In response to demands for lighter and crash-resistant car body, steel manufacturers have developed a suite of 3rd generation Advanced High Strength Steels (3gAHSS) with high strength and high ductility[1, 2] over the past decade. Application of these materials is crucial for the future of automotive design; however, when Zn-coated for corrosion protection and subjected to resistance spot welding (RSW), these novel steels can be prone to liquid metal embrittlement cracking[3, 4] leading to a drop in mechanical properties of welded joints (e.g. loss in tensile shear strength of up to 42% [5]), which may limit the use of these new steels in automotive applications.
Liquid metal embrittlement (LME) is a loss of ductility and strength of a solid phase in presence of a molten phase. Three conditions are needed for LME-formation: a susceptible microstructure, exposure to a liquid embrittling agent, and tensile stress [6–9]. This phenomenon has been observed for over 100 years [3]; however, the topic has renewed interest with the automotive industry’s increased use of Zn-coated 3gAHSS[2, 6–8].
Resistance spot welding is one of the most common processes in the car body assembly. The body-in-white (BiW) structure of an automobile contains at least 2000 spot welds, joining various materials with multiple sheet thickness [9]. During resistance spot welding, schematically shown in Fig. 1, sheets (1) are joined in a lap joint configuration. First, they are clamped by two highly conductive water-cooled electrodes (2), which are then loaded with electrode force (Fe). Subsequently, a high welding current passes between the electrodes, heating the joint by Joule heat, leading to melting at the interface between sheets and forming a molten nugget (3). Once the weld has grown to sufficient size, the welding current ceases, allowing the weld nugget to cool and solidify as heat flows into the cooled electrodes.
Several theories have been introduced to explain LME cracking during RSW of Zn-coated steels [10, 11]. It is generally agreed, that LME occurs because liquid zinc penetrates along the grain boundaries of steel, reducing its capacity to withstand stress. Susceptible microstructure, presence of liquid zinc and tensile stress are necessary for LME-formation; however, precise description of stresses, strains and temperatures under which LME-cracks form during the resistance welding process remain unknown.
To date most of the literature on LME cracking during RSW has focused on process changes that can mitigate cracking. As a result, several strategies have been proposed to optimize welding parameters, reducing LME. The use of higher electrode forces [12], multiple-pulse welding schedules [13–15], current ramping [16], using prolonged hold times (HT)[7, 12, 15, 17, 18] have all been shown to reduce LME cracking severity during RSW. In the presented studies, a classification by Choi et al.[19] was used (schematically shown in Fig. 1) to describe crack location within the weld zone. According to this classification Type A cracks are located along the flat indent area above or below the nugget, Type B cracks are located on the weld shoulder (starting at the radiused part of an electrode indent and continuing into the HAZ), and Type C cracks are located at the weld notch.
Type A (indent) crack formation has been investigated in[13, 20–22]. and a number of studies have concentrated on Type B[2, 14, 16, 19, 23–25] and Type C[12] cracks. The ability of Type A cracks to affect weld strength is still debated [5], however only quasi-static strength tests have been conducted. The effect of cracking on fatigue life or corrosion resistance of welds have not been investigated.
During automotive assembly, LME can be mitigated by modifying the welding schedule and a number of studies [7, 13, 17, 19] have been conducted to assess the influence of resistance spot welding parameters such as electrode force, current modulations and variations of hold times on LME occurrence during RSW.
Multiple authors [7, 12, 19, 26] simulate stress development in different regions of a resistance spot weld and concluded that the cracks form due to stress build-up at or after electrode release [7, 18, 19]. The later authors propose prolonged hold times to mitigate LME cracking. Hold time prolongation has been shown to mitigate LME due to active heat extraction by conductive heat transfer from the sheet to the electrode, thus reducing the time material spends in the LME-sensitive region. Another important factor is the support that the electrodes provide to the sheet surface. While the contact between the electrode and the sheet is maintained, friction in the contact prevents crack growth, even if cracks initiate during hold time [18].
A brief overview of main studies, investigating the influence of hold time on LME can be found in Table 1. Although variation of hold time has been shown to influence LME, the literature does not offer a clear understanding of how the hold time influences temperature and stress in welds with production-relevant schedules, leading to changes in LME severity in the manufacturing environment. In some cases, due to high number of variables investigated [15], it is hard Isolate how changes in each variable affect the stress-state and LME cracking independently. Other studies have been conducted using experimental conditions that make the application of their findings in manufacturing environment challenging (e.g. use of excessive weld time [7]). Expulsion, being another important factor to LME formation in the manufacturing conditions [27], is also often left unaccounted for. Therefore, a systematic study of hold time on LME formation will be carried out. This study uses manufacturing-relevant joint configurations and welding schedules with production-relevant heat input levels. The mechanisms linking hold time to LME will be accomplished by focusing on investigating temperature and stress development during and after the welding process.
Table 1
Detailed review of literature, studying the influence of hold time on LME
Reference
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Materials
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Welding shedules
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Main findings
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[7]
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DP1200HD EG in two 4-sheet stack-ups:
1 3g AHSS + 3 mild steel
1 3gAHSS + 1 mild steel
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Multiple pulses with the total welding time of 1520 ms, followed by hold times of 10, 200, and 800 ms
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Hold time of 800 ms leads to lower stress levels and temperatures, as compared to other hold times, thus reducing LME
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[12]
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GI coated 3G AHSS Q&P 980 + Q&P 1100
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AC welder, 8R50 dome shaped electrodes, 20 cycles weld time Imax-0.5 kA with subsequent PWHT, tilted electrodes 5°
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Hold times over 10 cycles help mitigate C-type cracking
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[15]
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GA-coated TRIP 590
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Varied hold time between 280 ms, 480 ms, and 680 ms in a DOE analysis, along with variation in electrode force, welding current, and welding time with dome shaped and radiused electrode tips.
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No influence of hold time on LME severity when increasing hold times from 280 to 480 ms, and a slight decrease in LME severity when increasing the hold time from 480 to 680 ms
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[17]
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TRIP 1180 steel with a sheet thickness of 1.6 mm GI and ZnMg coatings
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Three weld times 200, 400, 1000 ms, and three hold times 20, 300, 1000 ms.
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Prolonged hold time reduces Type C LME severity for weld time of 1000 ms (Type C LME is present for hold time of 20 ms, no Type C LME measured for 300 and 1000 ms)
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[18]
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Three 1.6-mm steels: HDGA TRIP 590, HDGI DP600, and HDGI mild steel.
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Matrix of weld currents and and electrode forces at hold times of 40, 100, and 1200 (2, 5, and 60 cycles @50 Hz)
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Increase of hold time from 40 and 100 to 1200 ms mitigates LME, however no difference in cracking severity for HT of 40 and 100 ms
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