Investigation on microstructural features and tensile shear fracture properties of resistance spot welded advanced high strength dual phase steel sheets in lap joint configuration for automotive frame applications

Chakkaravarthi Rajarajan; Paramasivam Sivaraj; Tushar Sonar; Selvaraj Raja; Nallusamy Mathiazhagan

doi:10.1515/jmbm-2022-0006

Article Open Access

Investigation on microstructural features and tensile shear fracture properties of resistance spot welded advanced high strength dual phase steel sheets in lap joint configuration for automotive frame applications

Chakkaravarthi Rajarajan , Paramasivam Sivaraj , Tushar Sonar , Selvaraj Raja and Nallusamy Mathiazhagan

Published/Copyright: April 13, 2022

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From the journal Journal of the Mechanical Behavior of Materials Volume 31 Issue 1

Abstract

The main objective of this research paper is to study the microstructural features and mechanical properties of resistance spot welded advanced high-strength steel of dual phase grade in lap joint configuration which is mainly employed in sheet form for fabrication of the automotive structure. Resistance spot welding (RSW) being a solid-state welding (SSW) process is used to overcome the problems in fusion welding of AHSS-DP steel such as heat affected zone (HAZ) softening, solidification cracking and distortion which significantly deteriorates the mechanical properties of AHSS-DP800 steel joints. The straight lap (SL-TSFL) and cross lap tensile shear fracture load (CL-TSFL) of spot joints were evaluated. Optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques were used to characterize the weld nugget. The X-ray diffraction (XRD) results are also presented for phase identification in the weld nugget. The fracture surface of failed TSFL specimens was analyzed using SEM. The lap joints made using RSW disclosed superior SL-TSFL, CL-TSFL, and WNZH of 21.7 kN, 17.65 kN, and 589 HV_0.5 The superior joint strength and hardness of the weld nugget zone are correlated to the evolution of lath martensite in the nugget zone.

Keywords: Resistance spot welding; advanced high strength steel; tensile shear fracture load; microhardness; microstructure

1 Introduction

The invention of new materials in the modern engineering world proves to have an impact on every engineering application. These materials help to achieve environment-friendly vehicles from emission reduction, fuel efficiency, weight reduction, and crashworthiness factors [1]. For these concerns, Advanced High Strength Steels (AHSS) were developed with the high strength-to-weight ratio at a moderate cost [2]. Among this category, Dual-Phase steels and their variants have proved that better formability and machinability for automotive structures [3]. Dual-phase 800 (DP800) steel consists of 70% ferrite and 30% marten-site phases which offers a high rate of work hardening, strength, and elongation. Thereby it proves its feasibility as high strength lightweight material for automobile structural frames [4]. The weight reduction in an automobile is required to achieve a lower rate of gas emission, fuel efficiency [5]. The high heat input in fusion welding leads to wider bead and heat affected zone (HAZ), solidification and HAZ cracking problems and distortion of alloy sheets [6,7,8,9,10,11,12]. In this investigation, resistance spot welding (RSW) is employed to overcome the heat input-related problems in fusion welding of AHSS DP800 steel such as HAZ softening, solidification cracking, micro fissuring in HAZ, and distortion which significantly lowers the joint performance [13]. It involves resistive heating of joining surfaces under pressure at a temperature less than melting point of parent metal. The RSW is widely used for joining sheet metal in automotive industries for fabricating automobile structural frames as it is economical, shop-friendly, and can be easily automated [14]. The weld nugget size and associated tensile shear strength are commonly used to check the quality of RSW joints. The microstructure and tensile shear strength of spot weld joints are influenced by RSW parameters such as welding current, welding time, and electrode pressure [15,16,17].

Manickam et al. [18] investigated the tensile shear fracture properties of friction stir spot welded AA6061 and copper alloy joints. Rajendran et al. [19] studied the fracture load properties of solution treated and artificially aged lap joints of AA2014-T6 alloy developed using friction stir welding process. Rajendran et al. [20] investigated the tensile shear fracture properties of riveted and friction stir welded AA2014-T6 lap joints. Liao et al. [21] studied the microstructure of RSW DP600 steel joints. The results revealed needle-like martensite in the fusion zone, which consists of primitive orthorhombic and face-centered cubic Chromium carbides (Cr₃C₂ and CrC). Zhao et al. [22] observed that Heat affected zone (HAZ) softening increases with the steel grades due to the higher martensite volume fraction of the base metal in stronger steels. Yuan et al. [23] investigated the nugget zone characteristics of dissimilar RSW (DP600 – DC54D) weld joints. The studies concluded, the nugget zone consists of lath martensite and intensive ferrite phases. The tensile shear strength has variable effects with changes in welding current. Kishore et al. [24] studied the weldability and failure behavior of bare and galvanized DP600 steel joints. The critical nugget diameter was found to be 4.4 mm nominal ensuring pull-out failure mode of fracture. With increased welding current and time, a diameter of nugget and load-carrying capability increased as well. Aslanlar et al. [25] analyzed the impact of welding time from 5 cycles to 15 cycles on resistance spot welded micro-alloyed steel. The optimized welding parameters yielded maximum tensile peel and shear strength values. The joint strength has increased with an indentation depth of approximately 15% of sheet thickness. Hernandez et al. [26] investigated the nanoindentation characteristics of HAZ and parent interface region in RSW DP steel joints. The results concluded the formation of tempered martensite (TM) in HAZ which led to lower hardness because of broken tempered martensite morphology towards the parent metal. Wan et al. [27] studied the effect of welding current parameters on spot-welded DP600 steel and found the direct effect of welding current on nugget size and strength of joint. It is confirmed through numerical model along nugget zones and temperature distribution of weld zone.

The experimental work on RSW of AHSS-DP800 thin sheets, so far, is limited. There is a lack of systematic investigation on evaluating the microstructural features of weld nugget zone and mechanical properties of AHSS-DP800 spot weld joints. This necessitates further research in RSW of AHSS-DP800 sheets. So, the main objective of this research paper is to evaluate the microstructural features of nugget zone using advanced characterization techniques and assess the mechanical performance for lap joint configuration required in automotive applications.

2 Experimental methodology

For this study, 1.6 mm thick cold-rolled AHSS-DP800 sheets were chosen as a parent metal. The chemical composition of AHSS-DP800 sheets is provided in Table 1. The mechanical properties of parent metal are provided in Table 2. The spot welds were developed employing the semi-automatic electrical Resistance Spot Welding machine. The photograph of RSW machine employed in this investigation is shown in Figure 1. A Conical type water-cooled electrode was used with 16 mm shank and 5 mm lid diameter respectively. The welding trials were performed to determine the working limits of process parameters.

Table 1

Chemical composition (wt.%) of AHSS-DP800 steel

C	Si	Mn	Cr	P	S	Ni	Mo	Ti	Fe
0.146	0.88	1.500	0.025	0.007	0.0036	0.027	0.0018	0.0016	balance

Table 2

Mechanical properties of AHSS-DP800 steel

0.2% offset Yield Strength (MPa)	Ultimate Tensile Strength (MPa)	Elongation in 50 mm gauge length (%)	Microhardness (HV)
604	832	26	295

Figure 1

Photograph of RSW machine employed in this investigation

Table 3 shows the process parameters for finding the optimal conditions of RSW parameters for joining AHSS-DP800 lap joints. Response surface methodology (RSM) is commonly employed for optimizing process parameters [28]. It is a set of mathematical equations used to develop a design matrix for predicting the responses [29]. RSM is also utilized to fit the empirical relationships to data obtained from the developed design matrix [30]. The RSW parameters to attain maximum strength and nugget zone hardness are enumerated in Table 4.

Table 3

Working limits of RSW parameters for joining AHSS-DP800 steel

Parameter	Notation	units	Levels

			−1.68	−1	0	+1	+1.68
Welding current	WC	kA	4	4.5	5	5.5	6
Electrode pressure	EP	MPa	3.5	3.75	4.0	4.25	4.5
Welding time	WT	s	0.5	1.0	1.5	2.0	2.5

Table 4

Optimized RSW parameters for joining AHSS-DP800 steel

Condition	WC (kA)	EP (MPa)	WT (s)	SL-TSFL (kN)	CL-TSFL (kN)	NZH (HV_0.5)
Experimental	5.0	4.0	1.50	21.70	17.65	589
Predicted	5.05	3.99	1.53	21.57	17.34	585

An empirical relationship was derived from the RSW parameters and validated by analysis of variance (ANOVA) to predict the SL-TSFL, CL-TSFL, and NZH of AHSS-DP800 sheet spot joints as given by equations 1, 2, and 3. The RSW parameters were designated as I, P, and T.

(1) SL-TSFL(kN)=−913.57+9.42(I)+350.27(P)− 10.26409(T)−0.086(I×P)+ 0.80200(I×T)+3.16(P×T)−0.101(I2)− 43.93(P2)−13.84(T2)

(2) CL-TSFL(kN)=−691.74+10.43(I)+214.64(P)+ 20.98(T)−0.63(I×P)+0.11(I×T)+ 0.98(P×T)−0.08(I2)−23.32(I2)− 10(I2)

(3) NZH(HV)=−26058.16+253.26(I)+10067.52(P)+ 229.3(T)+1.9(I×P)+11.25(I×T)+ 51(P×T)−2.77(I2)−1280.9(P2)− 328.22(T2)

The spot weld joints were made for the straight lap and cross lap joint configurations as per ASTM – ANSI/SAE/AWS /D8.9 – 13a standards. The dimensions of straight lap tensile (SL-TSFL) and cross lap tensile shear fracture load (CL-TSFL) testing specimens are illustrated in Figure 2. The photograph of typical SL-TSFL and CL-TSFL specimens of RSW AHSS-DP800 joints is displayed in Figure 3. The shear fracture test performed under the servo motor enabled a universal testing machine with a maximum capacity of 1000 kN. For each welding condition, three TSFL specimens were prepared and the average of three was reported as final reading. The nugget of spot weld was cross-sectioned and subjected to metallography specimen preparation. A Vickers microhardness testing machine was used to measure microhardness across the weld nugget. The microhardness of weld nugget was measured on mirror-polished specimens of AHS-DP800 steel RSW joints. The hardness measurement was recorded from the weld nugget cross-section employing 0.5 kg load and 15 sec dwell time. The RSW lap joint was cut along the longitudinal direction of spot weld and mirror polished to finer finish.

Figure 2

Dimension of TSFL specimens: a) SL-TSFL and b) CL-TSFL

Figure 3

Photograph of typical TSFL specimens of RSW AHSS-DP800 joints: a) SL-TSFL; b) CL-TSFL

The mirror-polished metallographic specimens were etched using Villella’s reagent for analyzing the macro and microstructural features. It was developed as a mixed solution of 1gram picric acid, 5 ml HCl and 100ml ethanol. The macrostructure of AHSS-DP800 joints was analyzed using stereo zoom microscope. The microstructural features of weld nugget and HAZ were characterized using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The metallographic specimens for TEM were prepared according to the standard procedures. The EDS line analysis of weld nugget was performed to study the elemental changes and X-ray diffraction (XRD) results are also presented for the phase identification in the weld nugget region. The crystallographic nature was revealed from selective area diffraction patterns through TEM.

3 Results and discussion

3.1 Tensile shear fracture load

Figure 4 shows the photograph of fractured SL-TSFL and CL-TSFL specimens of DP800 steel joints for the optimized condition of RSW parameters. SL-TSFL specimens showed tearing mode failure initiated in HAZ of joints. The CL-TSFL specimens showed a partially pullout type of failure mode in DP800 steel RSW joints. The joints welded using optimized RSW parameters showed maximum SL-TSFL and CL-TSFL of 21.70 kN and 17.65 kN with 7% and 9% elongation respectively. The SL-TSFL of RSW DP800 steel joints is 22.94% higher than the CL-TSFL of joints. Thus, the load-carrying capability of RSW DP800 steel joints is superior in straight lap joint configuration than cross lap joint configuration.

Figure 4

Failure of optimized joint of RSW AHSS-DP800: a) SL-TSFL and b) CL-TSFL specimen

The failure energy for SL-TSFL and CL-TSFL joints was evaluated from the failure energy absorption equation.

(4) Failure energy absorption (Q)=∫SminSmaxfdx=∑x=1XF(x)[y(x)−y(x−1)]

Whereas, S – displacement at load, x – number of sampling experiments, X – maximum fracture load in kN. The shear fracture load and failure energy corresponding to elongation of both SL-TSFL and CL-TSFL joints are illustrated in Figure 5. The joints were observed to get failed by the failure energy of 85 kJ and 77 kJ in SL-TSFL and CL-TSFL specimens. The fractured surface of SL-TSFL and CL-TSFL specimens was analyzed by SEM as shown in Figure 6. The failure occurred in the periphery of the weld nugget due to severe plastic deformation. The fractured surface showed finer dimpled features in SL-TSFL and CL-TSFL specimens. The size of dimples has a significant effect on the mechanical and metallurgical properties of the welded joints. The dimple and microvoid size observed in fractured surfaces of SL-TSFL specimens is much finer whereas it was observed to be slightly coarser in CL-TSFL specimens. The CL-TSFL specimens revealed the presence of cleavage facet regions. It is mainly attributed to the presence of carbides. The breakage of carbides during the tensile shear loading provides the crack imitation and propagation path resulting in the formation of cleavage facet region and river pattern.

Figure 5

Failure energy of RSW AHSS-DP800 joints: a) SL-TSFL and b) CL-TSFL

$Figure 6 SEM fractograph of TSFL specimens of optimized joint at lower and higher magnification: a) and b) LAP-TSFL; c) and d) CROSS-TSFL specimen$

Figure 6

SEM fractograph of TSFL specimens of optimized joint at lower and higher magnification: a) and b) LAP-TSFL; c) and d) CROSS-TSFL specimen

3.2 Microhardness

The microhardness of nugget zone (NZ) is directly propositional to SL-TSFL and CL-TSFL. Figure 7 shows the microhardness mapping of different regions of RSW DP800 steel joints. The hardness variation was observed in nugget zone (NZ), coarse-grained HAZ (CG-HAZ), and fine grained HAZ (FG-HAZ) of welded joints. The NZ showed higher hardness than HAZ and base metal. The NZ showed higher hardness of 589 HV_0.5 which is much higher than base metal hardness of 295 HV_0.5. The hardness variation was observed in HAZ of RSW joints due to change in grain size resulting in the formation of CG-HAZ and FG-HAZ. The CG-HAZ showed lower hardness than FG-HAZ. It is mainly attributed to the grain coarsening in CG-HAZ. The higher hardness of 584–589 HV_0.5 in the NZ was associated with the formation of martensite structure due to the severe cooling rate associated with the process. The martensite formation in dual-phase steel is approximately from 40–120°Cs⁻¹ [31, 32]. The severe elastic strain rate during the process assisted the softening of grains in CG-HAZ and initiated failure of the spot weld. The hardness plot and its corresponding contour maps revealed the hardness distribution around joints. It was evident from the distribution map that CG-HAZ regions showed less hardness when compared with FG-HAZ regions due to the fine grains associated with the latter. Due to the cooling rate of the process, the martensite formation was induced which attributed to the higher hardness in NZ compared to HAZ and base metal.

Figure 7

Microhardness mapping of weld nugget from the centre

3.3 Microstructural characteristics

Figure 8 displays the optical microstructure of DP 800 steel. It reveals the presence of ferrite along with martensite. The average grain size of ferrite phase and martensite phase is 7–9 µm and 3–7 µm respectively. The macrostructure of RSW spot joint of AHSS-DP800 steel is shown in Figure 9. The macrostructure showed no weld defects. The weld defects and imperfections were analyzed by the macrostructural characteristics of the weldments. Figure 10 displays the optical microstructure of DP800 steel spot weld at lower magnification showing different regions of weld. The optical microstructures of NZ, CG-HAZ, FG-HAZ at lower and higher magnification are shown in Figure 11. The SEM microstructure of nugget zone at lower and higher magnification, CG-HAZ and FG-HAZ are shown in Figure 12. The NZ microstructure shows dynamic recrystallization of grains exhibiting a columnar structure. This formation suggests the formation of martensite and bainite phases in the ferrite matrix during solidification. The superior TSFL and NZH in DP800 steel RSW joints are attributed to the evolution of needle/lath-like martensitic structure in nugget zone.

Figure 8

Optical microstructure of AHSS-DP800

Figure 9

Macrostructure of AHSS-DP800 RSW joints

Figure 10

Optical microstructure of weld nugget of RSW AHSS-DP800 joints showing NZ, CG-HAZ, FG-HAZ and BM regions

Figure 11

Optical microstructure of different regions of weld nugget of RSW AHSS-DP800 joints: a) NZ at 200X; b) NZ at 500X; c) CG-HAZ at 200X; d) CG-HAZ at 500X; e) FG-HAZ at 200X and f) FG-HAZ at 500X magnification

Figure 12

SEM micrograph of different regions of weld nugget: a) nugget zone at lower magnification; b) nugget zone at higher magnification; c) CG-HAZ; d) FG-HAZ

The CG-HAZ and FG-HAZ exhibit volumes of Tempered martensite (T α′) in the deformed region with columnar/needle/lath type martensite (α′) in the supercritical region. These phases induce the softening effect and accelerate the failure near the weld nugget. The region near the HAZ will exhibit high elastic distortion than farther regions. The average grain size of the martensite was in the range of 5–7 µm with a columnar profile. The grain morphology of the nugget and weld interface zones were classified based on grain sizes and orientations with columnar hard face martensite (α′) and tempered martensite structure (T α′) in alpha (α) ferrite matrix as discussed. The dissolution of high carbon content assisted the formation of these phases in fully deformed region [33]. The region formed due to heat dissipation next to HAZ regions composed of ferrite and Tempered martensite (Tα′) phases. The weld failure occurred in the subcritical cooling regions due to the presence of soft tempered martensite (Tα′).

Figure 13 displays TEM microstructural features of weld nugget: a) NZ; b) HAZ; c) SAD pattern of NZ and d) SAD pattern of HAZ. Figure 13a describes the columnar/needle/lath type martensite structure (α′) in the supercritical region of α matrix. Although Figure 13b indicates the sub-critical heat-affected (HAZ) region of dual-phase steel, which clearly shows the coarser grain structure. It contains a few volumes of Tempered martensite (T α′) in the deformed grain matrix. This will induce the softening effect nearer the weld joint, which is also used to correlate the failure pattern of the weldment. To reveal the crystallography orientations through the selective area diffraction patterns (SAD). it was observed from various regions of the spot weld such as dynamically recrystallization region (DRX), Coarser and finer grain heat affected regions (CG-HAZ & FG-HAZ). The nucleated grains are rapidly formed as a columnar grain at the inner core section associated with equiaxed grains at the outer core section. The observed diffracted crystallography patterns consisted of [101]_α′ and [200]_γ phases in Figure 13c and 13d. While the high cooling rate condition dissolution of high carbon content in the nugget zone forms carbide phases. This diffracted pattern revealed that intermetallic precipitates found were cementite (Fe₃C) and manganese carbide (Mn₃C). The cementite (η) phase was an orthorhombic crystal structure with lattice dimensions about a = 0.526 nm, c = 1.263 nm. Very few researchers were reported Mn content was induces the formation of retained austenite interweaves in the martensitic block. It will also assist the stability of the austenite phase in the subcritical region which induces the retained austinite and tempered martensite phase [33]. From the SAD patterns, the intermetallic precipitates cementite (Fe₃C) and manganese carbide (Mn₃C) were found which was formed due to the dissolution of high carbon content in the deformed regions [22]. Thus, cementite and manganese carbide structures were identified as orthorhombic and body-centered cubic with a = 0.526 nm, c = 1.263 and a = 0.458 nm, c = 0.963 nm lattice dimensions. The EDS results confirmed the Fe, Mn, Si, C elements and their percentage of the composition. The diffraction pattern and SAED patterns confirmed the presence of cementite and manganese carbide with [101], [200] and 2.00520 Å, 1.42582 Å from XRD and 101, , 200, from TEM results.

Figure 13

TEM microstructural features of weld nugget: a) NZ; b) HAZ; c) SAD pattern of NZ and d) SAD pattern of HAZ

3.4 Elemental and phase analysis

Energy Dispersive Spectroscopy (EDS) analysis was used to quantify the constitutional elements in the weldment. Figure 14 shows the EDS line scan analysis of weld nugget. The major elements in the spot-welded regions were Fe, Mn, C, and Si along with other traces. The X-ray diffraction (XRD) tool was used to categories the phase composition of the weldments. The XRD was working under the elemental diffraction angle and intensity of the atomic elements. The observed diffracted patterns peaks are in the form of two theta angles (2θ) vs. intensity. The XRD peaks were analyzed through pan analytical Xpert high score software to predict diffracted phases, miller indices, and D-spacing values. Figure 15 shows the XRD patterns of RSW DP800 steel joints. It discloses the presence of two peak intensity phases at the angle about 45.22° and 65.46°, and it was working under the Cu-Ka1 (1.540598 A). These peak intensities were rich in Fe (45.22°), and Mn (65.46°) assisted with minor elements presence of Carbon and Silicon. The possible phases found in a Nugget zone were Fe₃C (cementite) and Mn₃C (manganese carbide). The formation of this intermetallic phase is mainly due to the critical cooling rate that occurred in the nugget zone. Instead of this cementite phase, the weld failures occurred mainly due to the formation of Tempered martensite (Tα′) in the interfacial region of the coarser grain heat-affected region. Although the founded the miler indices and d-spacing values iron (Fe), manganese (Mn) rich phases about (101) and 2.00520 Å, (200) and 1.42582 Å. Also, the crystal diffraction is represented in pole figure texture in Figure 9b using the calculated ODF data in the diffraction. The crystal diffraction (XRD) phenomenon establishes a relation between atomic intensity (I) and deviation angle (2θ) [23]. The diffracted angles with corresponding miler indices and d-spacing values designated the phases as iron (Fe), Manganese (Mn), cementite (Fe₃C), and manganese carbide (Mn₃C).

Figure 14

EDS line scan analysis of weld nugget: a) Line scanning region; b) EDS spectrum

Figure 15

XRD Patterns for AHSS-DP800 joint

4 Conclusions

The AHSS DP800 sheet (1.6 mm thick) was joined successfully using the solid-state RSW process without cracking in welds and HAZ, porosity, and penetration defects encountered in resistance spot welding processes.
The RSW DP800 steel joints made using the welding current of 5.0 kA, electrode pressure of 4.0 MPa, and welding time of 1.50 s displayed maximum SL-TSFL of 21.7 kN, CL-TSFL of 17.65 kN, and NZH of 589 HV_0.5 respectively.
The AHSS-DP800 joints were fractured by failure energy of 85 kJ and 77 kJ in SL-TSFL and CL-TSFL specimens respectively. The performance of AHSS-DP800 spot joints is superior in straight lap joint configuration than cross lap joint configuration.
The weld failure occurred in CG-HAZ as a result of lower hardness than weld nugget and base metal. It is due to the evolution of soft tempered martensite (Tα′) in CG-HAZ.
The weld nugget microstructure consisted of a needle/lath-like martensitic structure along with a ferrite matrix due to the critical cooling rate which is mainly responsible for superior strength and hardness of AHSS-DP800 spot weld joints.

Funding information:
The authors state no funding involved.
Author contributions:
All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
Conflict of interest:
The authors state no conflict of interest.

Abbreviations

AHSS: Advanced High strength Steel
DP steel: Dual Phase Steel
RSW: Resistance Spot Welding
SSW: Solid State Welding
NZ: Nugget Zone
HAZ: Heat Affected Zone
TSFL: Tensile Shear Fracture Load
SL − TSFL: Straight Lap TSFL
CL − TSFL: Cross Lap TSFL
WNZH: Weld Nugget Zone Hardness
OM: Optical Microscopy
SEM: Scanning Electron Microscopy
TEM: Transmission Electron Microscopy
XRD: X-ray diffraction
FG − HAZ: Fine Grain HAZ
CG − HAZ: Coarse Grain HAZ

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Received: 2021-12-27

Accepted: 2022-02-17

Published Online: 2022-04-13

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/jmbm-2022-0006

Keywords for this article

Resistance spot welding; advanced high strength steel; tensile shear fracture load; microhardness; microstructure

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