Influence of process parameters and heat treatment on self-piercing riveting of high-strength steel and die-casting aluminium

This introductory paper is the research content of the paper "Influence of process parameters and heat treatment on self-piercing riveting of high-strength steel and die-casting aluminium" published by Elsevier.

1. Overview:

• Title: Influence of process parameters and heat treatment on self-piercing riveting of high-strength steel and die-casting aluminium
• Author: Chao Wang, Zhanpeng Du, Aiguo Cheng, Zhicheng He, Hailun Tan, Wanyuan Yu
• Publication Year: 2023
• Published Journal/Society: Journal of Materials Research and Technology
• Keywords: Self-piercing riveting, Die-casting aluminium, Cracking mechanism, Joinability, Mechanical response

2. Abstract

The application of steel/die-casting aluminum alloy is an inevitable trend in the development of automotive lightweight technology. The joining process of self-piercing riveting (SPR) is the key technology to guarantee the collision safety for the body. However, the crackings on joint button can be easily found owing to the low ductility of die-casting aluminum. This paper aims to investigate cracking mechanism and improve the SPR joinability by exploring methods of cracking inhibition. Parametric study is performed to explore the effects of heat treatment, process parameters on crack inhibition and forming quality of SPR with steel/die aluminum alloy. The results show that SPR joinability can be improved by larger elongation and lower yield strength by proper heat treatments, i.e., AlSi10MnMg-T6 and AlSi10MnMg-T7. Meanwhile, the depth and diameter of the die are the main factors affecting cracking generation and forming quality. Similar to the upsetting process, tangential tensile stress is generated on the bottom surface in the riveting process, it leads to cracking generation on the bottom surface. This paper further studies the effects of heat treatment and stack direction on joint quality and mechanical response of SPR joints. Tearing failure of the lower sheet is the main factor causing failure of steel-aluminium joint (steel is the top sheet). The heat treatment mainly affects the energy absorption value and has a relatively small effect on the peak force. The mechanical properties of the steel-aluminium joints are superior to those of aluminium-steel joints (aluminium is the top sheet).

3. Research Background:

Background of the research topic:

• Lightweighting is an urgent demand for the automotive industry to achieve energy saving and emission reduction [1].
• Hybrid use of aluminum and high-strength steel in body-in-white (BIW) is a practical method for lightweighting [2,3].
• Die-casting aluminium parts can reduce vehicle weight [2,3].

Status of previous research:

• Resistant spot welding (RSW) is unsuitable for joining steel-aluminum sheets [4].
• Self-piercing riveting (SPR) is a key joining process for aluminum and mixed-material lightweight structures [5].
• Previous studies have investigated the effects of process parameters (sheet thickness, rivet length, die geometry) on SPR joint quality and strength [6,7,8,9,10,11,12,13].
• Cracking in die-casting aluminum bottom sheets is a significant challenge, affecting corrosion resistance and joint strength [14-17].
• Existing methods to reduce cracking include process parameter optimization, adding ductile materials, and new SPR processes (F-SPR, L-SPR, TA-SPR) [15,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34].
• Heat treatment can affect the mechanical properties of casting aluminum alloys [35,36,37].

Need for research:

• Limited research exists on the combined influence of process parameters and heat treatments on cracking and forming quality.
• It's unclear how to select versatile rivets and dies for low-ductility die-casting aluminum.
• Comprehensive consideration of the combined effects of process parameters and heat treatment is lacking.
• The impacts of both heat-treated and non-heat-treated materials need investigation.

4. Research purpose and research question:

Research purpose:

• To investigate the cracking mechanism in SPR of steel/die-casting aluminum.
• To improve SPR joinability by exploring methods of cracking inhibition.
• To solve the crackings for die-casting aluminium part using suitable process parameters and heat treatment.
• To explore the effect of heat treatment and stack direction on mechanical response.

Core research:

• Effects of rivet length, die diameter, die depth, and heat treatments on joint quality.
• Analysis of cracking generation mechanisms using microscopy.
• Effects of heat treatment and stack direction on the mechanical response of SPR joints.

5. Research methodology

• Research Design: Parametric study, experimental investigation.
• Materials: High-strength steel HC340/590DP (1.6 mm thick), die-casting aluminum AlSi10MnMg and JDA1b (3.0 mm thick). Chemical compositions are detailed in Table 1. Material properties are in Table 3.
• Heat Treatments: Three heat treatment conditions for AlSi10MnMg (T5, T6, T7) were used, as listed in Table 2.
• Riveting Details: Servo SPR system (EPRESS GmbH), load-control mode, 80 kN max riveting force, 100 mm/s riveting speed. Semi-tubular rivets made of boron steel (H5). Three die types (flat die I, flat die II, pip die) with varying parameters (Table 5, Fig. 2).
• Experimental Setup: Aluminum as the bottom sheet. Detailed analysis scheme in Table 4.
• Data Collection:
  • Riveting experiments with varying parameters (rivet length, die diameter, die depth, heat treatment).
  • Metallurgical microscopy and scanning electron microscopy (SEM) analysis.
  • Quasi-static tests (shear, peeling, cross-tension) using a universal testing machine (ETM105D) at 3 mm/min.
• Analysis Method:
  • Joint quality evaluation: Forming quality (height of rivet head, undercut, bottom thickness, remaining thickness - Fig. 3(a), Table 6) and appearance quality (cracking grades - Fig. 4).
  • Joinability matrixes (Fig. 6).
  • Analysis of cracking mechanisms.
  • Comparison of mechanical properties (peak force, energy absorption).

6. Key research results:

Key research results and presented data analysis:

• Joinability: SPR joinability can be improved by larger elongation and lower yield strength through heat treatments (AlSi10MnMg-T6, AlSi10MnMg-T7) (Fig. 8).
• Die Depth and Diameter: The depth and diameter of the die significantly affect cracking generation and forming quality (Fig. 9). Larger depth and diameter improve forming quality but increase cracking risk.
• Rivet Length: Rivet length has a relatively small influence on cracking generation. Proper rivet length (6.0-6.5 mm) can improve forming quality.
• Cracking Mechanism: Tangential tensile stress generated on the bottom surface during riveting leads to cracking (Fig. 11, Fig. 12, Fig. 13, Fig. 14, Fig. 15).
• Stack Direction: Steel-aluminum joints (steel top, aluminum bottom) have superior mechanical properties compared to aluminum-steel joints (Fig. 20).
• Heat Treatment Effects: Heat treatment mainly affects energy absorption, with a smaller effect on peak force (Fig. 20).
• Failure Mode: Tearing failure of the lower sheet is the main factor causing failure of steel-aluminium joint (steel is the top sheet).

List of figure names:

• Fig. 1 Self-piercing riveting equipment.
• Fig. 2 Appearance and dimension parameters of the rivets and dies.
• Fig. 3 Schematic illustration of forming quality.
• Fig. 4 Schematic illustration of appearance quality.
• Fig. 5 Geometric dimensions of SPR joints and quasi-static test.
• Fig. 6 Joint quality with varied rivet and die combinations.
• Fig. 7 The central cross-sectional profile of SPR joints of S4#.
• Fig. 8 Joint quality with varied elongation and yield stress.
• Fig. 9 Effect of parameters of the rivet and die on joint quality.
• Fig. 10 Selection of dimensional parameters of the die: (a) depth and (b) diameter.
• Fig. 11 Characteristics of the two types of cracks on bottom sheet.
• Fig. 12 Three regions based on the degree of deformation.
• Fig. 13 Metallographic graphs of the cross-section of the SPR joint.
• Fig. 14 Cracking characteristics of three types dies.
• Fig. 15 Schematic illustration of bottom surface cracking: (a) piercing (b) flaring, and (c) danger zone and safety zone.
• Fig. 16 The cross-section profile, appearance quality, and forming quality of SPR joints (unit: mm).
• Fig. 17 Static failure modes for the SPR joints under quasi-static tests.
• Fig. 18 The failure mechanism and micromorphology of the joints: (a) shear joints, (b) peeling joints, (c) cross-tension joints, (d) macro fracture, (e) magnified view of joint J2#, and (f) magnified view of joint J4#.
• Fig. 19 Bottom sheet fracture of the cross-tension test: (a) macro fracture, (b) magnified area of joint J1#, (c) magnified area of joint J2#, and (d) magnified area of joint J5#.
• Fig. 20 The mechanical properties of SPR joints with different heat treatments and stack direction.

7. Conclusion:

Summary of key findings:

• SPR joinability is improved by larger elongation and lower yield strength achieved through heat treatment.
• Die depth and diameter are key factors influencing cracking and forming quality.
• Cracking is caused by tangential tensile stress on the bottom surface.
• Steel-aluminum joints have better mechanical properties than aluminum-steel joints.
• Heat treatment primarily affects energy absorption, not peak force.
• Tearing failure of the lower sheet is the main factor causing failure of steel-aluminium joint (steel is the top sheet).

The research provides a detailed understanding of the cracking mechanism in SPR of steel/die-casting aluminum joints.

It offers practical guidance for selecting appropriate process parameters and heat treatments to improve joinability and prevent cracking.

The findings contribute to the advancement of lightweight automotive manufacturing by enabling reliable joining of steel and die-casting aluminum.

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9. Copyright:

• This material is a paper by "Chao Wang, Zhanpeng Du, Aiguo Cheng, Zhicheng He, Hailun Tan, Wanyuan Yu": Based on "Influence of process parameters and heat treatment on self-piercing riveting of high-strength steel and die-casting aluminium".
• Source of paper: https://doi.org/10.1016/j.jmrt.2023.09.187

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