A Novel Shim-Assisted Resistance Spot Welding Process to Improve Weldability of Medium-Mn Transformation-Induced Plasticity Steel
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Abstract
Medium-Mn transformation-induced plasticity steels have great potential to significantly reduce vehicle weight and improve fuel economy due to their outstanding combination of high strength and excellent ductility. One bottleneck to the application is their poor weldability resulting from their high Mn contents. In this paper, three resistance spot welding set-ups, including no shim, an interstitial-free steel shim at the faying interface (shim-in) and shims against the electrodes (shim-out), were incorporated to investigate the weldability of Fe-7Mn-0.14C medium-Mn steel. Tensile-shear, cross-tension, and microhardness tests were used to evaluate the mechanical properties of the welds. Experimental results demonstrated that the failure mode of the welds transitioned from the interfacial fracture in the case of no shim to the desired nugget pull-out fracture in the shim-out set-up, resulting in dramatical improvements in both peak loads and their corresponding extensions during the tensile-shear and cross-tension tests. In contrast, the shim-in set-up made no improvement. What can contribute to such improvement was then discussed on the basis of observed morphologies and microstructures of welds.
Introduction
The application of advanced high strength steels (AHSS) to automotive industries can significantly reduce vehicle weight and improve fuel economy. Compared to the 1st generation and the 2nd generation AHSS, the 3rd generation (Gen.3) AHSS have a better combination of high strength and superior ductility; therefore, they receive an increasing interest from both steel and automotive industries.[1] Medium-Mn TRIP steels, which usually contain approximately 5 to 12 wt pct Mn, are considered one of the most promising candidates of the Gen.3 AHSS. They have a dual-phased microstructure of ferrite and retained austenite; the latter is often in the range of 20 to 50 pct.[2] Both strength and ductility could be improved due to the so-called transformation-induced plasticity (TRIP) effect, resulting from the transformation of the retained austenite to martensite during the tensile deformation.[3, 4, 5] It has been reported that the product of strength and ductility of many developed medium-Mn steels can be more than 30,000 MPa pct.[6]
The weldability of medium-Mn TRIP steels has to be assessed before they can be passed to commercial applications. In particular, the mechanical properties of welds play essential roles in crash performance. Some efforts have been recently made to investigate the microstructure and mechanical properties of welded medium-Mn TRIP steels after using different welding processes. For examples, a Fe-8Mn-0.06C (all are in weight percentage, unless mentioned elsewhere) steel was joined by gas tungsten arc welding, and the impact toughness of full martensite structure in weld was evaluated, which strongly depends on the sizes of the martensitic block and packet.[7] Gas metal arc welding was applied to Fe-6.5Mn-0.98C steel with the added pulse current, which can enhance the plastic deformation capacity of weld joint.[8] Laser welding of Fe-10.4Mn-0.15C steel produced a fusion zone (FZ) of welds, which were mainly composed of martensite and some interdendritic austenite, and no softening in the heat-affected zone (HAZ).[9] In addition, the precipitation of VC nanoparticles could lead to the increases of both microhardness and tensile strength of the simulated HAZ in the Fe-8.1Mn-0.98C steel.[10]
However, we are still facing a great challenge in designing a proper weld process to obtain the good-quality welds with the satisfied properties for the medium-Mn TRIP steels because the brittle and hard phase is frequently formed during welding due to their high Mn content, which leads to a high susceptibility to cracking.[11] In the automotive industry, moreover, resistance spot welding (RSW) is a primary and vital sheet metal joining process due to the two advantages: the high welding speed and the excellent suitability for robotization.[12,13] There are about 3000 to 5000 spot welds in a typical steel body-in-white.[14] During the RSW process, the thermal process destroys the designed microstructure of AHSS that has been formed during the previous rolling and annealing processes, which may deteriorate the mechanical properties of welds.[12] In particular, the interfacial fracture (IF) of weld after the RSW of AHSS is detrimental to the crashworthiness of the vehicles and shall be avoided.[15] Peterson has used a low carbon dilution sheet during the RSW of martensitic steel sheets to change the IF fracture mode.[16] However, there are few researches published about the RSW of medium-Mn steels, to our best knowledge.
In this paper, different RSW processes were used to weld a medium-Mn steel received from a commercial supplier after a trial industrial production. Three resistance spot welding set-ups, including no shim, an interstitial-free steel shim at the faying interface (shim-in) and shims against the electrodes (shim-out), were implemented to evaluate the weldability of medium-Mn steel. The effects of the interstitial-free steel shims at different locations on the mechanical performance were analyzed through the morphology and microstructure of weld, microhardness measurement, and tensile-shear tests.
Experimental Procedures
Chemical Compositions and Mechanical Properties of the Studied 7Mn Steel and IF Steel Shim
Steel Grade | Composition (Wt Pct) | Mechanical Properties | |||||||
---|---|---|---|---|---|---|---|---|---|
Mn | C | Si | P | S | Fe | UTS (MPa) | YS (MPa) | TE (Pct) | |
7Mn | 7.14 | 0.14 | 0.23 | 0.056 | — | bal. | 1100 | 1080 | 35–40 |
IF | 0.099 | 0.001 | 0.004 | 0.079 | 0.0067 | bal. | 294 | 140 | 44.1 |
Microstructures of the received 7Mn steel (BM, base metal)
Illustrations of three welding set-ups employed in the study. (a) No shim, (b) shim-in, i.e., a 0.8-mm-thick uncoated IF steel shim at the faying interface, (c) shim-out, i.e., two IF shims against the electrodes (F, electrode force; D, nugget; Tm, solidus temperature; Q, heat quantity; R, electrical resistance; Rheat, heat resistance; δ, material thickness; λ, thermal conductivity. All of them will be discussed later)
The resistance spot welding schedule used in the three welding set-ups
The dimensions of specimens (mm) for the tensile-shear test (a) and the cross-tension test (b), d = 18 mm
Results
Morphology and Microstructure of Welds
The nuggets and weld surface obtained in the three welding set-ups. (a) No shim, (b) shim-in, (c) shim-out
The comparison of nugget diameters and thickness obtained in the three welding set-ups
Microstructural and compositional examination across the welds. (a) Microstructures in the shim-out welding set-up, (b) mapping of Mn distribution by EPMA in the shim-out set-up, (c) the average Mn contents in BM, FZ of three set-ups measured by EDS
The microstructures of fusion zones examined by optical microscopes (a to c) and SEM (d to f). (a and d) No shim, (b and e) shim-in, (c and f) shim-out
Mechanical Properties
The microhardness distributions in the welded zones along diagonal direction
The load-displacement curves of spot welded 7Mn steel during the tensile-shear tests (a) and the cross-tension tests (b)
The comparison of measured mechanical properties among the three welding set-ups. (a) The average values of peak load and energy absorption, (b) the average values of ductility ratio
Fracture Modes
Interfacial fracture (IF) mode with the features of brittle, ductile and dendrite characteristics in the specimen subjected to the no shim welding. (a) Overall view of the IF, where (b) to (d) locations are magnified and shown, (b) intergranular brittle fracture, (c) transgranular brittle and dimple-like ductile fracture, (d) dendritic fracture
Pullout fracture (PF) having both the brittle and ductile fracture characteristics in the specimen subjected to the shim-out welding. (a) Overall view of the PF, where ‘b’ location is magnified and shown in (b), and further magnified in (c), to show the mixture of both the intergranular brittle and transgranular ductile fracture
The Nugget Size, Fracture Mode and Mechanical Properties of Welds for the Three Welding Set-Ups
Set-Ups | No Shim | Shim-In | Shim-Out |
Nugget diameter/thickness (mm) | 7.69/1.67 | 7.67/2.38 | 8.64/3.28 |
TS test | |||
Fracture mode | IF | PF | PIF & PF |
Peak load (kN)/extension (mm) | 16.0/1.5 | 12.3/1.3 | 22.8/2.7 |
CT test | |||
Fracture mode | IF | PF | PF |
Peak load (kN)/extension (mm) | 2.0/2.3 | 2.7/3.7 | 6.7/12.7 |
Discussion
In the shim-out set-up (Figure 2(c)), the temperature on the weld surface should be lowest among the three set-ups because the heat could dissipate faster from IF steel shim to the water-cooled electrodes due to its higher thermal conductivity and less heat is generated in it due to its lower electricity; moreover, the solidus temperature of IF steel is about 73 °C higher than that of 7Mn steel. All these factors lead to the temperature on the weld surface in the shim-out set-up being far below the solidus temperature, i.e., the solid-state IF steel shims with enough strength may exist during welding, which prevented the liquid expulsion. In addition, the IF steel shims outside 7Mn steel sheets were more ductile because they were still ferrite after the rapid cooling and can accommodate more strain induced by the thermal stress; moreover, they acted as extra thermal barrier, leading to lower cooling rate in 7Mn steel sheets and then reduced thermal stress. Both may contribute to no cracking observed in the shim-out set-up. In contrast, the 7Mn steel on the weld surface in both no shim and shim-in set-ups just transformed to the briite martensite during the rapid cooling; in these cases, the stress on the martensite directly imposed by electrodes together with thermal stress may easily cause cracking on the weld surface, as observed in Figure 5.
Among the three set-ups, the shim-out set-up led to the largest nugget size because the heat, including both Q1 and Q2 in Figure 2(c), was largest and the stacked sheets between the electrodes were thickest. Heat was actually generated from two IF steel shims, two 7Mn steel sheets and particularly the three interfaces as the relatively high electrical resistance, including two IF/7Mn steel interfaces and one 7Mn/7Mn steel interface. In contrast, the heat generated in the shim-in set-up was from two 7Mn steel sheets, one IF steel shim and two IF/7Mn steel interfaces; the heat generated in the no-shim set-up was just from two 7Mn steel sheets and one 7Mn/7Mn interface, which was smallest among them. In addition, the total thickness of the stacked steel sheets between the electrodes is smallest in the no-shim set-up and largest in the shim-out set-up, it is logical that the thickness of nugget formed during welding is smallest in the former case and largest in the latter, as shown in Figure 6.
The mechanical performance of resistance spot welds may depend on many factors, such as nugget size, microstructure, base metal strength etc.[23] However, the nugget size is the most important parameter governing the mechanical properties of spot welds.[13] In our case, the TS peak loads and extensions appeared to be determined mainly by the nugget diameters, whilst the CT peak loads and extensions were determined by the nugget thickness (Table II). This is logical since the peak load shall depend on how much volume of nugget can bear the load along the loading direction. This is also consistent with previous research results. For examples, some models for estimating the TS and CT strength of weld indicate that the peak load shall be in the square proportional to the nugget diameter in the interfacial fracture mode and in the direct proportional to nugget diameter in the pullout fracture mode.[24, 25, 26]
It is worthy of mentioning that the shim-assisted welding technology has been investigated before. For an example, Peterson[27] inserted a low carbon steel shim at the faying interface of 1330 MPa martensitic steel sheets, i.e., the shim-in set-up in this study, which produced a pull-out fraction and dramatically improved the peak load by over 300 pct. However, the same welding set-up did make any improvement in our case. This is probably because the studied 7Mn steel contains a much higher Mn content than the ordinary martensitic steel; thus, the dilution of C and Mn in the fusion zone by IF steel shim is not enough to bring down the C and Mn contents for avoiding martensitic transformation. This can be hinted by the observed martensite microstructure in the FZ of shim-in set-up, as shown in Figure 8.
Instead, we invented a new welding setup, i.e., shim-out set-up, which has been not presented before, to our best knowledge. Compared to the no-shim set-up, this novel welding set-up led to the increase of TS peak load by 50 pct (from 16 to 23 kN) and the increase of CT peak load by three 3 times (from 2 to 7 kN). Such a dramatical improvement is mainly attributed to the much larger diameter and higher thickness of nuggets achieved in this new set-up, which leads to the pull-out fracture.
Conclusions
- 1.
Both no shim and shim-in welding set-ups produced the expulsion and cracking on the weld surface, whilst the shim-out set-up led to no expulsion and good surface quality because the IF steel shim can maintain its solid state with a certain strength during welding due to its higher solidus temperature and better thermal conductivity than the medium Mn steel.
- 2.
Compared to the no-shim set-up, our invented shim-out welding set-up has significantly improved the overall mechanical properties of RSW of medium-Mn steel, including peak load, extension, energy absorption and ductility ratio. In particular, this led to the increase of TS peak load by 50 pct and the increase of CT peak load by three times. Such a dramatical improvement is mainly attributed to much larger nugget achieved in this set-up, which leads to the pull-out fracture.
- 3.
Compared to the no-shim set-up, the shim-in welding set-up did not improve the tensile-shear peak load and the energy absorption of the medium-Mn steel welds, which is out of our expectation. This is likely because the studied medium-Mn steel contains much higher Mn content than the conventional low-alloyed martensitic steel so that the microstructure in the FZ is still the brittle martensitic phase.
- 4.
The failure mode of the welds transformed from the interfacial fracture in the no-shim welding set-up to the desired nugget-pull-out fracture in the shim-out set-up due to the much larger nugget formed in the later case. This was because more heat was generated due to higher resistivity and more material was involved to melt during welding in the shim-out set-up to feed the larger nugget.
Notes
Acknowledgments
The authors gratefully acknowledge the financial support and technical support provided by Hundred-Talent program of Chinese Academy of Sciences and General Motors.
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