Automotive Innovation

, Volume 1, Issue 1, pp 70–75 | Cite as

Recycle-Friendly Aluminum Alloy Sheets for Automotive Applications Based on Hemming

  • Hongzhou Lu
  • Junping Zhang
  • Ni Tian
  • Xinli Song
  • Mingtu Ma
  • Guimin Lu
Article
  • 106 Downloads

Abstract

Unavoidably, Fe impurities are mixed into Al alloys during recycling of aluminum automotive parts. High Fe content recycle-friendly aluminum alloy sheets (RASs) are discussed in terms of their applications to car body parts and closures. RASs have advantages, such as they are low cost and light weight, they require less energy to process, and they reduce emissions when used in vehicles. The hemming ability of RASs was investigated through various quenching rates and hot-rolling reduction rates. The hemming factor (HF) values of low Fe content samples (0.1 and 0.3 wt% Fe content) were 2 and 3. The hemming ability of samples with an Fe content of 0.5 wt% was unacceptable for applications to automotive outer closures (i.e., HF values of 4 and 5). The HF values changed from 3 to 5 for even higher Fe content samples (0.8 wt% Fe), depending on the quenching and hot-rolling reduction rates. Lower quenching (air quenching) and lower hot-rolling reduction rates (6-pass hot rolling) both improved the hemming ability of the high Fe content samples. A low-cost and recycle-friendly aluminum alloy automotive sheet from end-of-life vehicles is presented, and its hemming properties are characterized.

Keywords

Recycle-friendly Lightweight Hemming Aluminum alloy Automotive sheets End-of-life vehicle 

Abbreviations

RASs

Recycle-friendly aluminum alloy sheets

ELVs

End-of-life vehicles

HF

Hemming factor

MPD

Micrometer-sized precipitate density

GBPD

Grain boundary precipitate density

PFZ

Precipitation-free zone

AQ

Air quenching

WQ

Water quenching

1 Introduction

Applications of aluminum alloy sheets in car bodies have increased, owing to the light weight of these materials, which contributes to improved energy savings and reduced emissions [1]. However, the manufacturability and high cost of aluminum alloy sheets limit their applications in aluminum car bodies and closures. Hence, the development of recycle-friendly automotive aluminum alloys is necessary [2], to reduce costs and energy consumption, as reported by Choate and Green [3]. In these cases, secondary metal produced from recycled products saves 93% of energy consumption compared with that required for primary aluminum production. However, the Fe content of scrap aluminum alloys can increase to unacceptable levels, owing to the iron pickup effect [4]. Wrought aluminum alloy compositions of recycled metals have been studied by Das and Gesing [2, 5], who indicated that recycled aluminum alloys typically have an Fe content of \(\sim \)1.00  wt%. A high Fe content is considered to reduce the formability of aluminum alloys [4, 6, 7]. Hemming is an important forming process for manufacture of lightweight aluminum alloy closures, where the final step in the forming route of a body closure is joining of the outer panel to the inner panel through a hemming operation. This joining method is cheap, easy to perform, and environmentally friendly; however, it requires severe \(180^{\circ }\) bending of the edges of the outer panel, which quite often results in cracking or complete tearing of the bent surface. The hemming properties of aluminum alloy automotive sheets with various Fe levels have been investigated [4, 8, 9, 10, 11]; however, the development of recycle-friendly automotive aluminum alloys with high Fe content has yet to be reported. Therefore, low cost [3] and recycle-friendly automotive aluminum alloy sheets with suitable hemming properties would be highly desirable.
Table 1

Chemical composition of aluminum alloys with various contents of Fe, wt%

Materials

Si

Mg

Fe

Mn

Al

0.1 wt% Fe-bearing alloy

1.05

0.54

0.10

0.069

Rest

0.3 wt% Fe-bearing alloy

1.04

0.50

0.30

0.070

Rest

0.5 wt% Fe-bearing alloy

0.97

0.56

0.50

0.071

Rest

0.8 wt% Fe-bearing alloy

0.99

0.56

0.80

0.071

Rest

2 Experimental Procedure

2.1 Chemical Composition and Observations

Four AA6022-type automotive aluminum alloys of 0.1–0.8 wt% in Fe were studied, as presented in Table 1. Before hot forging of the casting slabs, hot rolling from 40 to 4 mm was performed at the State Key Laboratory of Rolling & Automation, based on previous investigations [12]. Two hot-rolling operations were applied as follows: The reduction rates per pass for 6 passes were 20, 22, 28, 33, 42, and 43%, whereas the reduction rates per pass of 4 passes were 25, 33, 50, and 60%. We also conducted 6-pass cold rolling to 1 mm for the thick sheets. Subsequently, the samples were heat-treated to the T4P temper state through several consecutive steps, such as a solution treatment, quenching, and pre-aging steps. The samples were soaked for 30 min at the solution temperature of \(550^{\circ }\)C, followed by air quenching (AQ) or water quenching (WQ) at \(25^{\circ }\)C. The samples were pre-aged for \(100^{\circ }\)C \(\times \) 30 min immediately after quenching. To clarify the effects of Fe, other element contents were limited in the corresponding elemental composition, including Cu, Mn, and Cr.

The density of submicrometer grain boundaries of micrometer-sized precipitates was analyzed with a scanning electron microscope (Nano Nova SEM400), for samples electro-polished in a 4% perchloric acid solution. Quantitative analysis of the micrometer-sized second-phase particles was performed in the multiple observation fields with Image-Pro Plus software. The particle numbers per unit length of the grain boundary were used to characterize the grain boundary particle density. Precipitation-free zones and dislocations were characterized through transmission electron microscopy (TEM) with a JEM-2100 microscope, operating at 200 kV. Thin foils for TEM observation were produced from the mechanically polished films through electro-polishing in a twin-jet apparatus. The electrolyte used for the electro-polishing consisted of 10% \(\hbox {HNO}_{3}\) and 90% methanol. The controlled polishing parameters were: a voltage of 20 V and temperature of \(-20\,^{\circ }\)C.

2.2 Hemming Testing

Hemming testing of the recycle-friendly automotive aluminum alloy sheets (RASs) was performed to evaluate the corresponding hemming ability. Pre-stretching was performed on the samples prior to hemming testing, with pre-strains of 0, 2, 5, and 10%. The JIS H7701-2008 standard method for the hemming test of high-strength aluminum alloy sheets for automotive use was used. The hemming of a rectangular sample 50 mm \(\times \) 30 mm in size was fully folded through insertion of an intermediate sheet, with a bend line (BL) parallel to the rolling direction (RD) of the sheets. The bending test procedure was applied in two steps: 150\(^{\circ }\)–170\(^{\circ }\) and final pressing to \(180^{\circ }\). Two backup rolls and a punch pin were used during testing. The diameter of the backup rolls was 30 mm, and the radius of the punch pin was 0.6 mm. The surface roughness Ra of the backup rolls and the punch pin was below 0.8 \(\upmu \)m, as shown in Fig. 1. Moreover, the distance of backup rolls dr was 8 mm and the initial step pressing speed was 50 mm/min. For the final pressing to \(180^{\circ }\), the thickness of the inserted intermediate sheet was 1.2 mm and the final pressing speed was set to be 5 mm/min. The comparative hemming factor (HF) ranges from 1 (very good hemming ability) to 5 (very bad hemming ability). This parameter was used to scale the hemming quality of the tested sheets.
Fig. 1

Schematic diagram of RAS hemming test

Fig. 2

Bending surface of most deteriorated cracking samples for each alloy, under a air quenching and 6-pass hot rolling, b water quenching and 6-pass hot rolling, c air quenching, and 4-pass hot rolling, and d water quenching and 4-pass hot rolling

3 Hemming Testing Results

Figure 2 presents the bending surface morphology of the most deteriorated cracked samples for each alloy, under various quenching and pass hot-rolling processes. Distinct cracks appeared in all bending surfaces of the 0.5 wt% Fe-bearing alloy samples. Except for micro-cracks in Fig. 2a, c, no distinct cracks were visible for the 0.1 and 0.3 wt% Fe-bearing alloy samples. For the 0.8 wt% Fe-bearing alloy samples, distinct cracks appeared in the samples for which WQ was used, as observed in Fig. 2b, d; however, no cracks were visible in the samples for which the AQ and 6-pass hot-rolling methods were used, as shown in Fig. 2a.

According to the JIS H7701-2008 standard and the observations from Fig. 2, the HF values of the RASs with various quenching rates and hot-rolling reduction rates were measured and the results are presented in Fig. 3. The hemming ability of the samples with 0.5 wt% of Fe was unacceptable for applications to automotive outer closures (i.e., HF values of 4 and 5). The HF values of the low Fe content samples (i.e., 0.1 and 0.3 wt% Fe content) were 2 and 3; nevertheless, it was noted that the scale of HF values changed from 3 to 5 for high Fe content samples (i.e., 0.8 wt% Fe), depending on the quenching and hot-rolling reduction rates. The lower quenching (AQ) and lower hot-rolling reduction (6-pass hot rolling) rates improved the hemming ability of high Fe content samples.
Fig. 3

Hemming factors of RASs with various quenching and hot-rolling reduction rates

4 Mechanistic Discussion

The bendability difference of various iron content rass is affected mainly by grain size [13], grain orientation [14, 15], micrometer-sized intermetallic particles [8], shear bands [16], grain boundary particle density [17], and precipitation-free zones [18] (short for PFZs) adjacent to the grain boundaries. Quenching and hot-rolling reduction rates affected the hemming ability, as shown in Figs. 2 and 3. The quenching rate can affect the grain boundary particle density [17], whereas a lower quenching rate led to higher amounts of grain boundary particles and formation of PFZs. According to ref. [9], the hemming fracture mechanism might transform from a strain-localization-induced fracture mechanism [19] to a grain boundary particle PFZs-induced fracture mechanism [20]. At a higher quenching rate, the hemming fracture of Al alloy sheets was directly governed by micro-voids adjacent to micrometer-sized second-phase particles and shear bands [21]. The amounts of the AlFeSi micrometer-sized second-phase particles increased together with the iron content, as observed in Fig. 4. The micrometer-sized precipitate densities increased linearly along with the Fe content, reaching 12,000/mm\(^{2}\) at and Fe content of 0.8 wt%, whereas a high amount of crack initiation sources and crack propagation sources led to cracking. A higher quenching rate could lead to a greater amount of dislocations and dislocation tangle formations, as shown in Fig. 5c. The strain hardening increased as the dislocation density increased [22], according to Ref. [23]. The dislocation tangles induced the occurrence of inhomogeneous deformation in local areas and promoted formation of shear bands. Consequently, for high Fe content aluminum alloy sheets (such as 0.5 and 0.8 wt% Fe-bearing alloy), the higher micrometer-sized precipitate densities and dislocations (dislocation tangles) induced hemming cracking. At low quenching rates, the hemming fracture of the aluminum alloy sheets was directly governed by voids around the submicrometer-sized grain boundary particles and PFZs [20]. From Fig. 4, the grain boundary precipitate densities (GBPD) of the 0.8 wt% Fe-bearing alloy were lower than the GBPD of the other three alloys. This phenomenon might be attributed to silicon loss, consumed by the AlFeSi particles. Measurable grain boundary PFZs were observed in samples formed with lower quenching rates (air quenching), as observed in Fig. 5a, b. The low GBPD might contribute to the hemming ability of the 0.8 wt% Fe-bearing alloy.
Fig. 4

Micrometer-sized precipitate densities (MPD) and grain boundary submicrometer-sized precipitate densities (GBPD) together with iron content, determined from SEM observations

Fig. 5

Dislocation distributions in various samples, determined by TEM: a air quenching and 6-pass hot rolling; b air quenching and 4-pass hot rolling; c water quenching and 6-pass hot rolling

From Fig. 5, we observed that there were more dislocation density and dislocation tangles in Fig. 5b than in Fig. 5a. This phenomenon could be explained by the fact that higher hot-rolling reduction rates could lead to a higher dislocation density, as previously discussed. Higher amounts of dislocations induced additional shear bands. In this case, higher hot-rolling reduction rates damaged the hemming ability of the automotive aluminum alloy sheets.

The hemming properties of the aluminum alloy automotive sheets with various iron levels are summarized, together with results from Refs. [4, 8, 9, 10, 11] in Fig. 6. The bendability of the AA6111 aluminum alloy was investigated as a function of Fe content ranging from 0.06 to 0.68 wt%. Before forced-water quenching [10], the low-iron alloy (0.06 wt% Fe) could be bent fully, whereas it featured notable grooves on the surface linked with the shear bands. The high-iron alloy (0.68 wt% Fe) developed cracks when it was subjected to hemming. The AA5754 automotive sheet bendability was investigated as a function of Fe content (0.08 and 0.30 wt%). Before the forced-water quenching [4], the results demonstrated that the high Fe content alloy exhibited damage at the Fe-rich particles, which led to a considerable reduction in the bendability. Lievers et al. [11] studied the effects of Fe content on the bendability of three Fe variants of AA6111 sheets (including 0.06, 0.26, and 0.68 wt% in Fe) that were forced-air-quenched. The bendability was worst for an Fe content of 0.68 wt% and best for 0.26 wt%. The effects of the Fe content values (i.e., 0.1, 0.5, 0.8, and 1.0 wt%) on the mechanical properties of Al–1.0 wt% Si–0.5 wt% Mg–0.1 wt% Mn alloy T4 sheets were investigated [8], after forced-air quenching. The bendability was the worst for an Fe content of 0.5 wt%, whereas in samples with Fe content greater than 0.5 wt%, bendability improved or remained the same. This conclusion was similar to the findings of this work. Davidkov et al. [9] studied the bendability of two AA6016-type aluminum alloy sheets with various Fe contents (0.1 and 0.23 wt%), soaked for 55 and 5 s at the same solution temperature. For the samples with an Fe content of 0.23 wt% a shorter quenching was applied, which led to relatively good hemming properties. The low Fe content alloy (0.1 wt% Fe) developed cracks when it was subjected to hemming. Thus, on the basis of the results for the 6\(\times \times \times \) series alloys from Fig. 6 and those from this work, we deduce that:
Fig. 6

Estimated hemming properties of aluminum alloy sheets with various Fe contents

  1. (i)

    At the same quenching rate, with Fe content ranging from 0.06 to 0.3 wt%, the hemming properties of the alloy sheets with high Fe content were higher than those of alloy sheets with lower Fe content;

     
  2. (ii)

    Over the Fe content range from 0.5 to 1.0 wt%, the hemming properties were poorest for an Fe content of 0.5 wt%, but improved at 0.8 wt%;

     
  3. (iii)

    Air quenching was necessary to improve the hemming properties of the 0.8 wt% Fe content alloy sheets.

     
However, the air quenching rate must be controlled [17], at lower quenching rates (\(< 24^{\circ }\hbox {C} /\hbox {s}\)), because many large grain boundary particles formed, which induced bending cracking.

5 Conclusions

The hemming ability of samples with an Fe content of 0.5 wt% was unacceptable for applications to automotive outer closures, (i.e., HF values of 4 and 5). The HF values ranged from 3 to 5 in high Fe content samples (0.8 wt% Fe), depending on the quenching and hot-rolling reduction rates. Lower quenching (AQ) and lower hot-rolling reduction (6-pass hot rolling) rates improved the hemming ability of the 0.8 wt% Fe content samples. According to the obtained experimental results in this study and the corresponding discussion, low cost and recycle-friendly aluminum alloy automotive sheets from ELVs could be designed and characterized based on their hemming properties. The following further conclusions could be drawn:
  • The acceptable and optimum Fe content was approximately 0.8 wt% for the recycle-friendly automotive aluminum alloy sheets.

  • A low quenching rate should be applied to recycle-friendly automotive aluminum alloy sheets, and the quenching rate should be controlled within a reasonable range.

  • To reduce the dislocation density and shear bands, low hot-rolling reduction rates should be applied to recycle-friendly automotive aluminum alloy sheets.

Notes

Acknowledgements

The authors would like to acknowledge the financial contribution from the National Science and Technology Pillar Program during the 12th Five-year Plan Period (Grant No. 2011BAG03B00).

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Copyright information

© Society of Automotive Engineers of China (SAE-China) 2018

Authors and Affiliations

  • Hongzhou Lu
    • 1
  • Junping Zhang
    • 2
  • Ni Tian
    • 3
  • Xinli Song
    • 4
  • Mingtu Ma
    • 2
  • Guimin Lu
    • 1
  1. 1.School of Resources and Environmental EngineeringEast China University of Science and TechnologyShanghaiChina
  2. 2.China Automotive Engineering Research InstituteChongqingChina
  3. 3.State Key Laboratory of Rolling and AutomationNortheastern UniversityShenyangChina
  4. 4.Wuhan University of Science and TechnologyWuhanChina

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