Forming characteristics of 6xxx alloys

How do we measure the formability of our products?

From hoods and doors to fenders and deck lids, AMAG automotive sheets have wide-ranging automotive applications and meet numerous requirements. In addition to mechanical and forming characteristics, these sheets possess excellent corrosion resistance, process-stable weldability and bondability combined with outstanding surface quality for defect-free painting. Consistently, high-performance materials are essential in the modern automotive industry. OEMs require companies to complete complex homologation procedures before they can be considered as sheet metal suppliers. The material properties listed above are among those examined in extensive testing under laboratory and industrial conditions. The most stringent requirements pertain to formability. Modern automotive designs demand increasingly sharp edges, tighter bending radii and high draw depths. Materials must therefore withstand all kinds of forming operations without the development of necks or tears. If a press shop is unable to produce a proper car body part this usually casts doubt over the formability of the material. In this context, a materials supplier like AMAG faces three crucial questions:

How can we “measure” good formability and what are the key indicators?
How does formability change when materials are stored for prolonged periods by the customer?
And how can we adjust the chemical composition or processing of the material to positively influence forming properties?

In practice, forming processes often involve several steps and complex states of stress and strain. A hemming edge, which is used for various purposes including connecting the inner and outer parts of a hood, places different demands on a material than deep drawing. However, tensile testing - which can be carried out quickly and has been standardized in DIN EN ISO 6892-1 - has proven a suitable way of providing an initial idea of material behavior. It provides key parameters such as yield strength, ultimate tensile strength, uniform elongation, elongation at break, strain hardening exponent and plastic strain ratio (r-value), allowing initial assumptions about formability. For example, a n-value should be beneficial for stretch forming [1]. In addition to tensile testing, the Center for Material Innovation (CMI) can also conduct the test procedures listed below, which help to characterize the formability of automotive products:

Test procedure	Provides indication of:
Forming limit test	Onset of necking for different strain states
Bulge test	Hemmability
Bend test	Bending capacity / the edges on punched sheets
Erichsen cupping test	Stretch-forming capacity
Hole expansion test	Ductility of the edges on punched sheets

Table 1: Overview of test procedures conducted at the CMI

The assessment of a forming limit diagram is more time-consuming than the other test procedures listed in Table 1. In order to cover several strain states, from stretch forming to deep drawing, a number of different sample geometries must be produced and formed in advance. Each test specimen is represented by a data point in the forming limit diagram (FLD). The FLD is part of the material cards used for formability simulations of newly developed automotive components. While the FLD effectively depicts general differences between different alloys or states, it is less sensitive to smaller changes of materials and processes. Deep drawing capacity and hemmability are key properties for automotive alloys. The hemmability of a material can be quickly and precisely evaluated in a laboratory. This initially involves pre-stretching a sheet sample by 10% in the rolling direction (to simulate sheet forming) and then bending it 180° with the bending line perpendicular to the rolling direction. The radius around which the material is bent is gradually reduced until cracks appear. The so-called hemming factor (f), which is used to assess hemmability, is calculated by the sheet thickness (t) relative to the radius (r) around which the material can be hemmed without cracking:

unformbarkeit_EN_von-6xxx-abbildung1 — **Figure 1:** Images taken using an optical microscope of the hemming edges of a 1 mm material sheet.

This hemming test is a very good indicator of the hemming capacity of real components. In order to evaluate the deep drawing capacity of the material close to the industrial process AMAG has commissioned its own tool. The design for this complex tool AUDI AG and was developed to determine forming capacity in the course of material qualification procedures. The tool contains key elements of automotive bodywork parts, such as complex corner radii, sharp design ridges, door handle cups and draw beads. The research group of Lightweight and Forming Technologies at TU Graz constructed the tool and assists with test procedure. The formability tests are carried out on a 400 ton press that can achieve realistic industry conditions, including drawing speeds of up to 150 mm/s and blankholder forces of up to 1600 kN. These tests require industrially produced blanks measuring 1.0 mm x 435 mm x 472 mm. The surface structure (mill finish, EDT) and the type and volume of dry lubricant applied can be varied. The dry lubricant is applied electrostatically, which ensures that the dry lubricant is evenly distributed. The tool has a closed geometry and therefore a constant drawing depth. The forming capacity is assessed based on the maximum blank holder force at which a flawless component can still be formed. A component is considered flawless (also known as a “good part”) when no necking or crack formation is evident (Figure 2).

umformbarkeit-von-6xxx-abbildung2 — **Figure 2:** Formed component with different failure patterns (left: component with necking; right: component with cracking).

Ergebnismatrix_EN — **Figure 3:** Results matrix for the deformation tests. Each square represents a component, while the color indicates how that component was assessed.

Increasing the blank holder force reduces the material flow and fosters material thinning, so the formability requirements can be gradually increased. Figure 3 shows the procedure for characterizing forming capacity. At least three components are required for each classification in order to validate the limits for good parts and cracked parts. The area in between is a transition region and can contain components of all classifications.

Influence of natural aging on formability

AMAG automotive products BIW 6FO-100 and BIW 6ED-110 are both used for inner structure applications and are based on alloy system EN AW-6016. The product names are based on their applications (BIW = body in white), their alloy series (6 for 6xxx aluminium alloys) and a further application-specific code (FO for forming optimized; ED for electric drive). The final number denotes the material’s typical Rp0.2 yield tensile strength in T4 delivery condition [MPa]. The typical mechanical characteristics of both products are summarized in Table 2 and Table 3. BIW 6ED-110 is characterized by very good artificial aging capacity and still offers good formability in T4 delivery condition.

Product	Rp_0,2 [MPa]	R_m [MPa]	Rp_0,2/Rm	A_G [%]	A₈₀ [%]	n_4-6	r_2-20/(Ag-1)	Δr
BIW 6FO-100	95	208	0,46	23	27	0,31	0,66	0,31
BIW 6ED-110	113	239	0,47	22	26	0,29	0,63	0,20

Table 2: Typical mechanical properties after 10 days of natural ageing and test direction perpendicular to rolling direction. The planar anisotropy was calculated as: Δr = ½ (r0 – 2r45 + r90)

Due to its low T4 strength, BIW 6FO-100 offers a balanced profile of strength, formability and hemmability.

Product	Rp_0,2 [MPa]	R_m [MPa]	A₈₀ [%]	σ₀ [MPa]	k [MPa]
BIW 6FO-100	205	262	19	53,4	15,9
BIW 6ED-110	235	295	19	66,7	17,7

Table 3: Typical mechanical properties after paint bake simulation (2 % prestrain and 185 °C f or 20 min) and test direction perpendicular to rolling direction.

KLT2_EN — **Figure 4:** Yield strength progression over a natural aging period of 18 months

Influence of the natural aging period on formability

Both products underwent testing with the deep drawing tool over an 18-month period [2, 3]. They were also subjected to tensile tests to measure their mechanical characteristics. In Figure 4, the yield strength σy is plotted on a logarithmic scale against natural aging time and follows a linear path that can be described as follows [4]: σy= σ0+k logt.This formula combines the natural aging period (t), the yield strength after 1 h of natural aging (σ0) and the kinetics of natural aging (k). We can therefore determine a number of parameters for the two materials: BIW 6ED-110 is more highly alloyed with Si and Mg than BIW 6FO-100 and therefore has higher initial strength as well as faster kinetics. Figure 5 summarizes the results of deep draw testing on both products. The tests were always carried out with two different draw speeds. It is noticeable that the formability characterized by the maximum blank holder force for good parts remains almost constant over the entire natural aging period.

Ergebnis_A-Tool_100 mm-s_KLT_EN — **Figure 5:** Result with the deep drawing tool for two different drawing speeds over a period of 18 months. The diagrams show the maximum holding force for good parts as well as the transition zone. Above the transition zone, only cracked parts were obtained.

The long natural aging period and approx. 20-25% higher yield strength appear to have little negative impact. The mechanical properties for 6xxx automotive products are guaranteed for a six-month period. However, the results show that the material still has high capacity for deep drawing beyond this period. Despite its higher strength, BIW 6ED-110 performs better in deep draw testing compared to BIW 6FO-100. Failure due to necking or cracking always occurs at the same area of the formed part (Figure 3).

Umformbarkeit_1 — **Figure 6:** Kocks-Mecking plot of two products: BIW 6ED-110 and BIW 6FO-100

Simulations show that this area is subject to plane strain conditions. The material’s failure is induced by geometric instability. In order to find a explanation for the products’ different performance on the experimental tool, we must consider the following three factors in further detail that should be beneficial regarding formability:

A high and uniform r-value

The average r-value (rm) is similar in both materials, though 6ED-110 has lower planar anisotropy (Δr - see Table 2), which should have a positive impact on deep drawing capacity [1].

A high strain hardening capacity especially at high strains

Strain hardening is a competing process between accumulation of dislocations and dynamic recovery during plastic deformation.If we plot the strain hardening rate (dσ/dε) against the increase in yield stress (σ - σy), the result is a Kocks-Mecking plot (KM plot). The slope of the KM plot provides an indication of dynamic recovery, which may be influenced by solutes [5]. In general, microstructural features such as dispersoids, solutes and precipitates can change the strain rate and the course of the KM-Plot due to interactions with dislocations [6 to 8]. As Figure 6 demonstrates, BIW 6ED-110 has a significantly higher strain hardening rate over the entire strain range.

Umformbarkeit_2_DE — **Figure 7:** Considère criterion for diffuse necking

This leads to the retardation of diffuse necking that occurs when the decreasing rate of hardening comes to balance with the increasing flow stress (Considere criterion, Figure 7).

Positive strain-rate sensitivity

Positive strain-rate sensitivity (m) means that the yield stress increases as the strain rate increases. In terms of the formation of necking during deformation, the m-value is important after diffuse necking (dσ/dε = σ), as a positive m-value inhibits the formation of local necking (dσ/dε = σ/2 [9]. Compared to steel, aluminium has low strain-rate sensitivity. However, research conducted by Langille [10] shows that this actually can have a positive effect on its formability. The m-value was not determined for either product; however, if we compare the results for the different drawing speeds, an increase in the drawing speed from 10 mm/s to 100 mm/s reduced the maximum holding force for good parts by 22% in the case of BIW 6FO-100.For BIW 6ED-110, the maximum holding force was reduced by only 15%. Further investigation would be required to determine whether this effect can be explained by differences in strain-rate sensitivity. In laboratory tests, however, researchers identified that Si has a positive influence on the m-value, which would support better performance by the more heavily Si-alloyed BIW 6ED-110.

umformbarkeit-von-6xxx-abbildung8 — **Figure 8:** Comparison of folding edges after (180 days) for BIW 6FO-100 (top) and BIW 6ED-110 (bottom) with a hemming factor of 0.4

Although BIW 6ED-110 offers superior deep drawing performances compared to BIW 6FO-100, if we look at hemmability, BIW 6FO-100 has a significantly better hemming edge after six months of natural aging. The differences between hemming and deep drawing capacity are due to various failure mechanisms.While deep draw capacity is primarily determined by the factors described above, which have an impact on the material’s instability (diffuse/local necking), hemmability is primarily influenced by fracture behavior, i.e. by the formation, growth and coalescence of pores in the microstructure [11, 12]. Good hemmability therefore does not require good deep draw capacity, and vice versa. In summary, we can see that it is always important to consider different aspects when examining a material’s formability. AMAG has a wealth of material characterization methods at its disposal - and the experimental tool is an important addition. It can effectively determine the limits of a material’s formability and offers the opportunity to examine a number of different influencing factors (chemical composition, storage time, tribology) on the formability of a wide spectrum of automotive products.

Benefits for customers:

AMAG has a wealth of methods at its disposal for comprehensive characterization of formability, which enables us to select suitable products to meet our customers’ needs and define optimal applications for them.
The experimental tool allows us to test developments under industrial conditions before they are used by our customers.
Our close cooperation with university partners - such as the Institute of Materials Science, Joining and Forming (IMAT) at TU Graz - helps us to expand our expertise, which enables us to offer our customers the best possible support in relation to forming technology.

References

[1] Ostermann, F.: Blechumformung. In: Ostermann, F. (Hrsg.): Anwendungstechnologie Aluminium. VDI-Buch. Berlin, Heidelberg: Springer Berlin Heidelberg; Imprint: Springer Vieweg 2014, S. 505–546[2] Hodžić, E., Domitner, J., Thum, A., Shafiee Sabet, A., Müllner, N., Fragner, W. u. Sommitsch, C.: Influence of alloy composition and lubrication on the formability of Al-Mg-Si alloy blanks. Journal of Manufacturing Processes 85 (2023), S. 109–121[3] Hodžić, E., Domitner, J., Thum, A., Sabet, A. S., Müllner, N., Fragner, W. u. Sommitsch, C.: Influence of natural aging on the formability of Al-Mg-Si alloy blanks. Journal of Manufacturing Processes 94 (2023), S. 228–239[4] Esmaeili, S.: Effect of composition on clustering reactions in AlMgSi(Cu) alloys. Scripta Materialia 50 (2004) 1, S. 155–158[5] Embury, J. D., Poole, W. J. u. Lloyd, D. J.: The Work Hardening of Single Phase and Multi-Phase Aluminium Alloys. Materials Science Forum 519-521 (2006), S. 71–78[6] Chen, Y., Weyland, M. u. Hutchinson, C. R.: The effect of interrupted aging on the yield strength and uniform elongation of precipitation-hardened Al alloys. Acta Materialia 61 (2013) 15, S. 5877–5894[7] Cheng, L. M., Poole, W. J., Embury, J. D. u. Lloyd, D. J.: The influence of precipitation on the work-hardening behavior of the aluminum alloys AA6111 and AA7030. Metallurgical and Materials Transactions A 34 (2003) 11, S. 2473–2481[8] Poole, W. J., Embury, J. D. u. Lloyd, D. J.: Work hardening in aluminium alloys. In: Fundamentals of Aluminium Metallurgy. Elsevier 2011, S. 307–344[9] Ghosh, A. K.: The Influence of Strain Hardening and Strain-Rate Sensitivity on Sheet Metal Forming. Journal of Engineering Materials and Technology 99 (1977) 3, S. 264–274[10] Michael Langille: The Influence of Microstructural Components on the Formability of Aluminium Alloy Sheets, Thesis. Université Grenoble Alpes 2019[11] Zubeil, M.: Versagensprognose bei der Prozesssimulation von Biegeumform- und Falzverfahren, Meisenbach Zugl.: Erlangen-Nürnberg, Univ., Diss., 2014. Bamberg 2019[12] Le Maoût, N., Thuillier, S. u. Manach, P. Y.: Aluminum alloy damage evolution for different strain paths – Application to hemming process. Engineering Fracture Mechanics 76 (2009) 9, S. 1202–1214

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