J-2
Citation:
J-2. C. Ji, M. Kimchi, Y. Kim and Y. Park, “The application of pulsed current in resistance spot welding of zn-coated hot-stamped boron steels,” in Advances in Resistance Welding, Miami, FL, 2016.
RSW of Advanced High-Strength Steels
In general, if any type of AHSS [Dual-Phase (DP), Transformation-Induced Plasticity (TRIP), Complex Phase (CP), Ferrite Bainite (FB), or Martensitic (MS)] is used for the first time, the user should take the welding schedules applied to mild steel and then:
- Increase the electrode force by 20% or more depending on Yield Strength.
- Increase weld time as appropriate.
If these changes are insufficient, try these additional changes:
- A multi-pulse welding schedule (several pulses or post heating).
- Larger tip diameter and/or change the type of electrode.
- Increase the minimum weld size.
When resistance welded, AHSS require less current than conventional mild steel or HSLA because AHSS have higher electrical resistivity. Therefore, current levels for AHSS are not increased and may even need to be reduced depending on material chemical composition. However, most AHSS grades may require higher electrode forces for equivalent thickness of mild steels because electrode force depends on material strength. If thick mild steel or HSLA steel (of the same thickness) is replaced by an equivalent thickness of AHSS, the same forces may be required during assembly welding.
AHSS often have tighter weld windows (welding parameters that give acceptable welds) when compared to mild steels, as shown in the Figure 1.
Figure 1: Schematic weld lobes for AHSS, HSLA, and mild steel with a shift to lower currents with increased strength.
The current range (kA) for AHSS of 600-1400MPa during RSW is shown in Figures 2 and 3. The process window for Resistance Spot Welding of AHSS is influenced by the electrode force and welding time used in a major way. The current range increases by an average of 500 A for every additional 500 N of electrode force (Figure 2). The current range also increases by an average of 250 A for each additional 40 ms of welding time (Figure 3). Extra amounts of electrode force and welding time lead to increased current range, allowing for a wider process window.
Figure 2: RSW with AHSS, current range for varying electrode force (Cap Type B 16/6, 6-mm tip diameter, single pulse, 340-ms weld time, 250-ms hold time, plug failures.).I-6
Figure 3: RSW with AHSS, current range for varying weld time (Cap Type B 16/6, 6-mm tip diameter, single pulse, 3.5-kN electrode force, 250-ms hold time, plug failures.).O-1
A more extensive weld studyT-5 of three DP HDGA (45/45 g/m2) coated steels showed similar welding behavior for all three steels. The 1.6-mm-thick steels were DP 340/590, DP 420/780, and DP 550/980. To characterize the welding behavior of the steels, useful current ranges and static weld tensile tests were performed. The useful current range is the difference between the welding current required to produce a minimum button size (Imin) and the current that causes expulsion of weld metal (Imax). In this study, the 4√t as the minimum button diameter was used, where “t” is the nominal sheet thickness. This is generally used in the automotive and steel industries. The weld current range was 2.2 kA for the DP 340/590 and DP 420/780 and 2.5 kA for the DP 550/980 steel (Figure 4). These current ranges are sufficiently wide to weld successfully the DP steels. The study also found no weld imperfections, which means these three DP steels are weldable with simple, easy to use welding parameters.
Figure 4: Welding current ranges for 1.6-mm DP HDGA steels with minimum tensile strengths of 590, 780, and 980 MPa.T-5
Average reported weld hardness was 380 HV (Vickers Hardness) for the DP 340/590 and 415 HV for the other two. Again, all three DP steels had similar weld hardness distributions. The study also concluded that weld fracture mode alone is not a good indicator of weld integrity and performance. The load to fracture should be considered more important in judging weld integrity.
A second studyT-6 compared two 1.6-mm-thick HDGA (45/45 g/m2) steels: DP 420/700 and TRIP 420/700. The weld current range for 18 cycles weld time was similar: 1.4 kA for the DP 420/700 and 1.5 kA for the TRIP 420/700. The average weld hardness was 400 HV for both steels. The study concluded that acceptable welds with no imperfections can be produced in both steel grades. Both steel grades are readily weldable with easily adoptable welding parameters. Weld tensile strength differences between the two steels were small and not considered statistically significant.
Weld schedules (Figure 5) with pulsed current profiles for AHSS can have weld-current ranges similar to mild steel. Even though there is no increased tendency for weld expulsion with AHSS avoiding weld expulsion is highly desirable with AHSS. Loss of nugget material can affect weld-nugget size and strength.
Figure 5: Schematics of optimized weld schedules for AHSS.B-1
Post annealing (tempering pulse weld schedule) of TRIP steel may alter weld fracture mode and weld current range (Figure 6). However, since studies have shown that the occurrence of partial or IF fractures does not necessarily indicate poor weld quality, the use of pulsed current is not required to improve weld quality. Further, the effect of current pulsing on tensile and fatigue properties, as well as the electrode tip life, is not known. Therefore, users should perform their own evaluations regarding the suitability of such modified parameters.
Figure 6: Post annealing may enlarge weld current range.B-1
Additional work using Quench and Partition (Q&P) 980 showed less current required than conventional steels because it has higher electrical resistivity. Due to ultra-high base metal (BM) strength, it needs higher electrode force than conventional steels which have equivalent thickness. The weld lobe of 1.6-mm Q&P 980 is shown in Figure 7 with the pulsed weld time and force of 5.8 kN. The yellow zone of this figure shows the fracture mode of full button (FBF) when peel tested. Some pictures of these weld spots’ fracture mode are captured in Figure 8, for diameters of 6.0 to 7.7 mm.
Figure 7: Pulsed current profile and weld lobe of 1_6-mm Q&P 980.B-4
Figure 8: Fracture mode of weld spots in yellow zone.B-4
Hardness measurements and cross sections through the spot weld different zones can be identified as depicted in Figure 9. In a first step, the spot-welded joint can be subdivided into three zones: weld, Heat Affected Zone (HAZ), and BM. The weld is covered by the HAZ, where the melting temperature is not reached but high enough to change the microstructure. This region is dominated by inhomogeneous properties due to the different temperature and cooling gradients. Considering the hardness measurements of AHSS 3 sample, even a softening in the HAZ compared to the BM can be observed. Finally, the HAZ is surrounded by the BM, which does not show any local changes within the structure. These modifications of microstructure in the HAZ and weld are essential for the load-bearing capacity because the strength and ductility are drastically changed in comparison to the BM. Normally a high hardness is related to high strength and less ductility.
Figure 9: Hardness distribution through spot welds of various strength steels.P-7
Weld spot micrograph and microhardness of 1.6-mm Q&P 980 is shown in Figure 10, in which no weld defects, such as cracks, shrinkage void, pore, no fusion, deep indentation, etc. were found. Hardness testing is typically performed as shown in Figure 11 (diagonal traverse across the weld from BM of top coupon to BM of bottom coupon) using a suitable instrument for micro-indentation hardness testing (Vickers or Knoop).
Figure 10: Weld spot micrograph and microhardness of 1.6-mm DP 980.B-4
Figure 11: Typical cross-sectioned weld and hardness traverse.A-13
The different hardness values seen in a typical cross-sectioned weld depict different microstructural regions.P-8 Figure 12 shows the temperature distribution of a typical 2T weldment of hot stamped boron steel. At the solidified weld nugget, we see the highest temperatures and steadily decrease toward the unaffected base metal. The weld metal, coarse grain heat affected zone, fine grain heat affected zone, and unaffected base metal are made up of martensitic microstructure. The base metal has this microstructure due to the heat treatment (hot stamping) process. The weld metal, coarse grain heat affected zone, and fine grain heat affected zone are exposed to austenitizing temperature upon welding and are cooled rapidly reforming the martensite microstructure. The subcritical heat affected zone has a unique microstructure of over-tempered martensite. In this region, the peak temperature re below the Ac1, causing the base metal martensitic microstructure to decompose into ferrite and cementite. Micrographs of the different weld regions can be seen in Figure 13.
Figure 12: Temperature distribution of a typical 2T RSW of hot stamped boron steel.P-8
Figure 13: Different microstructures seen throughout the HAZ of a 2T hot stamped boron steel joint.P-8
P-8
P-8. Andrea Peer, Ying Lu, Tim Abke, Menachem Kimchi, and Wei Zhang “Deformation Behaviors of Subcritical Heat-affected Zone of Ultra-high Strength Steel Resistance Spot Welds.” in 9th International Seminar & Conference on Advances in Resistance Spot Welding. Miami, (3 2016). Paper No. 12.
Ultrasonic Assisted RSW
Recently research at the Ohio State University is developing methods that combine the benefits of ultrasonic welding and resistance spot welding. Y. Lu and co-workersL-10 developed the ultrasonic plus resistance spot welding process (U+RSW). In this process thin aluminium sheet was joined with steel sheet via ultrasonic welding, followed by RSW between the aluminium side of previously sheet and aluminium sheet as shown in Figure 1. In this study they used 1-mm-thick AA6061-T6 to 0.9-mm-thick AISI 1008 steel with 0.4-mm-thick AA6061-T6 as the insert.
Figure 1: Schematic diagram of U + RSW process. VD is the sonotrode’s vibration direction for USW of intermediate joint.L-10
The U+RSW method showed improved mechanical properties (Figure 2), tensile shear strength and energy absorbed prior to failure. It has shown a relatively thin intermetallic layer.
Figure 2: Effect of welding current on tensile shear strength and fracture energy of welded dissimilar joints of AA6061 to AISI 1008 steel. The welding time and the electrode force for RSW were kept constant at 0.083 s and 3.56 kN, respectively.L-10
Another research group have developed ultrasonic resistance welding process (URW), that apply in situ ultrasonic waves during the resistance spot welding process.S-17 The schematics of the process is shown in the Figure 3. Al-6061T6 and electrogalvanized TRIP 780 with dimensions of 70Lx 25Wx 1.6T mm3 and 70Lx 25Wx 1.3T mm3 respectively were used in this study.
Figure 3: URW Setup (a) Schematic illustration; (b) Flowchart of the URW process.S-17
The URW process have shown improved mechanical properties and defect free interface compared to the RSW process. In situ ultrasonic vibration facilitated the breakdown of the surface oxide and contaminants, which modifies contact resistance and heat generation rate accordingly. If melting occurs, ultrasonic vibration promoted wetting of the molten aluminium over the steel interface and resulted in the formation of defect-free, fully bonded interface as shown in Figure 4. Figure 5 compares the mechanical properties between the RSW & URW process. Comparing URW with traditional RSW, up to 300% increase in strength and more than 150% increase in displacement to failure is reported. SEM images of the fractured surface revealed that ultrasonic waves eliminated the eggcrate morphology generally observed in the RSW welds fractured surface, which is a typical representative of solidification cracking.
Figure 4: EDS line Scan of Al-6061 & TRIP 780 across interface for RSW at 11.4 kA (a-b) & at 13.4 kA (e-f) and for URW at 11.4 kA (c-d) & at 13.4 kA (g-h).S-17
Figure 5: Tensile test results (a) Joint shear strength from 11.4 kA (30 cycles) & 13.4 kA (20 cycles); (b) Energy absorption calculated via area under load-displacement curve for 11.4 kA (30 cycles) & 13.4 kA (20 cycles); (c-d) load-displacement curve for 11.4.S-17
Aluminium/Steel Using Interlayer
In order to reduce the thickness of brittle intermetallic compound (IMC) formed at Aluminium/Steel interface to improve the mechanical properties of resistance spot welding of Aluminium/Steel, different interlayers, such as Al-cladded steelS-16, O-3, Y-4, Al-Si alloyW-15, Al-Mg alloyI-6, pure Al W-16, Zn coating.M-10, U-4, A-21 Sun et al.S-16 has done resistance spot welding of 1.4-mm SAE 1008 mild steel with 2-mm 5182-O aluminium alloy using 1 mm/1.5 mm cold rolled aluminium-clad steel as a transition metal with Aluminium/Steel thickness ratio of 20/80 (thickness ratio was not optimized). The formation of IMC cannot be observed at the interface for cold rolled aluminium-clad steel. The electrodes used were Class 2 30-deg truncated cone with 8-mm surface diameter and 3-inch face radius on the aluminium side and 30-deg truncated cone with 8-mm flat surface diameter on the steel side. Two separate nuggets created the aluminium and steel sides with a final nugget diameters of 10.1 mm and 8.2 mm, on aluminium and steel sides respectively, at the end of the 3rd pulse (Figure 1). With increasing welding time from 1 pulse to 3 pulses, the intermetallic compound layer thickness increased from 0 µm to 8.5 µm. As a comparison, the IMC layer thickness was approximately 65 µm for resistance spot welding of 1-mm AA 5052 and 0.8-mm low carbon steel without transition metal. The static and dynamic strength of the RSW samples in cross-tension and coach peel test were comparable to those of the peak load of self-piercing rivets (SPR). However, lap shear tensile testing of RSW samples had lower energy absorption than dissimilar SPR samples due to different failure modes (interfacial fracture may occur). Fatigue strength of RSW samples were lower than those of SPR samples due to stress concentration and tensile residual stress at the notch tip.
Figure 1: Cross-sectional macrostructure of resistance spot welding of SAE1008/A5182-O with 3 pulses of 12 cycles and welding current of 13.6 kA.S-16
Oikawa et al.O-3 have done resistance spot welding of 0.4 mm-thick cold rolled steel with 0.6 mm-thick pure aluminium with 0.77 mm-thick aluminium-clad steel sheet as the insert metal. Fe-Al IMCs have not been noted at the Aluminium/Steel interface of aluminium-clad steel sheet and a peel strength of 32.1 N/mm can be achieved. IMC layer thickness was about 5 µm at Al/steel interface when aluminium alloy and steel was resistance spot welded with aluminium clad steel sheet as insert. However, the tensile shear strength (Figure 2) and U-tension strength (Figure 3) can be 3.6 kN and 1.5 kN which were similar to Al/Al resistance spot weld joint and were much higher than direct resistance spot welding of 2.4 kN and 0.6 kN. However, it should be noted that resistance spot welding with aluminium-clad steel added weight, and difficulties in hot rolled aluminium cladding process result in high cost. Thus, more investigations have been done recently on improving the cladding method and using thinner and lighter metals as interlayers.
Figure 2: Comparison of tensile shear strength of Al/steel joints by various methods (SP-spot welding; MC-mechanical clinching; RJ-rivet joining; AD-adhesion bonding).O-3
Figure 3: Comparison of U-tension strength of Al/steel joints by various methods (SP-spot welding; MC-mechanical clinching; RJ-rivet joining; AD-adhesion bonding).O-3
In order to reduce the IMC layer thickness, Zhang et al.W-15 used 4047 AlSi12 as an interlayer to suppress the IMC growth during resistance spot welding of 1 mm H22YD-Z100 Zn-coated high-strength steel sheets with 1.5 mm EN AW 6008-T66 aluminium alloy sheets. The effect of different interlayer thickness, i.e. 100 µm, 200 µm, 300 µm, 400 µm, on IMC growth behavior and mechanical properties were investigated. IMCs consisted of Fe2(Al,Si)5 and Fe4(Al,Si)13 and the thickness of IMCs reduced from 1.8 µm to 0.6 µm as the thickness of AlSi12 interlayer, increasing from 100 µm to 400 µm. But all were much thinner compared to 4 µm for spot welds without an interlayer. Nugget diameter reduced with increasing interlayer thickness and maximum tensile shear strength of 6.2 kN and pull-out failure mode were obtained with an interlayer thickness of 300 µm.
Figure 4: Fracture modes of H220YD high-strength steel/6008-T66 resistance spot weld joints (a) interfacial fracture without interlayer and (b) pull-out failure with 4047 AlSi12 interlayer thickness of 300 µm. W-15
Ibrahim et al.I-6 had investigated resistance spot welding of 2 mm A6061-T6 and 2 mm type 304 austenitic stainless steel with 80 µm thick Al-Mg alloy (80 wt % Al and 20 wt% Mg). Under the same welding condition, lap shear tensile strength was higher for welds with an Al-Mg interlayer than without the interlayer, and a peak strength of 8.4 kN can be obtained with welding current of 16.1 kA and welding force of 9.3 kN. A thin IMC layer of approximately 2 µm can be observed, which lead to a high strength. Fatigue strength of dissimilar RSW Aluminium/Steel samples were higher than Friction Stir Spot Welding samples. The failure mode depended on the load level. Pull-out failure occurred at high load (Pmax > 3 kN), shear fracture at 2.25 kN < Pmax <3 kN, upper Al sheet fracture at Pmax < 2.25 kN.
Figure 5: Relationship between peak load and number of cycles.I-6
Galvanized steel (GI) and Galvannealed steel (GA) are widely used in the automotive industry, and the presence of Zn layer may affect the weldability of Aluminium/Steel. Al-Zn eutectic liquid formed at Aluminium/Steel interface can efficiently remove oxide film and squeezed outside by electrode force. Thus, metallic surface of aluminium and steel can be revealed, and the Al-Fe IMC layer formed and a metallurgical bond was created.M-10, U-4 Arghavani et al.A-21 have investigated the resistance spot welding of 2 mm Al-5052 to 1 mm low carbon galvanized steel (ASTM A653 CS Type B-G60) (GI/Al) and compared that with the weldability of Al 5052 to uncoated steel DC01 (PS/Al). It showed the IMC layer at the GS/Al interface with the tongue-like layer of Fe2Al5 on the steel side and needle-like FeAl3 on the aluminium alloy side, as normally observed in Aluminium/Steel dissimilar welding (Figure 6).Z-3, U-4 GI/Al joints had thinner IMC layers at almost all the welding currents compared to PS/Al joints. And the IMC layer thickness in GI/Al increased initially with increasing welding current up to 12 kA and then decreased beyond it, which was due to heat consumption with Zn evaporation at higher welding current (I > 12 kA). Tensile shear strength of GI/Al welds were lower than PS/Al welds at welding current less than 12 kA since lower current led to incomplete nugget at GI/Al welds. However, tensile shear strength of GI/Al welds exceeded PS/Al welds at higher welding current since IMC layer thickness was over the critical value of approximately 5.5 µm for PS/Al joints at the higher welding current (Figure 7). Both GI/Al and PS/Al had an interfacial fracture mode with crack initiation at the peripheral regions of the weld and propagated through the IMCs layer/aluminium sheet.
Figure 6: SEM image of IMC layer in GS/Al specimen.A-21
Figure 7: (a) Ultimate tensile shear and cross-tension strength vs welding current; (b) schematics of load-displacement curve and fracture energy calculation for tensile shear testing; (c) Fracture energy vs welding current.A-21