Technical Paper
Warm HydroForming of the heat treatable aluminium alloy AC170PX

https://doi.org/10.1016/j.jmapro.2015.09.012Get rights and content

Highlights

  • Ageing is effective in the AC170PX for times larger than 400 s.

  • In the range 250–300 °C the UTS% is reduced when increasing the strain rate.

  • High strain rates and the temperature of 200 °C are optimal process conditions when forming the AC170PX.

  • Die cavity filling is improved from 50% to 67% and the thinning reduced.

  • Short forming times make the process less critical and industrially more attractive.

Abstract

In this work the Warm HydroForming process of a 6xxx series Al alloy has been investigated in order to find suitable process parameters for successfully forming a benchmark component displaying varying deformation ratios due to its geometry. Since the attention was focused on the 6xxx series alloy AC170PX, belonging to the group of the age-hardenable Al alloys, the process conditions affecting the ageing, such as temperature and duration of heating exposure, the rate to increase the oil pressure, had to be determined. A preliminary mechanical characterization on artificially aged specimens was thus necessary both at room temperature and in warm conditions to investigate the influence of the ageing phenomenon on the mechanical and deformation behaviour at temperatures ranging from 150 °C to 350 °C. Based on the results of the tensile tests, preliminary Warm HydroForming experiments were conducted by changing the most influencing process parameters: working temperature, heating time and the time needed to increase the oil pressure up to the maximum value. It was found that the optimum working temperature was 200 °C and that the exposure of the material to the employed warm-forming temperature had to be minimized. Final Warm HydroForming tests were thus carried out using different rates to increase the oil pressure for comparison purposes, revealing that employing higher rates, i.e. increasing the strain rate, was beneficial, in terms of die cavity filling and sheet thinning, to obtain sound products.

Introduction

In the last decades, car manufacturers have been facing increasingly stricter limitations in terms of polluting exhaust emissions. The Regulation (EC) No 715/2007 introduced the Euro 6 standard starting from January 2015: combined emissions of hydrocarbons and nitrogen oxides have to be thus capped at 170 mg/km that is almost one third less than the upper limit of the Euro5 regulation.

Downsizing the car engine can be considered as an attractive solution as well as reducing the weight of the vehicles structure, especially if a massive adoption of light alloys is taken into account. Aluminium (Al) alloys, for example, offer lower density and higher corrosion resistance than conventional steel grades, while keeping good enough mechanical properties.

Replacing steel by Al leads to reduce the car weight ranging from 40 to 60% and to improve at the same time the fuel efficiency [1]. In particular, different Al types are adopted according to the required properties: 6xxx series Al alloys are usually adopted when high strength and surface quality are needed, while 5xxx series Al alloy when structural applications in which energy absorption are important or when complex shaped parts have to be produced [2].

The above mentioned advantages are counterbalanced by the well-known poor formability of these alloys at room temperature due to the large amount of alloying elements to enhance mechanical properties [3]: poor formability limits the adoption of such alloys for car body and structural parts where high levels of elongation and ductility are needed. In order to overcome such limitations, different approaches can be used: (i) to adopt innovative stamping processes; (ii) to increase the working temperature.

Among innovative stamping processes, sheet HydroForming is surely a promising solution, especially for the manufacturing of highly complex parts since also allowing undercuts and homogeneous stretching of the material [4]. Such a technology finds several application in the automotive industry: engine cradle, rear axle and external body parts (roof and bonnet) are some of the applications [5]; in addition this process is proved to be convenient in the sense that it would allow manufacturing of a one-piece shell fender rather than stamping three pieces to compose the same fender via subsequent joining [6]. Researches on the hydroforming process demonstrated that, if the blank shape and the loading path are properly chosen, complex hollow shell structures can be produced by double sheet hydroforming process, even more easily and efficiently than traditional stamping and welding method [7]; the process appears to be versatile, in the sense that it can be combined with rapid tooling technique for the fast manufacturing of prototypes, thus shortening the product design process [8].

On the other side, the solution of increasing the working temperature to overcome the poor formability has been largely investigated in literature. Experimental campaign based on uniaxial tensile tests on both 5xxx and 6xxx series alloys demonstrated the benefits of the increase of the working temperatures: the total elongation was in fact enhanced, mainly due to the increased strain rate hardening accompanied by the reduction in flow stress and the increase in toughness of the material when compared with cold forming [9].

Therefore, the combination of such a forming process with the adoption of the warm condition is proved to be effective towards forming 6xxx Al alloys through reducing the forming forces as well as higher savings on tool mass [10], [11], thus making the hydroforming technology suitable for a wide variety of applications: (i) die cavity filling for a 5xxx aluminium alloy as well as its drawability have been observed to be up to 104% higher than conventional stamping when working in warm conditions [12], [13]; (ii) manufacturing complex components made of 6xxx Al alloy, such as the bipolar plates for fuel cell application, becomes possible in warm conditions [14]; (iii) reaching higher expansion ratios and thus manufacturing of tubular components out of otherwise low-deformation capacity metals like magnesium alloys becomes possible due to a better temperature distributions in this process [15]. Recent investigations about Warm HydroForming have also shown that it performed well when applied to most of the conventional steel grades: the temperature range employed has been proved to play a central role when non-symmetric austenitic stainless steel components were produced by using such a technology [16]. Interesting results in terms of higher drawability were obtained with a brand new flexible-media forming process, i.e. the solid granule medium forming, where metallic or non-metallic granules are selected as pressure-transfer medium to form the workpiece [17]. Great improvements were found in forming ultra-high strength martensitic steel components by employing warm temperatures up to 600 °C and adopting pressurized oil in combination with granular material, such as small ceramic spheres, as forming media [18].

On the other hand, most of the aluminium alloys adopted in forming processes are heat treatable. The heat treatments on a 6xxx age-hardenable alloy are widely reported in literature, with the aim of determining the ranges in which temperature and holding time had to be set to harden (or alternatively, soften) the material monitoring both the yield stress and the uniform elongation. Thus, even local heating affects not only the formability but also the mechanical properties in relation to the ageing phenomena. In this regard, it should be emphasized that applying local heating on an AA6060 tube-hydroformed component has been reported to be a promising solution to: (i) reduce the maximum pressure needed and (ii) avoid complete intermediate annealing operations, thus reducing the cycle time [19].

In a relevant study to our work, Ramesh et al. [20] focussed on the effect of natural ageing (T4), artificial ageing (T6), and T4 followed by T6 treatments and the resulting changes in various properties in two age-hardenable Al alloys: after a short-time ageing treatment, the increase of the mechanical properties was denoted while an over-exposition to the ageing temperature (i.e. 175 °C) resulted in the opposite phenomenon, overaging. Similar results were obtained on the AA6061 alloy: in particular, an incubation time of 15 min at the ageing temperature was elapsed before the onset of an increase in hardness, while maximum hardening was reached after about 200 min due to the formation of needle-shaped β precipitates [21]. The changes prior to the peak hardness at the atomic scale were reported in detail both at room temperature and 200 °C and it was shown that grain boundary Mg-rich clusters can be the precursors in the precipitation sequence during ageing, at least in severely deformed structures with enhanced mobility of magnesium atoms due to deformation-induced extra vacancies [22].

In this work the Warm HydroForming (WHF) process of a 6xxx series Al alloy in pre aged (T4 condition) has been investigated using an experimental approach. As benchmark, a stepped component characterized by two different heights, respectively, equal to 12 mm and 20 mm, was employed. Since the investigated material is an age-hardenable alloy, the specimens were characterized via mechanical and formability tests prior to the warm temperature forming tests. Both types of tests were conducted at room temperature following artificial ageing and at warm temperature conditions, i.e. 150–350 °C, in order to understand the effects of these treatments on the mechanical properties and deformation behaviour, more specifically formability in plane strain condition (FLD0), during a warm forming process. WHF experiments were conducted changing the parameters: (i) temperature, (ii) forming time which, in essence, changes heating time and deformation rate. Finally, further WHF tests were carried out using short forming times by using greater rates to increase the oil pressure in order to improve both the die cavity filling and the sheet thinning.

Section snippets

Investigated alloy

A single batch of AC170PX 1.2 mm thick was purchased in the T4 condition. The chemical composition is presented in Table 1.

Tensile tests on artificially aged blanks

The ageing phenomenon affecting the plastic behaviour of the present Al alloy was investigated via mechanical tests at room temperature on specimens previously aged in furnace (Nabertherm, precision of temperature measurement: ±3 °C). Specimens used for the mechanical tests were previously exposed at different ageing temperatures and ageing times in the range 150–400 °C (with

Evaluation of the ageing effect on the mechanical and deformative behaviour

The YS% and UTS% values of the specimens treated for the same ageing time have been given in Fig. 7: a comparison of the graphs in Fig. 7a and b indicates the strong effect of ageing on yielding.

Although the changes were more moderate, the UTS% values showed a similar trend to the YS% at all investigated ageing conditions. Since similar results in terms of both YS% and UTS% are produced by ageing times of 600 s and 1200 s, the attention was focused on the YS% and on the middle level of the

WHF tests changing temperature, heating time and forming time

In order to investigate how ageing affects the WHF process, an experimental campaign was carried out by changing the parameters (i) temperature, (ii) heating time and (iii) forming time. Tests were conducted according to the Central Composite Design (CCD) plan reported in Table 2.

For all tests, the BHF profile was increased linearly from 150 to 450 kN and a maximum oil pressure of 200 bar was employed.

Conclusions

In this study, the optimum working conditions for the Warm HydroForming process of the 6xxx series age-hardenable Al-alloy AC170PX have been established by means of an experimental approach.

Results of the preliminary characterization based on tensile and formability tests on both aged and as-received specimens can be summarized as follows:

  • the investigated Al alloy exhibited the maximum tensile strength at room temperature when performing the artificial ageing treatment in the temperature range

Acknowledgements

The authors wish to thank Prof. Tricarico and Prof. Kaya for their contribution in the discussion of results and Dr. Riccardo Brivio (Fontana Group) for his help in supplying the AC170PX. The research activities were carried out using the equipments funded by Region APULIA (project acronym: TRASFORMA, grant number: 28) and the Italian Ministry of Education, University and Research, MIUR (project acronym: SMATI, grant number: PON01_02584).

G. Palumbo is Associate Professor at the Politecnico di Bari, Department of Mechanics, Mathematics and Management. His research topics cover processes ranging from Casting and Laser applications to Sheet Metal Forming (warm deep drawing, hydroforming, superplastic forming, incremental forming), always using a numerical and experimental approach. Author of more than 100 publications, he is the Scientific Coordinator of the laboratory of Advanced Forming & Manufacturing, belonging to the Regional

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    G. Palumbo is Associate Professor at the Politecnico di Bari, Department of Mechanics, Mathematics and Management. His research topics cover processes ranging from Casting and Laser applications to Sheet Metal Forming (warm deep drawing, hydroforming, superplastic forming, incremental forming), always using a numerical and experimental approach. Author of more than 100 publications, he is the Scientific Coordinator of the laboratory of Advanced Forming & Manufacturing, belonging to the Regional Laboratory Network TRASFORMA. He is involved in national (www.bioforming.it, funded by Miur) and international (CNR/Tubitak agreement) projects aimed at investigating the manufacturing of sheet metal parts using lightweight materials (Ti, Al, Mg).

    A. Piccininni received the Master's Degree in Mechanical Engineering from the Politecnico di Bari in 2012; he has been working at the Politecnico di Bari since 2012 mainly on Finite Element simulations (sheet forming processes and residual stresses in casting processes) and experimental activities, such as warm hydroforming and deep drawing, residual stress measurements and mechanical/technological characterization of metals at elevated temperatures. He's currently at the Institute of Metal Research (Shenyang, China) as an academic research visitor to investigate the electromagnetic forming process.

    P. Guglielmi was born in Andria, Italy, in 1980. He received the degree in Mechanical Engineering from the Polytechnic of Bari, Bari, Italy, in 2012 (academic course: Sheet and Bulk metal forming processes). In 2012, he joined the Department of Mechanical Engineering, Mathematics and Management (DMMM), Polytechnic of Bari, as trainee in the framework of the PON project “SMATI”. He is now working at the Polytechnic of Bari as research fellow, involved in research activities mainly focused on material characterization and other experimental activities (hydroforming, casting processes and metallographic investigations).

    G. Di Michele was born in Foggia, Italy in 1988. She received the Master's degree in Mechanical Engineering from the Politecnico di Bari in 2013, with a dissertation titled: “Numerical/Experimental investigations about the Warm Hydroforming of the aluminium alloy AC170PX”. From January 2014 to April 2015 she worked at the Politecnico di Bari as PhD student: her main experimental activities were correlated to hydroforming process in cold/warm condition, mechanical/technological characterization of Al-alloys, investigation about the ageing phenomenon in 6xxx Al-alloys, metallographic analysis of not-ferrous alloys. Actually she is working as Repair Engineer at GE Avio Aero in Brindisi, Italy.

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