of the IN718 side compared to the IN718 BM for the H-36s compound appears to be mostly a result of grain refinement (2.8±1.5 μm) upon recrystallization, as shown in the GOS map in Figure 4.11(d). The SEM image of the fractured surface with microvoids and pits confirms the ductile fracture mode, as shown in Figure 4.13(c) for the H-0s joint.
Cortes R, Barragan ER, Lopez VH, Ambriz RR, Jaramillo D (2018) Mechanical properties of Inconel 718 welds performed by gas tungsten arc welding, International Journal of Advanced Manufacturing Technology. Kim MJ, Lee MG, Hariharan K, Hong ST, Choi IS, Kim D, Oh KH, Han HN (2017) Electrical current-assisted deformation behavior of Al-Mg-Si alloy under uni-axial stress, International Journal of Plasticity. Kumara SS, Muruganb N, Ramachandranc KK (2018) Microstructure and mechanical properties of friction stir welded AISI 316L austenitic stainless steel joints, Journal of Materials Processing Technology.
Li YF, Das H, Hong ST, Park JW, Han HN (2018) Electrically assisted pressure joining of titanium alloy, Journal of Manufacturing Processes. Roh JH, Seo JJ, Hong ST, Kim MJ, Han HN, Roth JT (2014) Mechanical behavior of 5052-H32 aluminum alloys under pulsed electric current, International Journal of Plasticity. Smith M, Bichler L, Gholipour J, Wanjara P (2016) Mechanical properties and microstructural evolution of in-service Inconel 718 superalloy repaired by linear friction welding, International Journal of Advanced Manufacturing Technology.