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The use of computer simulation to imitate physical processes has proven to be a time-efficient and cost-effective way of performing scenario testing for process optimisation in different applications. The finite element analysis (FEA) is the dominant numerical simulation method for analysing sheet metal forming processes. It uses mathematical tools and computer-aided engineering software programmes to predict forming processes. To improve the quality of output from the simulation, accurate material characterisation data that correctly model the behaviour of the material when it undergoes deformation must be provided. This paper outlines the stages of conducting material characterisation experiments, such as tensile, hardness, and formability tests, using the aluminium alloy AA1050-O. Sample preparation, the machine setup, and testing procedures for the material characterisation tests are given. Subsequent data preparation methods for input into an FEA software programme are also outlined. Implications of the testing results to a deep drawing process are examined while considering the formation of a rectangular monolithic component measuring 2300 mm by 1400 mm with a drawing depth of approximately 150 mm. The results from the characterisation tests indicate that the forming process for the product can be achieved using cold forming at room temperatures as a 25% strain was recorded before necking against an anticipated uniaxial strain of 5.93%. The aluminium alloy AA1050-O demonstrated a negligible strain rate sensitivity in the forming region, thus eliminating tool velocity from the key process parameters that should be considered during FEA simulations. A 50% increase in hardness was recorded after strain hardening.

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1. Introduction

Material characterisation experiments are essential in predicting material behaviour during forming processes. This behaviour helps in the intelligent optimisation of deep drawing during forming. The experiments in this study involved the determination of the mechanical properties of aluminium alloys used in the production of the product under review using a deep drawing process. Tensile tests and micro-hardness tests are the most critical tests used to evaluate the performance of materials [1]. In addition, formability experiments are essential for forming processes [2]. The tests provide input data for finite element modelling simulations. This approach improves the reliability of the results from finite element modelling simulations [3]. During forming, materials undergo bulk deformation that can result in defects that are difficult to predict and model, hence the need to carry out material characterisation tests [2]. The product under review is manufactured using AA1050-O-grade sheet aluminium, which is part of the AA1000 series, known as pure aluminium alloys, with over 99% aluminium [4]. While the material is considered very ductile, it is also susceptible to tearing when deep drawn [5]. AA1050 aluminium behaves differently under different conditions like strain and forming temperature [6]. Though aluminium is the second most used metal after iron, the accuracy of publicly available data for aluminium alloys is under scrutiny [7]. This could be due to variability in material properties based on production processes [8], especially the wide range of processing techniques for aluminium alloys [9]. Another reason for the reduced accuracy of aluminium alloy data is shorter historical data [7] compared to the long historical record of steel [10]. Aluminium alloy datasheets are therefore more representative of nominal data rather than exact values, and carrying out material characterisation tests prior to finite element analysis (FEA) is essential. In this study, the experiments were conducted at the Fraunhofer IWU in Germany and the University of Cape Town in South Africa. This paper presents the details of the experiments conducted, including specimen preparation, experimental setup, the equipment used, the procedures followed, applicable standards, and the results obtained. The results were further processed into acceptable formats for input into FEA packages. The implications of the results on the feasibility of the deep drawing process are also given. This is a big step towards the improvement of publicly available material datasheets for AA1050-O.

2. Overview of Material Characterisation Tests for Deep Drawing Process

Research in sheet metal forming processes has been spearheaded by developments in the automotive industry, especially the frequent demand for new shapes [11]. This has led to researchers seeking to better understand the relationship between material properties and the mechanics of sheet metal forming. These relationships are better understood using material characterisation experiments. Several characterisation experiments can be performed to ascertain the forming potential of different materials. According to Harhash [12], tensile tests are critical in evaluating the characteristics of materials. For steels, researchers can rely on mechanical properties given in product catalogues and handbooks [13]. Tensile tests serve as primary material characterisation tests prior to forming. Apart from tensile tests, researchers also explore using micro-harness tests in material characterisation experiments [14]. For example, Kim et al. [15] used micro-hardness tests to evaluate the properties of aluminium and steel laminates. In forming operations, it is essential to assess the micro-hardness before and after forming as the material undergoes strain hardening [16]. The evaluation of forming characteristics of sheet metal, especially for deep drawing, cannot be complete without the determination of the Forming Limit Curve (FLC). It is one of the most essential characterisation experiments in sheet metal forming [17]. A detailed review of these characterisation tests is given in the next section of this document, as these tests are critical to this study.

2.1. Tensile Tests

Tensile tests are easy to carry out, yet they provide essential data that can be used to make decisions regarding manufacturing. According to Harhash [12], tensile tests are useful when evaluating the forming potential of materials. Tensile tests involve stretching a standard sample and observing its behaviour during this induced stress. In other words, tensile tests are used to determine the behaviour of materials when exposed to forces [18]. Figure 1 shows an outline of a standard tensile test specimen. According to Marciniak et al. [19], the specimen has parallel sides with a reduced cross-section. The length should be at least four times the width. The tests and specimens are also standardised through the International Organisation for Standardisation (ISO), with the applicable standard being ISO 6892-1 standard [20].

The specimen is subjected to tensile loads measured by load cells. As the specimen stretches, extension parameters are measured by an extensometer [19]. The behaviour of the material when subjected to stress can be best described by its stress–strain curve, as shown in Figure 2. The extent of the deformation depends on the ductility of the material. This test is essential to this study as it determines the conditions required for deep drawing, whether cold, warm, or hot forming is used. The forming process happens in the plastic region. If the material fractures without sufficient stretch required for forming, forming the product at room temperature would be difficult. The yield stress, ultimate tensile stress, and strain hardening behaviour are all input parameters for Finite Element Modelling software programmes for forming processes like deep drawing. The ultimate tensile stress can be read from the stress–strain curve as the maximum stress the material can withstand, while the yield stress marks the beginning of plastic deformation or permanent change in the strain. This point can be determined using the 0.2% strain offset rule [21]. The strain-hardening parameters are determined using strain-hardening laws [22].

2.2. Micro-Hardness Tests

The evaluation of material hardness is essential for forming operations as hardness is related to the yield stress and material flow [23]. The importance of hardness in forming operations cannot be overemphasised; hardness can be controlled through tempering options to achieve the desired hardness [24]. Before heat treatment, hardness can be influenced at the alloying stage, with elements such as copper and magnesium playing a significant role in improving the hardness of aluminium alloys [25]. It is suggested that an increase in hardness reduces the ductility and formability of metals [26]. There are several techniques used to determine the micro-hardness of materials, namely the Brinell, Vickers, Knoop, and Rockwell methods [8]. These methods involve the use of a hardened indenter pressed onto the surface of a softer specimen until full plastic deformation occurs. The pressure is held constant for a specified time and released [23]. The indentation is measured to indicate the micro-hardness of the material. The most common micro-hardness measurement technique used by researchers appears to be the Vickers Hardness (HV) test. It was used to evaluate microstructure hardening during sheet rolling [14]. Rudnytskyj et al. [23] also used the Vickers Hardness test to investigate the relationship between flow stress and material hardness in the forming of AA6016 and AA6061 alloys. Another study on the processing of AA6061 at cryogenic temperatures also used the Vickers Hardness test to evaluate microstructure evolution [27]. Furthermore, the test was used to assess the influence of accumulative roll bonding and electromagnetic forming on the formability of copper sheets [28]. Like the tensile tests described earlier, the Vickers Harness tests are also standardised according to the International Organisation for Standardisation (ISO), with the applicable standard being the ISO-6507-1:2018 standard [29]. This test was also adopted in this study. Figure 3 shows the principle of the test and the indenter geometry. The test uses a diamond indenter driven by a load (F), with the hardness value being a function of the weight of the load and the diagonals of the indented impression labelled as d₁ and d₂.

2.3. Formability Tests

In the simplest sense, formability can be defined as the ability of the sheet metal material to be transformed into the desired shape without any hindrances. However, this definition is qualitative and subjective and suggests that formability cannot be quantified. Several methods can be used to carry out formability tests. According to Reddy et al. [18], formability can be alternatively evaluated using tensile tests, multiaxial tensile tests, the Erichsen cupping test, Olsen test, Fukui’s conical cup test, hole expansion test, limiting dome height test, Swift cup test, ear height test, hydraulic bulge test, draw bending test, or the Forming Limit Curve (FLC) test using the Nakajima Marciniak test. The FLC for material characterisation for forming operations like deep drawing is widely used by researchers. Emanuela and Marion [17] conducted a metallographic analysis on Nakajima specimens to evaluate failure developments. Another study used the Nakajima test to investigate anisotropy and thickness distribution for AA2024-T4 [30]. The Nakajima test can be conducted using simulation with reliable datasheets, particularly for steel alloys [31]. The Forming Limit Curve (FLC) is transformed into a Forming Limit Diagram (FLD) when the strain space of a sheet metal part is superimposed on the FLC. According to Zhang, Shao, and Lin [32], formability is a material property that can be evaluated using a Forming Limit Diagram (FLD), which is determined at various forming conditions. This means that formability is not a static property but should be defined with the given conditions. Therefore, the product’s geometry and process parameters need to be established to define formability or vice versa, meaning that to produce monolithic automotive accessories through deep drawing, there is a need to determine the properties that optimise the formability of the sheet metal into the monolithic component. The FLD approach can be used to determine the boundary material properties for safe forming. In the studies carried out by Harada and Ueyama [33], Harada et al. [34], and Satish, Feyissa, and Kumar [35], to mention a few, it was demonstrated that the formability of sheet metal can be improved by multistage drawing at elevated temperatures with intermediate heat treatment. This was also confirmed by Goud, Prasad, and Kumar [36]. These findings imply that elevated temperature forming can be explored to improve formability.

Like the tensile tests and the hardness tests described earlier, the determination of the Forming Limit Curve is also standardised by the International Organisation for Standardisation (ISO) with the applicable standard being ISO 12004-2 [37]. The standard outlines the test procedure and the dimensions of the specimen. Figure 4 shows a typical Forming Limit Curve. The curve is constructed by deforming various specimens until they fracture and then plotting the results on the major strain (ε₁) and minor strain (ε₂) just outside the fracture area [38]. In this study, this test was adopted for the material characterisation experiments and was further discussed in the section focusing on material characterisation experiments.

3. Chemical and Physical Properties of AA1050-O Aluminium

The material test data sheet for the AA1050-O alloy, provided by the supplier of the test pieces, shows the chemical composition, as indicated in Table 1. These data are consistent with what was reported by other researchers who studied similar materials. The consistency within the composition is mostly for aluminium, which is over 99%, though there are slight variations in the alloying elements. Some alloys of AA1050 do not contain copper [1], while some have significantly lower iron compositions [2]. Other AA1050 alloys contain additional alloying elements like chromium, vanadium, and gallium [39], with others having sodium, potassium, and nickel [40]. These similarities and differences in the alloying elements are also confirmed by data from the Handbook of Metals [41], other suppliers [4], and test laboratories [42].

Table 2 shows the physical properties of AA1050-O material according to the supplier’s materials datasheet. Likewise, the data are consistent with the Handbook of Metals [41] and test laboratories [42].

4. Determination of Tensile Properties for AA1050-O

Tensile tests remain one of the fundamental tests used in the determination of the basic behaviour of materials when exposed to forces [18]. The tensile tests were conducted according to the ISO 6892-1 standard [20]. The tests were used to determine the yield strength and the strain hardening flow curve, required as input into a finite element analysis software programme. These experiments were also used as pre-screening tools for the strain rate sensitivity of the material, as the testing speed was varied across the samples.

4.1. Tensile Test Specimen Preparation

Dog-bone test specimens were prepared for the tensile tests, using the dimensions shown in Figure 5, from an AA1050-O aluminium sheet metal plate of 1.5 mm thickness. The specimens were cut using water jet cutting to minimise the effects of superficial work hardening and heating that could result in the alteration in the material parameters [44].

Nine specimen samples were prepared and labelled S1, S2, S3, S4, S5, S6, S7, S8, and S9, as shown in Figure 6. This was to allow for the tracking of the samples and replication as recommended through the design of experiment principles by Montgomery [45], as well as Niedz and Evens [46]. The tensile tests were carried out based on uniaxial stress along the length of the specimen.

4.2. Tensile Test Experimental Setup

Figure 7 shows the tensile testing machine Zwick-Roell ZMART Pro, which was used to perform the tensile tests at the Fraunhofer IWU in Germany. A maximum load of 5 kN was used for the machine settings. During the test, the test speed was varied across three levels of 5 mm/min, 10 mm/min, and 15 mm/min. This was performed as a pre-screening test to determine the effect of the strain rate sensitivity of the material on the forming process. Three specimens were tested at each speed level. S1, S2, and S3 were strained at 5 mm/min, while S4, S5, and S6 were tested at 10 mm/min, and a speed of 15 mm/min was used for S7, S8, and S9. This was carried out to check the repeatability and consistency of the results.

4.3. Tensile Test Results

Figure 8 shows the samples after the tensile tests. Visual inspection shows that the testing speed had an influence on the location of the fracture point; for example, S1, S2, and S3 were tested at 5 mm/min, and the location of the fracture point was comparable. This was also observed for S4, S5, and S6 which were tested at 10 mm/min. However, for the specimens, S7, S8, and S9 tested at 15 mm/min, the location of fracture point S7 seemed to be an outlier. These differences in fracture points could only be attributed to different testing speeds. A trend was observed for the specimens tested at the same speed, though some outliers like S7 were also observed; the influences of other factors, like material inhomogeneity through microstructure variations and interstitial defects, are not discounted as these were not further investigated. However, the results of all specimen tests were considered as there were no outliers in terms of the quantitative data. The analyses focused on determining the yield strength, maximum tensile stress, and hardening exponents.

4.3.1. AA1050-O Engineering Stress–Strain Curves

Figure 9 shows the engineering stress–strain curves resulting from the tests. All nine specimens depicted the same behaviour. The maximum strain for AA1050 aluminium was 40.6%, with elongation at break, averaging between 40 mm and 50 mm for an average gauge length of 120 mm for the specimen. An average maximum tensile stress of 65.64 MPa was recorded. A safe engineering strain of about 25% was recorded before necking.

Table 3 shows a summary of the tensile test results. The results show that the testing speed does not affect the strain pattern of the material. Therefore, it was concluded that the strain rate sensitivity was not a key process parameter in the forming process. Hence, the punch velocity was dropped from further investigations.

4.3.2. AA1050-O Yield Strength

The yield stress was determined using the 0.2% offset yield stress. This is an acceptable method for the determination of yield stress for materials such as aluminium, where the yield stress cannot be determined visually [21]. Figure 10 shows the engineering stress–strain curves for the specimens and the 0.2% offset lines for each specimen. The 0.2% offset was determined using an elastic limit of 69 GPa given by the supplier on the material datasheet [43] as well as the data provided in the Metals Handbook [41] and by test laboratories [42]. The average yield stress for the material was determined to be 33 MPa.

4.3.3. AA1050-O True Stress–Strain Curves

The next stage was the determination of the strain–hardening curve of the material. This was facilitated by converting the engineering stress–strain into a true stress–strain curve. Figure 11 shows the true stress–strain curves of the specimens.

4.3.4. AA1050-O Determination of Hardening Exponents

To determine the strain–hardening curve of the material, there is a need to apply the hardening laws to the true stress–strain curves, as illustrated in Figure 11. One such hardening law is the pure power-law strain hardening law [21]. This is also described as the Hollomon power law that can be converted into the Ludwik power law [22].

(1) $σ = σ_{o} + K ε^{n}$

Equation (1) shows the hardening power law applied in this study. In the equation, $σ$ represents the true plastic stress, $σ_{o}$ is the yield stress, $K$ is a hardening coefficient, $ε$ is the true plastic strain, and $n$ is a hardening exponent. Figure 12 illustrates how the strain hardening coefficients and exponents were determined. The hardening parameters were determined by drawing straight lines based on each specimen’s natural logarithms of the true stress and strain. Equations of the linear trendlines were then used to determine the gradient corresponding to the hardening exponent, with the y-intercept corresponding to the natural logarithm of the hardening coefficient.

Table 4 shows a summary of the hardening parameters for each specimen. The average value of $K$ was determined to be 110.89 MPa, while $n$ produced an average value of 0.2116. These parameters were then used as input data into the finite element simulation software.

4.3.5. AA1050-O Input Data into FEA Software

Figure 13 shows the hardening curve required as input for a finite element analysis (FEA) simulation software. The deep drawing process simulation experiments are based on the results of the material’s behaviour, thus improving the accuracy and reliability of the model.

4.4. The Implications of the Tensile Test Results for the Deep Drawing Process

The tensile test aims to determine whether the plastic region is sufficient to cover the deformation required during forming. Assuming the one-dimensional elongation of the product shown in Figure 14, a starting length of 2300 mm would need to be stretched to the overall length of the final product of approximately 27 + 118 + 1394 + 703 + 45 + 49 + 27 mm, which is 2363 mm, as shown by the cross-sectional view of the long side in Figure 14. This operation requires a strain of 2.67%, which is achievable at room temperature. This confirms that cold forming is feasible for such an operation; otherwise, warm or hot forming would be required. The tests also confirm that AA1050-O has relatively low strain rate sensitivity in the forming region, as shown by the results of the tests at different speeds. This eliminates the need to further investigate parameters such as tool velocity when a finite element analysis simulation is performed.

Figure 15 shows the overall width dimensions of the same product. The total width is approximately 1483 mm. Assuming one-dimensional elongation and starting with a length of 1400 mm, which is the internal dimension for the product, requires the length to be stretched to 1483 mm. This operation requires a strain of 5.93%, which is also achievable at room temperature, further confirming that the product can be made via cold forming.

5. Determination of Micro-Hardness Properties of AA1050-O

The hardness properties of a material have a significant effect on the forming process in sheet metal operations [23]. Material hardness has an impact on the formability, tool wear, spring-back, surface quality, and forming forces [47]. Tool wear and material fracture can be reduced by using a lubricant in the forming process regardless of the material hardness [48]. In this study, the material hardness of the aluminium alloy AA1050-O was experimentally determined using the Zwick-Roell Vickers Hardness tester ZHV series. The experiments were conducted using an ISO-6507-1:2018 standard [29].

5.1. Micro-Hardness Test Specimen Preparation

The test specimens were prepared for hardness tests using the dimensions shown in Figure 16 according to an AA1050-O aluminium sheet metal plate of 1.5 mm thickness. The specimens were cut using water jet cutting to minimise the effects of superficial work hardening and heating that could result in the alteration in the material parameters [44]. Additionally, the specimens were polished with a P1000 fine sanding paper as recommended, according to the ISO-6507-1:2018 standard [29].

5.2. Micro-Hardness Test Experimental Setup

Figure 17 shows the Zwick-Roell hardness testing machine that was used for the hardness tests, conducted at the University of Cape Town in South Africa. A 0.5 mm diamond 136º indenter was used with a load of 1 kg. A waiting time of 15 s was observed before the readings were taken and the indenter moved to another place according to the testing standard [29].

5.3. Micro-Hardness Test Results

Figure 18 shows the sample results recorded during the hardness tests. Though the Vickers Hardness number was automatically determined by the machine, the calculation was also verified using Equation (2) [29]. In the equation, $F$ represents the weight of the load used, while $d_{1}$ and $d_{2}$ are the diagonals of the indented impression. The results show that the hardness of the AA1050-O sheet metal is uniform and ranges between 20 HV and 21 HV. These results show that there is no significant difference between hardness and length scales in a homogeneous material, as corroborated by Rudnytskyj et al. [23].

(2) $Vickers Hardness = 0.1891 \times \frac{F}{d_{1} \times d_{2}}$

The Vickers Hardness test was also performed on the samples that had undergone strain hardening during the tensile tests. Samples measuring 10 mm by 25 mm were cut off the residual tensile test specimens. The samples were polished using a P1000 fine sanding paper as recommended according to the ISO-6507-1:2018 standard [29]. The results from the test indicated that indeed, the material had undergone strain hardening, as shown in Figure 19.

5.4. The Implications of the Micro-Hardness Test Results for the Deep Drawing Process

The Vickers Hardness ranged from 30 HV to 33 HV for the strain-hardened samples. This indicates that the hardness of AA1050-O increases by at least 50% after strain hardening from the original values of 20 HV to 21 HV. The hardness values show that the material is very ductile and that forming can be achieved with relatively less force. The increase in hardness also shows that the material undergoes strain hardening and increases in stiffness after forming.

6. Determination of Formability Properties for AA1050-O

The Formability Limit Curve (FLC) test experiments were performed following the ISO 12004-2 standard guidelines [37]. The experiments which were conducted applied the Nakajima test to determine the formability of the AA1050 through the Forming Limit Curve. Similar tests were conducted by Afrasiab et al. [28], Turkoz et al. [49], and Dilmec et al. [50], though they were working on different materials.

6.1. Formability Test Specimen Preparation

The test specimens of AA1050-O with a 1.5 mm thickness were prepared. The specimens were cut using water jet cutting to minimise the effects of superficial work hardening and heating that could result in the alteration in the material parameters [44]. Three specimens were prepared for the tests using the dimensions shown in Figure 20. Only one size of specimen geometry was used because the full description of the forming limit diagram is not critical. This was because the finite element software used in the simulation can predict the forming limit diagram using the tensile test results. The dimensions of the specimen also conform to the ISO 12004-2 standard used [37]. Li et al. [51] used comparable specimen dimensions in their study.

Three samples were prepared and labelled L1, L2, and L3, as shown in Figure 21. This was carried out to allow for the tracking of the samples and replication following the design of experiment guidelines by Montgomery [45] and by Niedz and Evens [46]. The samples were first marked with a grid before being loaded onto the machine. The deformation of this grid indicates the strain that occurs within the material after deformation. According to ISO 12004-2, the maximum grid size should be 2.5 times the material thickness, and in general, as a rule of thumb, the grid size should be between 1 mm and 2 mm [37]. A grid size of 2 mm was applied to the samples. The grid was applied by spray painting to avoid microstructural changes in the material. After that, the blanks were loaded onto the machine.

6.2. Formability Test Experimental Setup

Formability tests were carried out according to ISO 12004-2 standards for Forming Limit Curve (FLC) tests [37]. Figure 22 shows the sheet metal testing machine that was used to carry out the formability tests at the Fraunhofer IWU in Germany. The machine was mounted with digital cameras to facilitate the measurement of deformation. A 100 mm diameter hemispherical punch was used at a speed of 1.0 mm/sec to deform the material until it fractured. The test was carried out at a room temperature of approximately 24 °C.

6.3. Formability Test Results

Figure 23 shows the samples after the Nakajima tests. The results from the three tests were plotted in the Forming Limit Curves shown in Figure 24. A total of six strain paths and six strain points were identified in order to estimate the Forming Limit Curve. These points were based on a position-dependent method, as illustrated by ISO 12004-2 [37]; it was used based on the prevailing conditions just before necking.

Figure 24 shows the Forming Limit Curve for AA1050. The curve also shows the boundary lines as defined by the major strain $(ε_{1})$ and minor strain ${(ε}_{2})$ . The right-side boundary is defined by the line $ε_{1} = ε_{2}$ and is known as the equi-biaxial state of strain, while the left-side boundary is defined by the line $ε_{1} = - ε_{2}$ and defines the deep draw strain limit, as suggested by Emmens [11].

6.4. The Implications of the Formability Test Results for the Deep Drawing Process

The Forming Limit Curve is input into the finite element analysis software and determines the forming limit diagram of the process. Formability results are juxtaposed to this curve to determine the percentage of the sheet metal in different forming regions. This includes, for example, safe forming, thickening, excessive thinning, wrinkling, insufficient stretch, and fracture. Figure 24 shows the foundation and classification of all deep drawing defects in the finite element analysis. It shows the regions in which deep drawing is feasible and infeasible, as well as the shear region and the region in which the material would tear. The forming process parameters would need to be optimised to produce a good quality product. Optimisation can be achieved using FEA before verification through physical forming experimentation or prototyping. Figure 25 shows an example of an FEA simulation, showing formability parameters such as thickening, compression, insufficient stretch, safe forming, risk of splits, excessive thinning, and splits. The simulation was performed using AutoForm Forming R8 programme.

7. Conclusions

It is important to conduct material characterisation tests to improve the accuracy of material datasheets for use in an FEA simulation. Apart from enhancing the quality of FEA results, material characterisation tests can be used as screening experiments to investigate the feasibility of forming processes at room temperatures as well as the effect of strain rate sensitivity during forming. In this paper, the mechanical properties of AA1050-O were determined, and some were confirmed through physical experiments. Tensile, micro-hardness, and formability tests were identified as key experiments required prior to FEA simulations. The data from the physical experiments require further preparation and processing for subsequent upload into a finite element analysis simulation. Examples of the preparation and processing steps were outlined in the paper. This process improves the quality of data input into FEA programmes, thus also improving the reliability of the output from the process. The tests revealed that forming was achievable at room temperature and that the strain rate sensitivity was minimal in the forming region for the example product used. The forming temperature and tool velocity were thus excluded as key process parameters that required no further investigations during the FEA simulations. AA1050-O exhibited good formability characteristics with a yield stress of 33 MPa and an ultimate tensile stress of 65.64 MPa at a safe plastic strain of 25% before necking. The material also showed good formability properties judging from its hardening exponent of 0.2116; additionally, an increase of 50% in hardness after strain hardening gives strength to the components that are formed from this material. This means that this alloy is easier to process using different sheet metal forming processes for a variety of non-load-bearing applications in the automotive, aerospace, food, and chemical industries. While the data may be generalised for AA1050-O alloys, the level of variation in the alloy composition of the same grade of aluminium alloys may be a limitation to the generalisation of the results of this study.

Author Contributions

Conceptualisation, B.S.; methodology, B.S.; validation, M.N. and B.S.; formal analysis, B.S. and M.N.; investigation, B.S.; resources, S.M., A.S. and M.N.; data curation, B.S.; writing—original draft preparation, B.S.; writing—review and editing, B.S. and S.M.; visualisation, B.S.; supervision, S.M. and A.S. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

All data are contained in the article.

Acknowledgments

The authors acknowledge Stellenbosch University, University of Cape Town and Fraunhofer Institute of Machine Tools and Forming Technology, IWU. The views expressed and conclusions drawn are those of the authors and are not to be attributed to the institutions.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures and Tables

Figure 1. Typical tensile test specimen.

Figure 2. Typical stress–strain curve.

Figure 3. Typical stress–strain curve (source: ISO-6507-1:2018 [29]).

Figure 4. Typical forming limit diagram (source: Holmberg, Enquist, and Thilderkvist [38]).

Figure 5. AA1050 test specimen dimensions with 1.5 mm thickness.

Figure 6. AA1050-O tensile test specimens.

Figure 7. Tensile testing at Fraunhofer IWU in Germany.

Figure 8. Tensile test specimens after tensile testing.

Figure 9. AA1050-O engineering stress–strain curve.

Figure 10. Determination of yield strength for AA1050-O.

Figure 11. AA1050 true stress–strain curve.

Figure 12. Determination of hardening parameters for AA1050-O.

Figure 13. Average flow curve of AA1050-O, input data into FEA.

Figure 14. Cross-sectional view of long side of typical product.

Figure 15. Cross-sectional view of short side of product.

Figure 16. AA1050-O hardness test specimens.

Figure 17. The Vickers Hardness testing machine at the University of Cape Town in South Africa.

Figure 18. AA1050-O Vickers Hardness values before strain.

Figure 19. AA1050-O Vickers Hardness values after strain.

Figure 20. AA1050-O Nakajima test specimen dimensions.

Figure 21. AA1050-O Nakajima test specimens.

Figure 22. Sheet metal forming testing machine at Fraunhofer IWU.

Figure 23. AA1050-O specimens after Nakajima tests.

Figure 24. AA1050-O Forming Limit Curve.

View Image - Figure 25. An illustration of an FEA simulation showing (a) a forming limit diagram and (b) a visual image of the different formability parameters on a deep-drawn part.

Figure 25. An illustration of an FEA simulation showing (a) a forming limit diagram and (b) a visual image of the different formability parameters on a deep-drawn part.

Table 1

Chemical composition of aluminium alloy AA1050-O (source: EuroSteel [43]).

Element	Al	Mg	Si	Fe	Cu	Zn	Ti	Mn	Others
Weight %	≥99.5	0.05	0.25	0.4	0.05	0.07	0.05	0.05	0.03

Table 2

Physical properties of AA1050-O aluminium (source: EuroSteel [43]).

Physical Properties	Value
Density	2.71 g/cm³
Young’s modulus (E)	69.0 GPa
Poisson’s ratio	0.33
Melting temperature	645–660 °C
Thermal conductivity	210–230 W/(K m)
Coefficient of thermal expansion (20–200 °C)	23.6 × 10⁻⁶/K
Electrical conductivity @ 20 °C	34–36 Ms/m

Table 3

Summary of tensile test results.

	Test Speed	Gauge Length L₀	Ultimate Tensile Stress R_m	Elongation at BreakA_t	Thickness a₀	Widthb₀	AreaS₀
	mm/min	mm	MPa	%	mm	mm	mm²
Specimen 1	5	122.66	64.30	40.79	1.56	20	31.2
Specimen 2	5	119.64	65.06	41.83	1.56	20	31.2
Specimen 3	5	122.85	65.08	40.74	1.56	20	31.2
Specimen 4	10	124.40	67.44	39.43	1.56	20	31.2
Specimen 5	10	124.40	65.68	40.23	1.56	20	31.2
Specimen 6	10	122.35	64.85	40.90	1.56	20	31.2
Specimen 7	15	121.80	66.09	41.09	1.56	20	31.2
Specimen 8	15	122.76	65.40	40.77	1.56	20	31.2
Specimen 9	15	122.05	66.90	39.90	1.56	20	31.2

Table 4

Summary of AA1050-O hardening parameters.

	HardeningExponent (n)	ln K	R²	K (MPa)
Specimen 1	0.2092	4.6833	0.9985	108.13
Specimen 2	0.2134	4.7040	0.9984	110.39
Specimen 3	0.2046	4.6868	0.9992	108.51
Specimen 4	0.2139	4.7391	0.9982	114.33
Specimen 5	0.2116	4.7099	0.9979	111.04
Specimen 6	0.2126	4.6982	0.9980	109.75
Specimen 7	0.2117	4.7148	0.9986	111.59
Specimen 8	0.2156	4.7123	0.9972	111.31
Specimen 9	0.2116	4.7270	0.9984	112.96
Average	0.2116	4.7084	0.9983	110.89

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