Strain rate sensitivity of the aluminium-magnesium-scandium alloy-Scalmalloy®

This work investigates the strain rate sensitivity of the aluminiummagnesium-scandium alloy Scalmalloy, which is used extensively for additive manufacturing of lightweight structures. This high strength aluminium alloy combines very good weldability, machinability and mechanical strength: it can be heat-treated to reach nominal ultimate tensile strengths in excess of 500 MPa. We report tensile tests at strain rates ranging from 10−3 /s to 103 /s at room temperature. It is well known that Al-Mg alloys exhibit a negative strain rate dependency in combination with serrated flow caused by the Portevin-Le Chatelier effect, which describes the interaction of Mg solutes with dislocation propagations. In contrast, in Al-Sc alloys, the flow stress increases with increasing strain rate and displays positive strain rate dependency. Additionally, the presence of Sc in the form of Al3-Sc provides a fine-grained microstructure which allows higher tensile and fatigue strength. This research shows how these combined effects interact in the case of Scalmalloy, which contains both Mg and Sc. Tests are performed at quasi-static, intermediate and high strain rates with a servohydraulic testing machine and a Split-Hopkinson tension bar. Local specimen strain was performed using 2D Digital Image Correlation.


Introduction
Selective Laser Melting (SLM) of metal structures in Additive manufacturing (AM) has the ability to manufacture complex geometries without special tooling. This favours the production of lightweight, high strength materials which are increasingly utilised in various fields of automotive and aircraft industry [1][2][3][4]. The dynamic loading due to crash and impact are frequently encountered in these applications. It is therefore a necessity to know the strain-rate dependency of the material to quantify crashworthiness.
This work studies the Scandium modified Aluminium-Magnesium alloy AA5028 which has been developed and patented by APworks and sold under the trade name Scalmalloy. According to the manufacturer, it is based on the AA5xxx series Al-Mg alloys with an Mg content of 3.2 -4.8 wt.-% and Sc 0.02 -0.04 wt.-% [5]. The addition of Scandium improves the strength, weldability and corrosion resistance [6]. The yield stress of the as-built material is approximately 350 MPa which can be increased by heat-treating to approximately 550 MPa [5]. These numbers, combined with the low mass density of 2.7 g/cm 3 , make Scalmalloy an interesting candidate for lightweight crash-absorbing structures. Its strain-rate sensitivity, however, is difficult to predict. Al-Sc alloys exhibit positive strain rate sensitivity with increased yield stress at elevated strain rates [7]. In contrast, Al-Mg alloys are known to exhibit negative strain rate sensitivity, at least up to moderate strain rates of 10 3 /s [8]. To answer the question on how these effects combine in Scalmalloy, we study its dynamic behaviour at four different strain rates ranging from 10 −3 /s to 1000 /s using servo-hydraulic and split-Hopkinson testing methods. Both as-built (non-heat-treated) and heat-treated specimens are investigated, and the resulting data is analysed in terms of the strain-rate dependency of Ultimate Tensile Strength (UTS) and strain at failure.

Scalmalloy
A commercial Laser Beam Melting system (EOS M400) equipped with a 1 kW laser unit (YLR-series, Continuous Wave (CW)-laser, wavelength 1070 nm) produced a total of 60 specimens from a single feedstock with dimensions of 20 × 120 mm 2 . The specimens were printed in a 90 degree orientation to the build plate. Building platform heated to 40°C acted as the base to print the specimens in layers of 60 µm. According to APworks recommendation, half of the batch was heat treated for 4 hours at 325°C. The specimen geometry shown in Figure 1 was CNC milled to dimensions. This geometry is chosen because it serves well with the constraints of our split-Hopkinson tension bar.

Quasi static tensile testing
The quasi static tensile tests were performed on the machine Instron 8801 system with 50 kN load cell at room temperature. The cross-head velocities of the machine are chosen to achieve strain rates of 0.001 /s, 0.1 /s and 10 /s. Nominal specimen strain was calculated using 2D Digital Image Correlation (DIC) with the commercial code GOM correlate software, with a resolution of 1 pixel = 0.026 mm. An airbrush gun is used to create a fine speckle pattern with an approximate speckle size of 0.1 mm. High speed camera (Photron SAZ) with 1024 × 1024 resolution recorded the deformation images. A virtual extensometer (on 2D DIC images) was defined over the parallel gauge region to calculate the nominal strain. The nominal strain could be evaluated up to the point of failure. From prior experience, the accuracy of the nominal strain measurement is better than 2% of the indicated value. Additionally, we evaluated the local strain in the necking region. This, however, was only possible for small local strains, as DIC then failed to correctly track deformation field. The beginning of deviation between local strain and extensometer strain was used to define necking strain, i.e., the end of uniform elongation. In this work, we always address nominal (engineering) stress, i.e., the ratio of force over the initial value of the specimen's gauge length cross-section.

Dynamic tensile testing
A split-Hopkinson tension bar (SHTB) was used to perform dynamic testing at strain rate of 10 3 /s. Figure 2 shows the configuration of SHTB apparatus. Compared to other SHTBs, this setup is optimized for low velocities, low forces and a long pulse duration of 1.2 ms. Input and output bars are 16 mm diameter aluminium rods. The striker is a hollow aluminium tube of 40 mm outer diameter and 20 mm inner diameter. Strain gauge 1 on the input bar and strain gauge 2 on output bar measure the incident wave, ε inc , and transmitted wave, ε tra respectively. Force is measured via strain gauges on the bars, connected diagonally in a Wheatstone bridge circuit to eliminate bending information. The Wheatstone bridge circuit is driven in constant voltage mode and its output is increased by a factor of 100 using an amplifier. This signal is recorded by a data acquisition (DAQ) card operating at 10 MHz and 16 bit resolution. The conversion factor from strain to force is established via a calibration procedure by placing a force censor between input and output bars, a static load is applied onto the bars. More details on the setup is described in [9]. Similar to that of quasi-static experiments the specimens are speckled with white and black spray paint with speckle resolution of 1 pixel = 0.067 mm. A high speed camera (Photron SAZ, 640 × 280 pixels and 100,000 FPS) is employed for recording the deformation images. Specimen strain was computed from 2D DIC in the same manner as that of the low strain rate experiments.

Tensile tests results
The nominal stress-strain curves are shown in Figure 3 for as-built and heat treated specimens. At strain rate 10 −3 /s, obvious serrations in the curves are visible for as-built specimens. These are presumably due to the Portevin-Le-Chatelier (PLC) effect, which describes the locking of a moving dislocation due to solute Mg atoms [8]. At higher strain rates, this effect diminishes as the diffusion speed of solute Mg becomes slow compared to the dislocation speed.
The characteristic properties like ultimate tensile strength σ max , strain at failure ε f , yield strength and necking strain are reported in Table 1. Note that the value of the strain rate at yield is uncertain for the SHTB experiment as yielding occurs during the rise of the loading pulse. Our quasi-static data is in agreement with prior work on Scalmalloy [10], where an Ultimate Tensile Strength (UTS) of 334 MPa and 540 MPa was reported for as-built and heat-treated specimens, respectively. The failure behaviour is brittle for the heat-treated specimens, with almost no necking. Whereas, as-built specimens exhibit limited necking with diameter at the failure location of 75% to the initial value.

Strain rate dependency
The characteristic quantities given in Table 1 are analysed to quantify the strain rate sensitivity. The maximum stress is well represented by a relationship proportional to the negative logarithm of the strain rate: The constant of proportionality for as-built is a = −4.9 ± 0.3 MPa, and for heat-treated is a = −2.9 ± 0.3 MPa. Uncertainty estimates are obtained from the residual error of the Levenberg-Marquardt algorithm used for data fitting. The dependence of failure strain on strain rate does not follow this simple Johnson-Cook type behaviour. Instead, a Cowper-Symonds relationship appears to fit the data better: For as-built (AB), we find ε f ail,0 = 0.223 ± 0.005, α = 0.009 ± 0.007, and β = 0.51 ± 0.10. For heat-treated (HT), we find ε f ail,0 = 0.109 ± 0.004, α = 0.009 ± 0.007, and β = 0.23 ± 0.13. In the strain rate interval 10 −3 to 10 /s both (AB and HT) the strain at the end of uniform elongation (beginning of neck formation) and the failure strain are unaffected. However at 10 3 /s, both failure strains increase. It can be said that this increase in ductility is the result of adiabatic heating. In conclusion, Scalmalloy exhibits a weak negative strain rate sensitivity for the UTS. The failure strain, however, increases along with the strain rate as shown in Figure 4.

Conclusion
This work investigates the strain rate sensitivity of the Sc-Al-Mg alloy Scalmalloy used for additive manufacturing purposes. Tensile tests ranging from 10 −3 /s to 10 3 /s were performed with servo-hydraulic and split-Hopkinson methods. It was found that the Ultimate Tensile Strength (UTS) decreases weakly (≈ 5 MPa / decade of strain rate) with increasing strain rate, while the yield strength remains constant. Whereas, the failure strain increases along with rising strain rate, which is likely a consequence of adiabatic heating. In general, thermal softening leads to increase in ductility and reduction of strength. In Scalmalloy, UTS decreases slightly with strain rate, which might be the combined effect of both strain rate hardening as encountered with metals, and thermal softening. However, the fact that yield stress is unaffected by strain rate, c.f. It is interesting to see how different strain rate effects of Sc and Mg combine in one aluminium alloy: the addition of Mg mainly leads to negative strain-rate sensitivity [8] at ≈ 10 3 /s of strain rate, while the addition of Sc incurs a pronounced positive strain rate sensitivity [11] for the UTS. For Scalmalloy, with alloying values of 4.4% Mg and 0.73% Sc by weight [10], these effects appear to mutually cancel out. We note that the observations made here only apply to strain rates of 1000 /s and below. One may speculate that, for even higher strain rates, a changeover from positive to negative strain rate exists, as is the case for the Al-Mg 5021 alloy [12].