Behaviour of resistance spot welded thermomechanically rolled high strength steels under tensile shear and cross-tension loads

doi:10.21203/rs.3.rs-3604489/v1

Over the past decades the vehicle manufacturers have been striving to reduce vehicle weight to minimize energy consumption and emissions in line with the key priorities of the environmental objectives of the European Union. These goals can be met by using high strength steels although their application raises numerous challenges in welding. In general, the railway vehicle body is made of ribs-stiffened shell structure which bears the load together with chassis. By the change of mild steel to high strength steel in these structural elements, joined with resistance spot welding (RSW), significant weight reduction can be achieved. Thanks to the results of material development now it is possible to use thermomechanically rolled high strength steels with relatively low carbon equivalent which can be ideal for the RSW of railway vehicles. In this paper the results of tensile shear (STS) and cross-tension (CTS) tests of resistance spot-welded joints made of t=3 mm thick S700M thermomechanically rolled high strength steel are presented. The main research scope was to elaborate a complex resistance welding process including the determination of optimal welding parameters based on STS and CTS tests. The optimal technological parameters were nearly the same for the two loading circumstances, however, a bit shorter cycle was justified during the optimization for the cross-tension strength. The value of the tensile shear force exceeded the minimum value specified by the AWS recommendation D8.1M by more than 22% with an electrode indentation depth of less than 5%.

railway industry

resistance spot welding (RSW)

thermomechanically rolled steels

tensile shear test

cross-tension test

Nowadays, the need to reduce the unladen weight of vehicles has come to the fore with railway vehicle structures. With the reduction of the unladen weight significant energy consumption can be saved, thus reducing emissions. Efforts to reduce the weight of vehicle structures have prioritized the use of high strength steel. The investigation of resistance spot welded (RSW) joints in high strength steels (HSS) and advanced high strength steels (AHSS) is a current research topic. In numerous studies the spot weldability of different automotive grades as DP, TRIP, TWIP etc. are examined in terms of parameter optimization for continuous and pulsed technology considering different loading conditions (static, dynamic, cyclic) [1–6]. In general, it is challenging to find a parameter combination which is ideal for all loading conditions. The design of RSW for quasistatic load is generally performed based on tensile shear and cross-tension tests where the optimum parameters are often different for the two testing circumstances. The shear tensile and cross-tension loads can be estimated by standardized and recently developed calculation methods [1, 2], although the applied RSW technology has still a crucial role for the final load bearing capacity of the spot-welded joint.

Material developments in recent years have been providing an opportunity to use high strength thermomechanically rolled steels in railway vehicle manufacturing. Generally, the parts of the railway vehicle body from thermomechanically controlled processed (TMCP) steels are welded by arc welding technology. However, the presence of TMCP technology in the production of high strength steel and thereby the reduction of carbon content and carbon equivalent has opened the possibility to use these structural steel grades in resistance spot-welded structures. Since the major focus of the researchers is the automotive sector in the field of RSW of HSS and AHSS, there is insufficient experience with the resistance spot weldability of thermomechanically rolled high strength steel. Therefore, the optimal welding technology parameters need to be determined for TMCP grades by technological experiments. In our previous investigations [7, 8] the motivation and the preliminary results of research work were presented in connection with the development of resistance spot welding technology for Alform® 700 M TMCP high strength steel, as possible future application in railway vehicle structures at the vehicle end and side walls. The present publication focuses on the parameter optimization based on the tensile shear and cross-tension tests of RSW joints of this steel grade.

The requirements for vehicle structures call for the design of structures that are adequate in terms of strength and fatigue, with the lowest possible weight, can be manufactured economically and are easy and cheap to maintenance. The body structure of railway vehicles generally made by ribs-stiffened shell structures; this structure bears the load together with chassis. Each structural element of shell structure participates in insurance of the structure’s stiffness, and they help to presence the forces parallel to the longitudinal axis of the railway vehicle.

The requirements of standard EN 15085-3 [10] Annex F summarize the details of the quality requirements for resistance-welded joints which should be accounted for the designing of railway vehicle structures under static loading. The standard defines two significant parameters for weld quality. On the one hand, it specifies the requirements for the minimum value of the shear tensile force as a function of the tensile strength of the base material, and on the other hand, it classifies the spot-welded joints based on the limits for the depth of electrode indentation (max. 10% deformation in surface quality class 2).

It is important to note that the requirement for a minimum value of the shear tensile strength (Table F4 of EN 15085-3) cannot be applied to steel materials with a tensile strength exceeding 620 MPa. Unless otherwise specified recommendation AWS D8.1M [11] was considered. The recommendation guides the value of the minimum acceptable shear tensile strength (STS_min [kN]) (1):

$${STS}_{min}=\frac{4\bullet (-\text{6,36}\bullet {10}^{-7}\bullet {R}_{m}^{2}+\text{6,58}\bullet {10}^{-4}\bullet {R}_{m}+\text{1,674})\bullet {R}_{m}\bullet {t}^{\text{1,5}}}{1000}$$

1

Where:	STS_min [kN]	minimum value of shear tensile strength;
	R_m [MPa]	base material tensile strength;
	t [mm]	material thickness.

The investigated Alform^® 700 M thermomechanically rolled high strength steel, a product of the Voestalpine (Austria) [12], can be classified as S700MC according to EN 10149-2 [13]. The verified mechanical properties and chemical composition of the t = 3 mm plates (Charge Nr.: 862432 / Blech-Nr.: 576545) used in our experiments are detailed in Table 1 and Table 2.

Table 1

Mechanical properties of base material
Grade	Yield strength R_eH [MPa]	Tensile strength R_m [MPa]	Elongation A₈₀ [%]	Hardness (HV0.2)
Alform® 700 M	767	818	18.0	240

Table 2

Chemical composition of base materials
Grade	Main alloying additions in base materials (wt. %)
Grade	C	Si	Mn	P	S	Al	Nb	V	Ti	Mo
Alform® 700 M	0.068	0.025	1.88	0.007	0.001	0,064	0.048	0.007	0,138	0.006

The investigated steel has low carbon content and the applied microalloying elements and the thermomechanical rolling result fine-grained tough microstructure. The calculated value of the carbon equivalent is 0.181% according to [14] (2):

$${CE}_{RSW}=C+\frac{Si}{30}+\frac{Mn}{20}+2\bullet P+4\bullet S\le \text{0,24}\%$$

2

From relation (2), we can conclude that the actual carbon equivalent of the base material is 25% below the limit value, therefore welding difficulties the deterioration of the mechanical properties of the joint are not expected due to the chemical composition.

The relationship between the tensile strength of the base material (R_m [MPa]), the measured shear tensile force (STS [kN]), and the plate thickness is detailed in the literature based on the results of approximately 100 publications [15]. For Alform® 700 M thermomechanically rolled high strength steel, the calculated value of STS_{min(Alform 700)} = 30,38 kN for the t = 3 mm thick examined samples is shown in Fig. 1.

Welding experiments were performed in the Welding Laboratory of the University of Dunaújváros, on a P.E.I.-Point PFP 281 type inverter, medium frequency, three-phase resistance welding equipment (nominal power: 114 kVA, maximum welding current: 13.2 kA). A simple work cycle was used during the experiments. A CuCrZr electrode was used with a truncated cone end (type A according to the standard EN 25184). The diameter of the endplate was determined in Ø10 mm which was taken into account in the relation d_e = 5√t [mm].

The experimental design aims to determine which input parameters (input factors) influence the objective function. To design the optimal technological parameters the Box-Wilson method was used based on the response surface [16]. The basis of the Box-Wilson experimental design method is that the optimal range is in the direction of the largest gradient compared to the design centre recorded. To create the experimental design Minitab 17 software was used which provided a good opportunity for interface-based modelling and statistical evaluation of the results. The standardized variables used in the experimental design and the width of the various intervals were determined based on the preliminary experiments (Table 3).

Table 3

Standardized variables and ranges of variation
Quantity	Standardized variables	Measuring unit	Centre of the plan	Delta	+ 1	-1
Welding time (t_h)	X₁	period	18	2	16	20
Current (I_h)	X₂	kA	11	1	12	10
Electrode force (F_e)	X₃	daN	1200	100	1300	1100

Two series of experiments were performed. In the first case, the suitability of the welded joints was evaluated based on shear tensile force, electrode indentation depth, and nugget diameter, while in the second case, the values of cross-tension force, electrode indentation depth, and nugget diameter. In both cases, the complex objective function was the maximum available tensile force while minimizing the electrode indentation depth. Splashing was observed for both series of experiments on specimen No. 4 (t_h = 20 per, I_h = 12 kA, F_e = 1200 daN). The results of this specimen were considered unsatisfactory due to splashing, however, we performed the tests on these specimens, too.

After the spot welding the penetration depth of the electrode on the specimens (e_t [mm] according to the standard EN ISO 17677-1 [17]) was measured with the help of a dial gauge. The tensile shear test according to EN ISO 14273 [18] and cross-tension tests according to EN ISO 14272 [19] were performed. After determining the combined optimum, macro- and microstructural images were taken of the joints welded with the optimal settings (Fig. 3 and Fig. 7). HV0.2 microhardness tests according to EN ISO 14271 standard [20] were performed. By integrating the area under the tensile diagrams, the work absorbed until fracture can be calculated.

4.1 Tensile shear test

The results of the performed tensile shear tests are detailed in Table 4.

Table 4

Results of the first series of experiments (shear tensile force and electrode indentation depth)
Nr.	Current (I_h)	Welding time (t_h)		Electrode force (F_e)	Electrode indentation depth (e_t)		Shear tensile force (F_TS)	Complex desirability
Nr.	[kA]	[per]	[ms]	[dN]	%	µm	[kN]
1	10	18	360	1100	7	210	22.9	0.39
2	12	16	320	1200	10	300	25.2	0.31
3	12	18	360	1100	11	330	38.6	0.42
4	12	20	400	1200	13	390	45.4	0.25
5	12	18	360	1300	12	360	39.9	0.30
6	11	16	320	1100	9	270	27.6	0.38
7	10	18	360	1300	7	210	21.2	0.26
8	11	20	400	1100	10	300	38.3	0.48
9	10	16	320	1200	5	150	19.4	0.33
10	11	18	360	1200	10	300	31.7	0.44
11	11	16	320	1300	9	270	27.2	0.39
12	11	18	360	1200	8	240	28.5	0.44
13	11	20	400	1300	11	330	33.9	0.34
14	10	20	400	1200	9	270	25.8	0.30
15	11	18	360	1200	9	270	28.5	0.44

The joints partially unbuttoned, or in some cases, sheared. During the tests, the force-displacement values belonging to the complex optimum were recorded. We observed a relatively large plastic deformation indicating tough failure. Based on the results of the first experiment series the optimal welding parameters were determined (Table 5).

Table 5

Optimal welding parameters based on the shear tensile tests
Technological parameters	Measuring unit	Value
Welding time (t_h)	ms	380
Current (I_h)	kA	10.9
Electrode force (F_e)	daN	1100

The results of tensile shear tests for joints with optimal welding parameters detailed in Table 6.

Table 6

Results of testing with optimal welding parameters
Nr.	Current (I_h)	Welding time (t_h)		Electrode force (F_e)	Electrode indentation depth (e_t)		Shear tensile force (F_TS)	Complex desirability	Failure mode
Nr.	[kA]	[per]	[ms]	[dN]	%	µm	[kN]
16/1	10.9	19	380	1100	4.1	125	37.3	0.54	buttoned
16/2	10.9	19	380	1100	4.1	125	37.2	0.54	buttoned
16/3	10.9	19	380	1100	4.5	135	36.9	0.50	buttoned
16/4	10.9	19	380	1100	4.3	130	37.1	0.52	buttoned
16/5	10.9	19	380	1100	4.5	135	37.0	0.50	buttoned
Average:					4.3	130	37.1	0.52
Standard Deviation (σ):					0.1788	4.4721	0.1414	0.0178

The result obtained during the tensile shear test of a welded joint made with optimal welding parameters was compared with the value calculated by the AWS D8.1M recommendation (Table 7).

Table 7

Comparison of the result of a tensile shear test with the value of the minimum acceptable shear tensile strength
Minimum acceptable shear tensile strength value according to AWS D8.1M recommendation	The average value of the shear tensile force of welds made with optimal welding parameters	Difference
STSmin [kN]	FTS [kN]	%
30.38	37.10	+ 22.1

Figure 2 shows the value of the complex desirability as a function of welding time and current. The value of the complex desirability determines the range of technological parameters where the value of the set objective function can be satisfied simultaneously. If the weighting of the target variables is equal (i.e., both values are equally important to us), then the complex desirability can be calculated as the geometric mean of each individual desirability.

Figure 4 shows the hardness curve (hardness measurement traverse on oblique line). No significant hardening was observed in the weld nugget and in the HAZ due to the relatively low carbon equivalent. The maximum hardness value was HV0.2 = 310 Vickers.

4.2 Cross-tension tests

The results of the cross-tension tests are detailed in Table 8.

Table 8

Results of the second series of experiments (cross tension force and electrode indentation depth)
Nr.	Current (I_h)	Welding time (t_h)		Electrode force (F_e)	Electrode indentation depth (e_t)		Cross-tension force (F_CT)	Complex desirability
Nr.	[kA]	[per]	[ms]	[dN]	%	µm	[kN]
1	10	18	360	1100	7	210	14.47	0.52
2	12	16	320	1200	10	300	13.88	0.33
3	12	18	360	1100	11	330	21.17	0.43
4	12	20	400	1200	13	390	16.65	0.18
5	12	18	360	1300	12	360	18.72	0.27
6	11	16	320	1100	9	270	13.75	0.36
7	10	18	360	1300	7	210	11.47	0.33
8	11	20	400	1100	10	300	15.67	0.36
9	10	16	320	1200	5	150	11.05	0.41
10	11	18	360	1200	10	300	22.90	0.62
11	11	16	320	1300	9	270	11.90	0.14
12	11	18	360	1200	8	240	18.60	0.61
13	11	20	400	1300	11	330	14.01	0.21
14	10	20	400	1200	9	270	13.70	0.26
15	11	18	360	1200	9	270	18.90	0.61

In the case of the cross-tension test, the joints were destroyed either by tearing in the centreline of the point seam (interface failure) or by tearing the welded nugget (plug failure). Figures 5a-b show an example of each failure mode according to EN ISO 14329 standard [21].

Based on the results of the second series of experiments the optimal welding parameters were determined too (Table 9).

Table 9

Optimal welding parameters based on the cross-tension tests
Technological parameter	Measuring unit	Value
Welding time (t_h)	ms	360
Current (I_h)	kA	10.6
Electrode force (F_e)	daN	1170

The effect of welding technology parameters (t_h [per], I_h [kA], F_e [daN]) on the value of the complex desirability, as well as on the cross-tension force (F_CT [kN]) and the electrode indentation depth (e_t [µm]) is illustrated in Fig. 6.

The results of cross-tension tests for joints with optimal welding parameters detailed in Table 10.

Table 10

Results of testing with optimal welding parameters
Nr.	Current (I_h)	Welding time (t_h)		Electrode force (F_e)	Electrode indentation depth (e_t)		Cross-tension force (F_CT)	Complex desirability	Failure mode
Nr.	[kA]	[per]	[ms]	[dN]	%	µm	[kN]
16/1	10.6	18	360	1170	9.3	279	22.4	0.66	plug failure
16/2	10.6	18	360	1170	9.4	282	22.3	0.66	plug failure
16/3	10.6	18	360	1170	9.9	297	21.8	0.62	plug failure
16/4	10.6	18	360	1170	9.6	288	22.1	0.64	plug failure
16/5	10.6	18	360	1170	9.8	294	21.9	0.62	plug failure
Average:					9.6	288	22.1	0.64
Standard Deviation (σ):					0.2280	6.8410	0.2280	0.0178

In Fig. 7 the microstructure of RSW joint prepared with the determined optimal parameters (cross-tension test specimen No. 16/6) is shown including the base material, heat affected zone (HAZ) and weld nugget.

In terms of the base material martensite and cementite islands embedded in a bainite matrix can be observed. The average grain size is in the range of 5–10 µm, which gradually coarsens in the HAZ. The weld nugget microstructure has a casted, dendritic characteristic. Overall conclusion, that heat input has a significant effect on the microstructure of the welded joint. By integrating the area under the diagrams recorded during the cross-tension test, it is possible to calculate the work absorbed until fracture (Fig. 8).

In the present paper the results of tensile shear and cross-tension tests of resistance spot-welded joints made of Alform® 700 M thermomechanically rolled high strength steel used in the production of railway vehicle structures are summarized. The primary goal was to carry out studies related to the development of optimal resistance spot welding technology for tensile shear and cross-tension loading. The load-bearing capacity of a welded joint made with the specified technological parameters is expected to be as close as possible to the original material while meeting the requirements for railway vehicle structures. The non-equilibrium microstructure of the base material requires a high degree of manufacturing discipline which is a cost-increasing factor during production. The softening of the complex (extra fine) microstructure characteristic of thermomechanically rolled steel can be avoided in the case of short-term, intensive heat input (using a short cycle). Minimizing heat input leads to achieving welded joints that meet the requirements. The following findings can made related to the resistance spot weldability of t = 3 mm thick Alform® 700 M thermomechanically rolled steel considering tensile shear and cross-tension loading:

Since the production technology of thermomechanically rolled steel does not lead to an equilibrium microstructure, it is necessary to limit the amount of heat input when welding these steels to avoid an undesirable decrease in strength properties. Based on the performed experiments the complex (extra fine) microstructure characteristic of thermomechanically rolled steel can be mostly preserved with short-term, intensive heat input, which accounts for the application of short cycle. To weld the joints that meet the quality requirements heat input must be minimized.

For both complex objective functions, we determined the optimal welding parameters: in case of tensile shear test the optimal parameter combination was t_h=380 ms, I_h=10.9 kN, F_e=1100 daN; for cross-tension test t_h=360 ms, I_h=10.6 kN, F_e=1170 daN. The optimal technological parameters were nearly the same for the two loading circumstances, however, in the case of optimization for the cross-tension strength a bit shorter cycle is justified.

During the cross-tension test of the joint made with the optimal technological parameters, the failure occurs at almost 40% lower force than in the case of the tensile shear test, while the electrode indentation (deformation) is approximately twice compared to the optimum of the tensile shear test. Therefore, it is recommended to take the required minimum value of shear force (STS_min [kN]) into account when designing railway vehicles.

When using the optimal technological parameters determined based on the results of the performed series of experiments, the value of the tensile shear force exceeds the minimum value specified by the AWS recommendation D8.1M by more than 22% (with an electrode indentation depth of less than 5%).

Based on these results, the railway vehicle structure can be redesigned for resistance spot welded joint configurations. In conclusion, it is justified to carry out further research (eg: fractographic tests; calculation, comparison and assessment of energy absorbed until fracture; dynamic loading test, etc.) to support the results.

Conflict of Interest

The authors declare that they have no conflict of interest.

Májlinger K, Katula L, Varbai B (2022) Prediction of the Shear Tension Strength of Resistance Spot Welded Thin Steel Sheets from High- to Ultrahigh Strength Range. Periodica Polytech Mech Eng 66(1):67–82. https://doi.org/10.3311/PPme.18934)
Májlinger K, Katula L, Varbai B (2023) Global Approach on the Shear and Cross Tension Strength of Resistance Spot Welded Thin Steel Sheets. Periodica Polytech Mech Eng 67(4). https://doi.org/10.3311/PPme.23184)
Şahin S, Hayat F, Cölgecen OC (2021) The effect of welding current on nugget geometry, microstructure and mechanical properties of TWIP steels in resistance spot welding. Weld World 65:921–935. https://doi.org/10.1007/s40194-021-01083-6)
Al-Mukhtar AM, Doos Q (Volume 2013) The Spot Weldability of Carbon Steel Sheet. Adv Mater Sci Eng. Article ID 146896 http://dx.doi.org/10.1155/2013/146896)
Spena PR, de Maddis M, Lombardi F (2015) Mechanical Strength and Fracture of Resistance Spot Welded Advanced High Strength Steels. Procedia Eng 109:450–456
Den Uijl N, Moolevliet T, Mennes A et al (2012) Performance of Resistance Spot-Welded Joints in Advanced High-Strength Steel in Static and Dynamic Tensile Tests. Weld World 56:51–63. https://doi.org/10.1007/BF03321365)
Alden S, Meilinger Á (2022) Investigation of Resistance Spot Welded Joints Made on Ultra-high-Strength Steel Sheets, Lecture Notes in Mechanical Engineering. Veh Automot Eng 4:981–994
Borhy I (2018) : Possibilities for resistance spot welding of thermomechanically rolled high strength steels in railway vehicle construction, Proc. of 29. Int. Welding Conf. (Miskolc, May 24.-26.), Török I. ed., (ISBN 978-963-358-160-5), 2018, pp. 169.-180 (in Hungarian)
Borhy I, Tóth T (2018) Optimisation of resistance spot welding technology for thermomechanically rolled high strength steels, Hegesztéstechnika (ISSN 1215–8372. 29(4):43–46 (in Hungarian)
EN 15085- (2023) 3:2022 + A1:2023, Railway applications. Welding of railway vehicles and components. Part 3: Design requirements,
AWS D8.1M (2013) : Specification for Automotive Weld Quality – Resistance Spot Welding of Steel, American Welding Society, 2013
Voestalpine (2014) : High-strength and ultra-high-strength thermomechanically rolled fine-grained steels, Alform® high-strength leaflet,
EN 10149-2 (2014) : Hot rolled flat products made of high yield strength steels for cold forming. Part 2: Technical delivery conditions for thermomechanically rolled steels, 2014
Oikawa H, Sakiyama T, Ishikawa T, Murayama G, Takahashi Y (2007) : Resistance Spot Weldability of High Strength Steel (HSS) Sheets for Automobiles, Nippon Steel Technical Report, No. 95,
Varbai B, Sommer C, Szabó M, Tóth T, Májlinger K (2019) Shear tension strength of resistant spot welded ultra high strength steels. Thin-Walled Struct 142:64–73. https://doi.org/10.1016/j.tws.2019.04.051)
Box GE, Wilson KG (1951) On the Experimental Attainment of Optimum Conditions. J Roy Stat Soc 13:1–41
EN ISO 17677- (2021) 1 Resistance welding. Vocabulary. Part 1: Spot, projection and seam welding,
EN ISO 14273 Resistance welding (2016) Destructive testing of welds. Specimen dimensions and procedure for tensile shear testing resistance spot and embossed projection welds,
EN ISO 14272 Resistance Welding (2016) Destructive testing of welds. Specimen dimensions and procedure for cross tension testing of resistance spot and embossed projection welds,
EN ISO 14271 (2017) Resistance Welding – Vickers hardness testing (low-force and microhardness) of resistance spot. projection and seam welds
EN ISO 14329 Resistance welding (2004) Destructive tests of welds. Failure types and geometric measurements for resistance spot, seam and projection welds,

Behaviour of resistance spot welded thermomechanically rolled high strength steels under tensile shear and cross-tension loads

Status:

Version 1

Abstract

Figures

1 Introduction

2 Requirements for welded railway vehicle structures

3 Welding experiments

4 Materials tests

4.1 Tensile shear test

4.2 Cross-tension tests

5 Summary and conclusions

Declarations

Conflict of Interest

References

Status:

Version 1