TMMOB Metalurj i ve Malzeme Mühendisleri Odas ı Eğ i t im MerkeziBildir i ler Kitab ı

23119. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2018

Towards Saving Some of Your Fuel

Zeynep Begüm Kurt¹,  Tanya Aycan Başer², A. Arslan Kaya³

¹Toksan Yedek Parça İmalat Ticaret ve Sanayi A.Ş, R&D Center, 16140, Bursa, Türkiye²Tofaş Türk Otomobil Fabrikası A.Ş, R&D Center, 16110, Bursa, Türkiye

³Metallurgy and Materials Eng. Dept, Engineering Faculty, MSKU, Mugla Turkey

Abstract

This study aimed to save weight without compromising on the functions and the mechanical requirements of the trunk lid goose-neck hinges made of steel by substituting with tubular extrudites made of AA6082 alloy. The key problem involved the behavior of the welded joints following bending and press forming operations in the alloy employed that showed natural ageing. A thorough optimization study finally led to a weight saving ratios of about 52-56% depending on the tubular profile shape.

1. Introduction

AA6xxx series offers medium-to-high strength levels and occupies a central position in the banana-chart of aluminum alloys (Fig.1) [1]. Better-known as an age-hardenable aluminum alloy series, the specific yield strength of 6xxx alloys can be comparable to the ultimate tensile strength of some boron steels (Fig.2) [1]. Semi-products made of 6xxx alloys necessitates T4 condition prior to shaping into the final forms, while the final mechanical properties achieved through artificial ageing [2-4]. A6082 with its excess Si offers better formability and strength combinations as compared to its sister alloys in the same series that contains higher Mg/Si ratios [4]. It is also worth noting that

chemical composition ranges for any member of an Al alloy group, as is the case for AA6xxx, is quite wide and consequently the specific composition of any Al alloy may show compositional differences as compared to its equivalent depending on the producer. Furthermore, the alloys of the series show room temperature ageing as well, and consequently the artificial ageing response can be different than expected [5]. Such differences reflect upon the properties of the products. In addition, there exist differences in downstream processing conditions from one company to the other during part productions. Consequently, production lines may well have to experiment to come up with the best solution for their case.

Trunk lid hinges come in three types, of those, the goose-neck type is the one which is more frequently used. This type of hinge, being in tubular or hollow profile form, is at one end attached to the trunk lid and the other end attached to the body with a bracket that enables free movement in the rotational axis. Goose-neck trunk lid hinges, being more voluminous and therefore heavier as compared to other types, are suitable both for substitution with new mechanisms that provide the same functionality as well as for weight-reduction attempts.

This study aimed to substitute the existing two types of, tubular and hollow square profile, goose-neck steel materials with AA6082-T4 in order to meet the same requirements after forming operations (bending and pressing) and to achieve some weight saving in assembled state.

2. Experimental

Naturally aged semi-products purchased from Onat Profil Co. and having the composition given in Table 1 were employed. Welded joints on A6082-T4 samples were made by using TPS 320i C PULSE /4R/FSC type MIG machine, employing 76Amp and 30V with Al 4043, and 73 Amp and 30V Al 5356 filler wire of 1mm Ø. The chemical

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232 IMMC 2018 | 19th International Metallurgy & Materials Congress

composition and the mechanical properties of the filler material are given in Table 2 and 3.

Figure 1. Banana-chart of aluminum alloys used in automotive.[1]

Figure 2. Comparison of specific strength of 6xxx series with some steels. [1]

Table 1. Composition of AA6082 extrudites. (wt.%)

Si Fe Cu Mn Mg Ti Al 0.85 0.25 0.04 0.041 0.79 0.01 97.40

Table 2. Al 4043 Filler wire chemical composition.

MIG wire Mg Mn Cr Ti Si Al

Al 4043

4.5-6.0 Bal.

Al 5356

4.50-5.50

0.05-0.20

0.05-0.20

0.06-0.15

Bal.

2.1 Structural Analysis

The mechanical properties, namely the yield and tensile strengths and elongation, of the weld zones of aluminum parts joined via MIG welding are about 50% lower as compared to the base metal. Therefore the computer based analysis was undertaken assuming, including the safety margins, a yield strength value of 240MPa and 10% elongation for the heat affected zones (HAZ) of the hinges.

Table 3. Mechanical properties of filler wires.

FillerWire

Yield Str.(MPa)

UTS(MPa)

ElongtionA5 (%)

Al 4043 > 40 > 120 > 8

Al 5356 > 110 > 235 > 17 2.2.1 Mechanical Tests

Tensile tests were conducted to assess the effect of welding on the mechanical properties of the joints.

TMMOB Metalurj i ve Malzeme Mühendisleri Odas ı Eğ i t im MerkeziBildir i ler Kitab ı

23319. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2018

The sample dimensions of the welded plates of AA6082-T4 were 2x100x300mm.

2.3 Strengthening Approach

In order to compensate the drop in the strength in HAZ, two different heat treatment procedures were employed following MIG welding. In the first approach, welded parts were initially solutionized at 530 °C for 1 h followed by air cooling. Subsequently, the parts were artificially aged at two different temperatures, namely 200 °C and 180 °C for different length of times. In the second approach, the welded parts were only subjected to artificial ageing at two different temperatures, i.e. 200 °C and 180 °C, for various time lengths without any prior solutionizing treatment.

2.4 Testing Procedures

2.4.1 Mechanical Tests and Microstructural Analysis

Tensile mechanical properties of welded samples were tested by using ZWICK /ROELL tensile test machine, and hardness values obtained via HV.01. Microstructures of weldments were examined on polished and etched sections via light microscopy.

To determine the penetration level of weldings, 12 samples taken from different locations were immersed in acid solutions prior to penetrant tests. The minimum acceptance level 0.2mm penetration is per standard tests specifications. Fig.3 shows examples of penetration depth in cross sections of the profile and bracket.

2.4.2 Performance Tests

The heat treated and non-heat treated goose-neck hinges were subjected to structural performance tests such as wind test under 500N force, side-loading, gas-spring tests in open and closed conditions, and fatigue tests for 25000 cycles.

3. Results and Discussion

Prior to any strengthening applications, the weldments made with Al 4043 and Al 5356 wires yielded a rupture force of 10950N and 15000N, respectively. Following a series of strengthening treatments after welding with 4043 wire, it was observed that the rupture force increased with increasing ageing time and temperatures. It was found that the solutionizing treatment conducted at

530°C did not lead to a noticeable hardness change for weldings with 4043 wire. A maximum rupture force of 13964N was observed after the ageing treatment at 200°C for 1h. The maximum rupture force obtained with 5356 wire was 21100N after an ageing treatment at 180°C for 5h. Since the acceptance critera for the welded structure were 240MPa yield and 320MPa tensile strength levels with an elongation value of 10% it was realized, also considering the cost, that the welding with Al4043 wire was successful enough following an artificial ageing treatment at 200°C for 1h.

The microstructures of the weld cross sections are given in Fig.1. Examinations on HAZ revealed that the transition zone was more uniform and less prone to cracking and pore formations along weld zones with 4043 wire as compared to those with 5356 wire.

The hardness measurements results are presented in Fig.2. Although, the maximum hardness value of 85-90 Hv was observed in HAZ of weldings with 5356 wire after ageing at 200°C for 1h, the hardness variation along the weld zone was greatest showing a range of 40-85 Hv as compared to the variation range of 60-75 Hv in non-heat treated as well as artificially aged conditions of weldings with 4043.

Penetration tests showed that weld penetration was greater than the required minimum of 0.2mm in all weldments. Fig.3 shows that weld penetration was 2mm between the hollow square extrudite profile material and the case.

Performance tests such as wind test under 500N force, side-loading, gas-spring tests in open and closed conditions yielded all equivalent values, giving less than 0.5mm plastic deformation. Prototypes also passed the standard fatıgue tests conducted at 0.8-6 Nm for 25000 cycles.

4. Conclusions

Having been found acceptable per technical requirements, the goose-neck hinges produced by using extruded AA6082 alloy having a square hollow profile gave a weight reduction of 52% while the tubular profile 58% as compared to the current steel material. Such a weight reduction has been calculated to be equivalent to a fuel saving of 0.01L per 100Km.

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Figure 1. Light microscopy images of the weld zones: (a-c) with welding wire Al 4043: a) o heat treatment, b) 530°C-1h + artificial ageing at 200 °C-1h, c) Artificial ageing at 200 °C-1h; (d, e) with welding wire Al 5356: d)

No heat treatment, e) Artificial ageing at 200 °C-1h.

Figure 2. Hardness measurements on welded samples.

Figure 3. Weld penetrations in the cross sections of hollow profile extrudites.

References

[1] D. Wieser, A. Förderreuther, P. Oberhauser, O. Rebuffet, B. Sprengers, D. Szegda, 7th Annual,

Global Automotive Lightweight Materials Europe (GALM), Munich, Germany, April 25 -26, 2018.

[2] T. A. Ba er, “Alüminyum Ala ımları ve Otomotiv Endüstrisinde Kullanımı”, MMO, Mühendis ve Makine Dergisi, 635, 2012.

[3] F. Vollertsen, A. Sprenger, J. Kraus, H. Arnet, J. Mater. Process. Technol. 87, 1–27, 1990.

Weld metal

bBase Metal

Weld

Base Metala

Weld metal

Base Metalc

Weld metal

Base Metalc

Weld metal

Base Metale

Al 4043 non-heat treatmentAl 4043 200 °C-1 h Arficial Aging Al 4043- 530 °C-5h+200 °C-1 h Artificial Aging Al 5356 Non-heat treatmentAl 5356- 200 °C- 1h Artificial Aging

cas

cas

profil

profile

TMMOB Metalurj i ve Malzeme Mühendisleri Odas ı Eğ i t im MerkeziBildir i ler Kitab ı

23519. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2018

[4] P. Mukhopadhyay, ISRN Metallurgy, v. 2012, doi:10.5402/2012/165082.[5] M. Murayama, K. Hono, Acta Mater. 47 1537-1548, 1999.

[6] J. Royset, T. Stene, J. A. Saeter, Mater. Sci. Forum Vols. 519-521 239-244, 2006.

[7] I. Dutta, S. M. Allen, J. Mater. Sci. Lett. 10 323-333, 1991.

[8] G. A. Edwards, K. Stiller, G. I. Dunlop, M. J. Couper, Acta Mater. 46, 1998.