3.1. Mechanical Properties of Mg-Al-Ca-Mn Alloy
The stress–strain curves of Mg-Al-Ca-Mn alloy under compression at the strain rates of about 3200/s and about 5500/s are shown in
Figure 4, and the corresponding mechanical property parameters are given in
Table 1. The Mg-Al-Ca-Mn alloy shows a strong strain rate sensitivity when it is compressed in the ED direction (
Figure 4a). When the strain rate increases from about 3200/s to about 5500/s, the yield strength increases from 178.48 to 196.44 MPa, and the compressive strength increases from 572.40 to 739.42 MPa, increasing by about 30%. The total compressive strain related to the first compressive deformation also increases from 10.18% to 15.56%.
Note that when the samples are compressed along the TD (
Figure 4b) or the horizontal 45° direction (
Figure 4c), the change of the stress with increasing strain under the two strain rates are nearly coincident with each other, indicating a strain rate insensitive as compared to those samples compressed along ED. The corresponding values of yield compressive strength, compressive strength, and total compressive strain are all summarized in
Table 1.
By comparing the stress–strain curves of the three compression directions, it can be found that when compressed in the ED direction, the material shows the largest yield strength and compressive strength, a strong strain rate sensitivity, and the least maximum deformation, although the deformation degree changes the most significantly with the increase of strain rate; when compressed in the horizontal 45° direction, the maximum deformation of the material is the largest, but the deformation degree changes the least with the increase of strain rate; and when compressed in the TD direction, the strength of the material is similar to that when compressed in the horizontal 45° direction, but the maximum deformation is lower than that when compressed in the horizontal 45° direction, and the maximum deformation increases significantly with the increase of strain rate.
The difference of mechanical properties shown in the stress–strain curves is essentially the difference of deformation mechanisms, which is related to the deformation of different samples’ direction [
25]. The mechanical properties of Mg-Al-Ca-Mn alloy when compressed in the ED direction are obviously different from those in the other two directions, indicating that the mechanism leading to the material deforming in the ED direction is different from that in the other two directions.
With Φ10 mm × 10 mm samples, the mechanical behavior of the alloy under lower strain rates can be acquired with the same experimental setup. The stress–strain curves of Mg-Al-Ca-Mn alloy under high strain rate compression at the strain rate about 700/s and about 1400/s are shown in
Figure 5, and the corresponding mechanical property parameters are given in
Table 2.
According to
Figure 5, the Mg-Al-Ca-Mn alloy shows strong strain rate sensitivity when it is compressed in the ED direction. When the strain rate rises from 700/s to 1400/s, the yield strength grows from 87.90 to 94.37 MPa, and the compressive strength increases from 113.42 to 231.59 MPa. The deformation greatly increases from 3.01% to 5.65% with strain rate, and the hardening rate in the elastic stage also increases. The material is subject to secondary hardening after yielding. When the strain rate reaches 1400/s, the material undergoes secondary hardening, of which the hardening rate is apparently higher than that when the strain rate is 700/s. Then, the material turns soft obviously before it hardens again, of which the hardening rate is generally the same as that of the secondary hardening.
The strain rate sensitivity is relatively weak when compressed in the ND direction, and the curves almost coincide with each other before material yielding, without any marked change for the yield strength. The material is subject to secondary hardening after yielding. The hardening rates are approximately the same under the two strain rates. When the strain rate is 1400/s, the compressive strength and the deformation are significantly improved compared with those when the strain rate is 700/s.
The samples compressed in the vertical 45° direction also manifest low strain rate sensitivity. The yield strength does not obviously increase with the rise of strain rate, and the two stress–strain curves almost coincide with each other before yielding. The secondary hardening rates after yielding are close to each other, but the secondary hardening at the strain rate of 1400/s lags behind that at the strain rate of 700/s. The compressive strength and the deformation increase sharply with strain rate.
Distinct progressive variation can be found for the mechanical properties of Mg-Al-Ca-Mn alloy when compressed in the three directions of ED, vertical 45°, and ND. The largest values of yield strength, compressive strength, and deformation appear when the material is compressed in the ED direction, which is followed by the compression in the vertical 45° direction, and the smallest values appear when compressed in the ND direction. This indicates that the dominant compressive deformation mechanism transits from one to another when the load direction rotates 90° from ED to ND.
Figure 6 collects the stress–strain curves of Mg-Al-Ca-Mn alloy obtained in this study when the material is compressed in the ED direction at the strain rate of 700/s, 1400/s, 3200/s, and 5500/s separately. It can be seen from the figure that the Mg-Al-Ca-Mn alloy displays a strong strain rate sensitivity with the rise of strain rate when it is compressed in the ED direction and the deformation also increases significantly with the rise of strain rate. According to the pole figures in
Figure 3, the texture of Mg-Al-Ca-Mn alloy is weak with obvious deflection observed, which indicates that the grains in the matrix are oriented in random, and there are many grains showing soft orientations favorable for slip. A previous research also evidences that [
26] when the slip-dominated deformation occurs in magnesium alloy, the material exhibits significant strain rate sensitivity. Therefore, it can be preliminarily determined that the deformation mechanism of Mg-Al-Ca-Mn alloy when compressed in the ED direction is slip-dominated. When the strain rate is more than 3200/s, the stress–strain curves of Mg-Al-Ca-Mn alloy also show softening and secondary hardening after yielding, but the softening is not as obvious as that when the strain rate is 1400/s. A possible reason may be that the dynamic hardening effect is more obvious, although the material undergoes softening when the strain rate increases to a certain value. As a result, the stress–strain curve reflects a decreased slope and a decreased hardening rate, but the strength of the material is still rising.
3.2. Energy Absorption Performance of Mg-Al-Ca-Mn Alloy
In a crush resistance test using an automotive crash box, the common parameters for evaluating the energy absorption performance of a material are the peak load Pmax, total absorbed energy E, average crush force Pm, crush force efficiency CFE, specific energy absorption SEA, and average load Pavr. These parameters are expanded around the relationship y = P(s) between the crush force P and the deformation length s. Therefore, in order to evaluate the energy absorption performance of the Mg-Al-Ca-Mn alloy in this study, it is necessary to convert the stress–strain curve y = σ(ε) into the crush force–deformation length curve y = P(s).
The calculation of engineering stress and strain is as follows:
where P is the load; A is the original cross-sectional area of a sample; L
0 is the original length of a sample; and L is the length of a deformed sample.
Assuming that the sample volume V keeps unchanged during the deformation, the instantaneous cross-sectional area A(t) = V/(L
0 − s(t)). Yet, the engineering stress–strain is calculated based on the original cross-sectional area A. Therefore, in order to accurately convert the stress–strain curves into crush force–deformation length curves, the engineering stress–strain curves need to be converted into true stress–strain curves. The conversion equation for its corresponding time t is:
where P(t) = σ
true(t)∙A(t) and s(t) = L
0∙ε
true(t).
The P-s curves in each direction after conversion are shown in
Figure 7. The parameters for evaluating the energy absorption performance of the Mg-Al-Ca-Mn alloy when high strain rate is compressed in the three directions can be obtained through calculation. The results are shown in
Table 3,
Table 4 and
Table 5.
It can be seen from
Table 3,
Table 4 and
Table 5 that the total absorbed energy E, the average load P
avr, and the specific energy absorption SEA of Mg-Al-Ca-Mn alloy are higher when the strain rate increases. The total absorbed energy E reaches 1.98 J, and the specific energy absorption SEA reaches 53.66 J/g, which are the highest values for the Mg-Al-Ca-Mn alloy compressed in the three directions in this study. By comparing it with
Figure 4a, it is found that the reason why the total energy absorbed by the Mg-Al-Ca-Mn alloy is rapidly increased with strain rate when compressed in the ED direction is that it has a higher strain rate sensitivity. As the strain rate increases, the compressive strength increases greatly, corresponding to the increase of the crush force P in the P-s curve in
Figure 4a. The strain also increases significantly with strain rate, corresponding to the increase of the deformation length s in the P-s curve. In a physical sense, the integral area of the P-s curve is the total absorbed energy E. Therefore, a higher strain rate sensitivity means a substantial increase in the total energy E absorbed by the material.
Since the automotive crash box is generally compressed along the ED direction under service conditions, and the aforementioned Φ3 mm × 3 mm sample compression test has proved that the Mg-Al-Ca-Mn alloy shows the highest energy absorption performance in the ED direction compared to the other test results when testing at a large strain rate, so the following will not discuss the energy absorption performance in the TD and horizontal 45° directions but rather investigate the energy absorption performance in the ED direction and the underlying cause for its change. The P-s curves of Φ10 mm × 10 mm Mg-Al-Ca-Mn alloy samples compressed in the ED direction are shown in
Figure 8. Parameters to evaluate the energy absorption performance when the Mg-Al-Ca-Mn alloy is compressed at high speed in the three directions such as the total absorbed energy E, the peak load P
max, the average crush force Pm, the crush force efficiency CFE, the specific energy absorption SEA, and the average load P
avr are calculated. The results are listed in
Table 6.
Compared with
Table 3, the total absorbed energy of the Φ10 mm 10 mm sample is higher than that of the Φ3 mm × 3 mm sample, and with the rise of strain rate, the crush force efficiency decreases, indicating a significant influence of the sample size on the energy absorption performance. The specific energy absorption SEA is defined as the energy absorbed by the unit weight of material, which is less affected by the material size. It can intuitively reflect how the energy absorption performance of the Mg-Al-Ca-Mn alloy changes under different strain rates. Therefore, the specific energy absorption was employed in this study to compare the sample’s energy absorption performance of the Mg-Al-Ca-Mn alloy.
Figure 9 plots the changes of specific energy absorption (SEA) with strain rate when the Mg-Al-Ca-Mn alloy is compressed in the ED direction. It can be seen that when the alloy is compressed under a relevantly lower strain rate, the specific energy absorption rises slowly. However, when the strain rate increases, the specific energy absorption rises exponentially.
Based on the aforementioned study, it can be concluded that when compressed in the ED direction, as the strain rate increases, the total absorbed energy E, the crush force efficiency CFE, and the specific energy absorption SEA of the Mg-Al-Ca-Mn alloy are all greatly improved. The Mg-Al-Ca-Mn alloy shows the highest overall energy absorption performance in this direction compared to the other test results. The reason is that the Mg-Al-Ca-Mn alloy exhibits a higher strain rate sensitivity when compressed in the ED direction. Thus, the Mg-Al-Ca-Mn alloy in this study has a good development potential and a wide application prospect when used as an energy-absorption material for vehicles.