Effects of Rivet and Die Geometries on Quality Factors and Mechanical Properties of SPR Joints with Dissimilar Material Combinations

Hee-Seon Bang; Seon-Myoung Hong; Dong-Won Choi

doi:10.5781/JWJ.2025.43.3.9

Abstract

This study investigated the effects of rivet and die shapes in Self-Piercing Riveting (SPR) joints on quality characteristics (head height, interlock, bottom thickness) and tensile-shear load for various combinations of aluminum alloys and steel sheets aimed at automotive lightweighting. Materials used included SGACC, SABC1470, SGAFC590, aluminum alloy A6014P-T4F, cast aluminum A356, and extruded aluminum A6N01. Three rivet types (C-type, HD2-type, HDZ-type) and four die shapes (FM, KA, DZ, SM) were employed. Surface and cross-sectional analyses revealed that SM dies were optimal for joints with aluminum bottom sheets, while FM dies were preferable for steel bottom sheets. Among rivet types, HDZ-type exhibited the best joining performance across all conditions. Tensile-shear tests were conducted under a non-standard double shear condition. In most cases, tearing failure occurred in the base materials, while in cases with steel as the bottom sheet, tearing was accompanied by rivet leg detachment. Analysis using an artificial intelligence prediction model (Xception) showed that the interlock exhibited the highest correlation with tensile-shear load, followed by head height and bottom thickness in terms of prediction accuracy.

Key words: Dissimilar materials, Multi-layer, Self-Piercing Rivet(SPR), Deep learning, Quality factors

1. Introduction

Interest in vehicle weight reduction has surged in the automobile industry as it faces strengthened environmental regulations, demand for higher fuel efficiency, and global policy changes to reduce carbon emissions. In particular, expanded dissemination of electric vehicles (EVs) and hybrid vehicles has further highlighted the importance of weight reduction technology to secure mileage¹^,²⁾.

To respond to this demand for weight reduction, the method of mixing and using various materials (e.g., aluminum alloys, high-strength steel, and composite materials) beyond the existing steel plate-oriented structure has been expanded. Conventional welding methods, however, have limitations in joining between dissimilar materials with different physical and mechanical properties. For instance, aluminum alloys and high-strength steels have different thermal properties, and the formation of intermetallic compounds may degrade weld strength. Therefore, there is growing interest in mechanical joining technology that can consider both cost efficiency and process stability, and the self-piercing riveting (SPR) method has attracted attention as a representative alternative technology³⁾.

The SPR process inserts rivets through two or more plates at the same time without the pre-hole process. It can ensure excellent mechanical strength and durability especially for joining between non-ferrous metals, such as aluminum⁴⁾. It is also applicable to composite materials with limited heat input, such as CFRP, because it is a mechanical joining method that does not use heat. Accordingly, the automobile industry has introduced SPR technology as a strategy to achieve both vehicle weight reduction and the efficiency of production processes, and it has been applied to various parts, including the lower body frame, doors, and hood⁵^,⁶⁾.

For the SPR process, the quality factors of the joints (head height, Interlock, and bottom thickness) vary depending on rivet and die geometries as well as material combinations, which is closely related to the mechanical properties of the joints, especially tensile-shear loads⁷⁾. Therefore, it is important to systematically analyze the effects of rivet and die geometries as well as material combinations on the quality factors and mechanical properties of the joints⁸⁾.

This study experimentally investigated the effects of rivet and die geometries on the quality factors of SPR joints with steel and Al material combinations, which are applied for vehicle weight reduction, and the tensile-shear loads of single and double shear structures. In addition, an artificial neural network model was constructed based on the derived quality factor data to determine the main quality factors closely related to the tensile-shear loads of the joints.

2. Research Method

This study aimed to compare SPR joining performance for rivet and die geometries by material combination and identify the relationship between quality factors and tensile-shear loads.

2.1 Materials used and joining conditions

To examine SPR joining characteristics for various material combinations, SGACC (1.2 mm), SABC1470 (1.2 mm), SGAFC590 (2 mm), aluminum alloy A6014P-T4F (1.4 mm), aluminum casting alloy A356 (3 mm), and aluminum extrusion A6N01 (4 mm) were used as listed in Table 1. The rivets and dies used are listed in Table 2. For rivets, the following three types were used: C-type with a sharp end, HD2-type with a relatively rounded end, and HDZ-type with a rivet body-to-leg ratio of 1:1. In the case of dies, four types were applied: the FM die with the flat bottom, the DZ die with a conical shape in the center, the SM die with the rounded dome-shaped bottom, and the KA die with the bottom protruded in the form of a ring. The pressure was selected by measuring the head height after SPR fastening and determining whether the quality factor criteria were met and there were cracks in the bottom sheet.

Table 1

Material combinations by layer position

Case	Top	Middle	Bottom
1	SGACC	SABC1470	A365
1	1.2 mm	1.2 mm	3 mm
2	SABC1470	-	A6N01
2	1.2 mm	-	4 mm
3	SABC1470	SGAFC590	A6N01
3	1.2 mm	0.7 mm	4 mm
4	SGACC	-	A6N01
4	1.2 mm	-	4 mm
5	SGACC	SGAFC590	A6N01
5	1.2 mm	1 mm	4 mm
6	A6014P-T4F	SGAFC590	SGAFC590
6	1.4 mm	1.4 mm	2 mm
7	A6014P-T4F	SABC1470	A365
7	1.4 mm	1.5 mm	3 mm

Table 2

Types of dies and rivets used in process

Type		Specification
Die	FM	095 0 020
		095 0 024
		095 2 028
		100 2 018
		100 2 220
	KA	110 2 115
		100 2 022
		100 2 125
	DZ	110 2 025
	DZ	1110 2 050
	SM	095 0 190
		120 0 312
		120 0 058
		100 0 085
		110 0 353
Rivet	C	7 mm, 7.5 mm, 8 mm
	HD2	7 mm, 8 mm
	HDZ	6 mm, 7 mm

2.2 Joint quality evaluation method

Exterior characteristics according to the rivet and die geometries for each material combination were examined in terms of the presence or absence of cracks in the bottom sheet and the head height, one of the quality factors.

To investigate the cross-sectional characteristics according to the rivet and die geometries, the quality factors (head height, interlock, and bottom thickness), separation (a phenomenon that the fastening capability decreases due to the minimal spreading of the rivet leg during rivet fastening), buckling (a phenomenon that the rivet leg is bent), and cavity (empty space not filled with the material) were mainly examined.

To evaluate the joining strength for each material combination, the tensile-shear load test was conducted in accordance with the ASTM D 1002/5868 standards. The specimens were prepared in a size of 100 mm (L) * 25 mm (W) and joined with an overlap of 25 mm. The test speed was set to 10 mm/min. In addition, the SPR process is a single shear structure in which the rivet leg is combined with the bottom sheet to support the load, but an attempt was made to examine the rupture behavior under double shear conditions to simulate situations where the middle sheet is fixed or acts as the central position of load transfer in the actual structure. Therefore, the three-layer combination was performed under double shear conditions as shown in Fig. 1(b). The test results need to be analyzed separately from the SPR joining characteristics of the single shear structure.

Fig. 1

Schematic diagram of tensile-shear test specimen, (a) Single shear, (b) Double shear

2.3 Data preprocessing and artificial intelligence model

A deep learning model was constructed to identify factors that are most closely related to the combined shear load transfer capacity of SPR joints based on the quality factor data as shown in Fig. 2. The model used the “Xception” model, which utilizes a convolutional neural network⁹⁾. A total of 1,120 data were used by augmenting the data under 14 conditions. The quality factors (head height, interlock, and bottom thickness) for each material combination were used as input data, and the output data were the tensile-shear loads of the double shear structure. The entire data were divided into 70% for training, 15% for validation, and 15% for testing to perform predictions.

Fig. 2

Schematic of strength prediction model using quality parameters

3. Research Results

3.1 Joint appearance by material combination

Table 3 and 4 summarize the bottom of SPR joints according to the die and rivet geometries for each material combination. When the bottom sheet was steel, the FM die with flat die geometry exhibited the most excellent joint appearance. The KA, DZ, and SM dies showed no crack in the bottom sheet, but joining failed and buckling occurred. This appears to be because plastic deformation is difficult in the complex die geometry or the curved dome shape as steel has lower elongation and higher strength than aluminum due to its properties.

Table 3

Surface characteristics of cracked SPR joints by material combinations

Case	Bottom side
1
2
3
4
5
6
7

Table 4

Sound surface characteristics of SPR joints by material combinations

Case	Bottom side
Case	SM 110 0 353	SM 095 0 190
1
2
3
4
5
7
6	FM 100 2 220
6

When the bottom sheet was aluminum casting alloy and extrusion, the use of the dome-shaped SM die exhibited the most excellent joint appearance. For the FM die, the casting alloy and extrusion at the bottom stuck to the die after fastening, resulting in the tearing phenomenon. This seems to be because the bottom shape of the die is flat and the contact point where the sides and bottom of the die meet has a rectangular shape. In the case of the DZ and KA dies, cracks occurred in the bottom sheet. This appears to be because the bottom sheet thickness was higher than the depth of the die and the bottom sheet could not secure the volume required for deformation during plastic deformation.

The HDZ-type rivet secured excellent joint appearance. The rivet appears to have excellent high-strength material joining capacity due to the high rivet strength and the rivet body-to-leg ratio of 1:1.

When the C-type and HD2-type rivets were used, cracks occurred in the bottom sheet because the rivet body-to-leg ratio was longer than that of other rivets. As the rivet strength was low, joining failed and buckling occurred for high-strength material combinations, such as SABC1470. This indicates that the geometry and strength of the C-type and HD2-type rivets lead to somewhat low joining capacity for high-strength steel material combinations, making them inadequate for high-strength steel material joining¹⁰⁾.

3.2 Cross-section and quality factors

Cross-sectional evaluation was performed for the specimens of the die (SM 110 0 353, SM 095 0 190, and FM 100 2 220) and HDZ-type rivet conditions for each material combination that met the criteria during exterior evaluation. The cross-sections according to material combinations and die geometry are shown in Table 5 while the measurements of the quality factors are summarized in Table 6.

Table 5

Cross-sections according to material combinations and die geometry

Table 6

Quality factors measurements according to material combinations

Case	Head height (mm)	Interlock (mm)		Bottom thickness (mm)
1	0.21	a₁	0.25	1.41
1	0.21	a₂	0.31	1.41
2	0.22	a₁	0.32	1.76
2	0.22	a₂	0.29	1.76
3	0.10	a₁	1.03	2.33
3	0.10	a₂	0.86	2.33
4	0.04	a₁	0.26	1.35
4	0.04	a₂	0.26	1.35
5	0.27	a₁	0.38	1.90
5	0.27	a₂	0.37	1.90
6	0.07	a₁	0.04	0.72
6	0.07	a₂	0.04	0.72
7	0.26	a₁	0.16	1.06
7	0.26	a₂	0.25	1.06
Criteria	-0.2≤K≤0.3	a_1,2≥0.15		t≥0.2

For cases 1, 2, 4, no cracks or pores were observed from the SM 095 0 190 die, resulting in excellent cross-sectional characteristics. In the case of the SM 110 0 353 die, however, it was not filled with the aluminum material and pores occurred. This appears to be because the spatial volume of the die was larger despite the same dome shape¹¹⁾.

For case 6 in which the bottom sheet is steel, the spreading angle of the rivet was similar to that of other material combinations, but the length of the interlock could not meet a quality factor criterion of 0.15 mm. This appears to be because the strength of the bottom sheet was high compared to other combinations and the bottom sheet thickness was low compared to other combinations¹²⁾.

For case 3, the strength of the top and middle sheets was high compared to other material combinations, but excellent cross-sections were observed from the SM 110 0 353 and SM 095 0 190 dies. This indicates that the formation of the interlock is most closely related to the bottom sheet among the top, middle, and bottom sheets¹²^-¹⁴⁾.

3.3 Tensile-shear load characteristics

When the tensile-shear load characteristics of single and double shear structures were examined under the rivet and die geometry conditions that exhibited excellent characteristics in the cross-sectional and quality factor evaluation, they were found to be similar to the maximum tensile-shear load values of the fractured base metal for each material combination as shown in Fig. 3. This indicates that single shear and double shear structures also have sufficient load bearing capacity.

Fig. 3

Tensile-shear loads of SPR joints for various material combinations

The fracture results of the tensile-shear specimens are presented in Table 7. Fracture occurred in the base metal under all conditions except for case 6. For case 6, the fracture of the top sheet base metal and rivet leg separation occurred. This appears to be because the load was concentrated on the interlock due to the fracture of the top sheet material and the length of the interlock was short.

Table 7

Fractured surfaces of SPR joints after tensileshear test

Case	Fractured surface
1
2
3
4
5
6
7

3.4 Load transfer capacity characteristics by quality factor

The artificial intelligence model training results are shown in Fig. 4 and 5. It was determined that overfitting was prevented because the training and validation data converged to zero. Among the quality factors, the interlock exhibited the highest coefficient of determination (R²) of 0.99, followed by the heat height (0.945). The bottom thickness showed a relatively low value of 0.886.

Fig. 4

Loss curves for training and validation data

Fig. 5

Predicted tensile-shear loads based on SPR quality factors, (a) Bottom thickness, (b) Head height, (c) Interlock

This indicates that the interlock and head height can be used as important control factors in the SPR joint quality evaluation and design stages. In particular, the interlock’s high prediction accuracy shows that it is a valid quality factor for process optimization and quality control even under single shear and double shear conditions. It is also believed that the use of the bottom thickness factor led to the lowest predicted value because predictions were affected by the bottom sheet thickness.

4. Conclusions

This study investigated the characteristics of self- piercing riveting (SPR) joints through quality factors and tensile-shear strength evaluation according to rivet and die geometries for each dissimilar material combination, and the following conclusions were drawn.

1) When the joint appearance and cross-sectional geometry were analyzed according to the die geometry, the most excellent appearance and cross-section were observed from the dome-shaped SM type die for the aluminum bottom sheet and from the flat FM type die for the steel bottom sheet.
2) In the case of rivets, the HDZ-type rivet exhibited excellent appearance and cross-section for all material combinations. For the C-type and HD2-type rivets, cracks occurred in the bottom sheet and joining failed.
3) In the tensile-shear test results for single shear and double shear structures by material combination, base metal fracture occurred in most of the combinations, but top sheet base metal fracture and rivet leg separation occurred in the combinations with the steel bottom sheet.
4) When the correlations between the quality factors and load transfer capacity of SPR joints were examined, the interlock exhibited the highest coefficient of determination (0.99) under single and double shear conditions, followed by the head height (0.945). This confirmed their applicability as the key factors of quality prediction models in the future. These results, however, were focused on joining performance and quality sensitivity analysis under the same conditions, and have limitations in generalization under single shear conditions.