3.1. Structure-Property Relationship (Effect of Material Modification)
Some of the favourable aspects of selecting forging as a “near net-shape” manufacturing process is the reduction in defects common with casting (such as closure of pores), and in the case of the AZ family of Mg alloys, the refinement in size and distribution of second phase particles [
1]. Furthermore, the thermomechanical processing can act as a grain refinement mechanism and given sufficient temperature and strain rate, can promote dynamic recrystallization of the cast materials characteristic dendritic microstructure. This recrystallization process often has implications on the texture direction as well as its overall intensity [
3]. The effect that forging of AZ80 Mg has on the resulting microstructure is linked to several factors, of which, the process parameters (thermal and mechanical), material flow, and starting microstructure are considered to be most significant. More specifically, there exists a relationship between the forging temperature, strain rate, total strain and the level of material modification that occurs as a result of forging. As-cast materials, undergo the most significant modification of microstructure, as if sufficient conditions are met, the material will recrystallize from a cast-dendritic microstructure to a more equiaxed polycrystalline microstructure similar to other wrought forms of AZ80 Mg [
26,
27]. To further highlight the benefits of forging, General Motors prototyped a similar suspension component to that presented in this current study using a low-pressure die-casting process of AZ91 Mg alloy and the resulting yield strength and failure elongation were 92.2 MPa and 3.4% which can be considered to be in a class inferior to the forged materials investigated here [
28].
Figure 3 illustrates the relationship between ductility and yield strength of as-forged AZ80, processed via a variety of different forging temperatures, rates, geometries and starting microstructures. Furthermore, since local thermomechanical history of these aforementioned state variables can be highly non-uniform in nature throughout the forging process, being both a function of time and location within the die-forging, the reported properties have a wide range of temperature, strain rate and total deformation. Thus, they have been grouped into categories of starting microstructure for clarity in illustrating the structure-property relationship of forged AZ80 Mg. It can be observed that, in general, the strength is superior when the starting material for the die forging has an extruded microstructure. Relative to cast-forged material, the ductility is also enhanced, but to a lesser degree. The present study data which also utilized extruded base material, falls within the range of data from other studies within literature. The general trend that can be observed that higher strength material generally is less ductile. There exists one reported cast-forged condition which is grossly inferior to all others presented (σ
Y = 110.7 MPa, ε
FAIL = 7.2%), the reasoning being that its forging condition was not sufficiently favorable to recrystallize the cast-dendritic microstructure and modify the properties significantly from the base as-cast material (σ
Y = 92.8 MPa, ε
FAIL = 7.4%) [
16].
From a fatigue perspective, the structure-property relationship plays a similar role, where increased yield strength generally has a positive effect on improving fatigue life. However, since the nature of fatigue damage in AZ80 Mg is highly dependent on the nature of plastic deformation mechanisms, in certain cases there exists an intersection point in the property curves where the effect of the material modification becomes more complex and dependent on both the life domain and type of loading [
17]. For example,
Figure 4 highlights the traditional strain-life (ε-N) property curve that is generated using fully reversed strain-controlled fatigue tests for a variety of different forms of AZ80 Mg, cast [
3], extruded [
17], cast-forged [
3] and extruded forged [
17]. Please note that the forging conditions vary between each forged variety, as once again, the data have been grouped into categories of starting microstructure for clarity in illustrating the structure-property relationship of forged AZ80 Mg. It can be observed that there is a dramatic improvement in the life in as-cast material once forged, over the entire range of presented strain amplitudes. For the extruded material, however, it is well-known that the base material is anisotropic, thus the fatigue properties differ in each material direction, with the extrusion direction (ED) exhibiting a higher yield strength resulting in longer life in the high cycle fatigue (HCF) regime, and the transverse direction (TD) having enhanced ductility corresponding to lower cyclic stresses and longer life in the low cycle fatigue (LCF) regime. The transition threshold (or curve intersection point) between the ED and RD demonstrates when the effects of strength and ductility on the fatigue damage mechanism are approximately equal. Once forged, the extruded-forged material exhibits an enhancement in life in the HCF regime, displaying the longest life at strain amplitudes ε
A < 0.4%. The individual data for the current study are shown to illustrate the amount of variability relative to the fitted ε-N property curve generated using the well-known Coffin-Manson approach. This variability is linked to both the stochastic nature of fatigue, and more importantly, the variation in thermomechanical histories for the location for which each sample was extracted from the forging. This interaction of the process parameters upon the resulting as-forged properties will subsequently be discussed in more detail in
Section 3.2. Furthermore, the applications of utilizing strain amplitude as a damage parameter for forged AZ80 Mg will be discussed in
Section 3.3.
Similarly,
Figure 5 illustrates the relationship between total strain energy density (total SED: summation of plastic and elastic energy per cycle, for the stabilized response), and Number of Reversals to Failure. The same AZ80 Mg material conditions are presented, cast [
3], extruded [
17], cast-forged [
3] and extruded forged [
17]. Once again, the individual data for the current study are shown to illustrate the amount of variability relative to the fitted E-N property curve generated using the well-known Jahed-Varvani approach [
29]. Similar observations can be drawn to those from the ε-N properties, with the extruded-forged material exhibiting a longer life for a given strain energy density in the HCF regime, with a similar transition threshold between plastic and elastic behaviour being the salient deformation mechanisms. Of particular interest, however, is the apparent similarity in damage tolerance of the as-extruded material in both material directions (ED/RD) in the LCF regime. More specifically, the damage tolerance of the as-extruded material is insensitive to the material direction and can be considered to be isotropic at high levels of plastic deformation (E
T > 2.0 MJ/m
3), at least on the basis of using strain energy density as a fatigue damage parameter. This observation delves deeper into the structure-property relationship of AZ80 Mg, as within the same material condition, we can observe the effects of texture on the fatigue properties, and in what domain of fatigue (LCF/HCF) they are most significant. This variability in the individual data for the current study is once again linked to variation in thermomechanical histories for the location for which each sample was extracted from the forging, and the associated variations in the damage tolerance of the resulting material. The applications of utilizing total strain energy density as a damage parameter for forged AZ80 Mg will be discussed in
Section 3.3.
3.2. Process-Property Relationship (Effect of Process Parameters)
Intrinsic to the forging process are a number of state variables (primarily thermal and mechanical in nature) which can vary as a function of time (i.e., from beginning to end of the deformation), and geometry (location, and local material flow conditions). These temporal and spatial variations lead to differences in the properties throughout the forged product and depending on the process parameters and the component geometry, can be highly 3-dimensional in nature. The influence of the local forging direction (or direction dominated by compressive loading) on the resulting texture of the as-forged material was previously explored by Gryguć et al. [
16,
17] in medium-scale I-beam style forgings. Those studies concluded that the favourable texture modification due to forging was the alignment of the c-axis of the hexagonal-closed-packed (HCP) crystal to be parallel with the local forging direction (or direction of dominant compressive loading), which is often normal to the die surface in relatively thin forgings at areas closer to the surface of the forged component. Depending on the process temperature, and local thermal history (accounting for the heat of deformation), the material recrystallization process may be highly non-uniform and heterogeneous in nature, resulting in spatially varying microstructure, texture and thus mechanical properties.
Figure 6 illustrates the relationship between the primary thermal process parameter (the target forging temperature) and the resulting yield strength for a variety of forged AZ80 Mg. Similar AZ80 Mg material conditions are presented, cast [
3], extruded [
17], cast-forged [
3] and extruded forged [
17], and they are once again grouped into categories of starting microstructure for clarity in illustrating the process-property relationship of forged AZ80 Mg. It can be observed that there is a general trend (albeit it, quite weak) where lower forging temperatures produce material with higher strength. This relatively poor correlation will subsequently be discussed in relation to the recrystalization mechanisms present during the forging process. For wrought forms of base material (i.e., extruded), forging at higher temperatures can result in grain growth (resulting in lower strength) and lower temperatures the potential for shear cracking (from reduced ductility) [
30]. Of the studies presented, all were isothermal in nature (forging dies and billet are at the same temperature), and the forging temperature window ranges 250–450 °C. Beneath 250 °C, edge cracking and poor formability result as only a few deformation mechanisms become active, and large strain is not achievable [
31]. At temperatures higher than 427 °C, incipient melting of the Mg
17Al
12 eutectic phase will occur near the grain boundary [
13,
32]. This propensity towards incipient melting at higher processing temperatures for AZ series Mg alloys limits them from being extruded quickly, as compared to other structural metals, the processing window is quite narrow; however, this is not as limiting with die forging as the local deformation rates are typically smaller [
33]. Chaudhury et al. suggest for AZ80 Mg, a non-isothermal forging process with a temperature window of 290–400 °C for the billet, and 205–290 °C for the forging dies to avoid these aforementioned types of defects that can result in the as-forged material [
30] which agrees well with the trend seen in the current study.
The stress-controlled fatigue response of the closed-die forged AZ80 Mg automotive suspension component is illustrated in
Figure 7 for the two target forging temperatures that were investigated in this current study. It can be easily observed that for virtually all of the tests performed, for a given stress amplitude, the lower forging temperature (300 °C) provides a longer life in comparison with those components which were forged at 450 °C. In general, the lower forging temperature of 300 °C shows an improvement in life of ~3× longer at 180 MPa and ~7× longer at 140 MPa, with the more dramatic improvement in the HCF as the superior strength contributes to lower fatigue damage and less cyclic plasticity. However, the level of variation in the life for a given stress amplitude increases at the lower forging temperature, indicative of a less uniform fatigue properties throughout the forged component, which can be attributed to a similar effect observed by Gryguć et al. [
24] where AZ80 forgings done at lower temperatures exhibit more bimodal grain structures, with spatially varying precipitate content throughout the forging, and higher temperatures lead to increased microstructural homogeneity, lack of precipitate content and overall grain growth. This increase in variability in the fatigue performance of die-forged AZ80 Mg at lower temperatures is obvious here in the current study where the forgings were air-cooled with no additional heat treatment. However, other researchers have investigated controlling the precipitation of second phase particles of AZ80 Mg by immediately quenching the component following forging, then followed by a subsequent aging (T5) treatment, with promising results [
30]. In general, a significant component of the variability observed in the properties of the as-forged components stems from the air-cooling following forging, and the resulting variability in local cooling rates throughout the complex geometry of the forged component itself. For this current study, the as-fabricated condition (-F) was intentionally selected as it provides insight into the nature of the resulting microstructure from the forging process itself, providing a better basis for comparison of the thermal process effects. Furthermore, to maintain consistency with the majority of the studies on forged AZ80 in the literature that have been presented here for comparative purposes, we have also characterized the properties in the as-fabricated conditions. The variability resulting from air-cooling, and the effect of artificial aging on both the mechanical properties and fatigue damage mechanisms of closed-die forged AZ80 is an effect that complicates the forging process design and is the focus of current and ongoing work. Furthermore, the pre-heating of the billet prior to step 2,3 and 4 of forging (bending, flattening and die-forging), introduces some unintentional yet complex thermal history resulting in a “pseudo”-aging or solutionizing treatments to the billet prior to forging depending on the target temperature, as the billet is held at various temperatures for a few hours at each step. There inevitably is some impact on the precipitates as a result of this complex thermal history prior to forging, which is further complicated by the fact that the starting microstructure prior to the die forging is then significantly mechanically deformed which can break-up and refine the precipitates, compounding with the effects of DRX. Despite this, the thermal history is only part of the thermomechanical processing history, and the effects of the process variables which are mechanical in nature will be subsequently discussed.
Figure 8, illustrates SEM micrographs highlighting the precipitate morphology of two different locations within the forging that were tested at the same stress amplitude yet exhibited a significantly different number of cycles to failure. The precipitates are continuous in nature and range in both size and morphology, from smaller lozenge/blunted asymmetric hexagonal structure to larger slab-like structures, and they can be observed in the brighter contrast regions in the micrographs with the darker background being the parent matrix. Their chemistry is well-known to be Mg
17Al
12 which is quite common in AZ series Mg alloys whose Al content exceeds the critical limit of 6 wt%, thus resulting in precipitation of these intermetallic [
34,
35]. These findings agree well with those from Lai et al., who performed artificial aging studies on extruded AZ80 Mg and found that smaller lozenge-shaped precipitates (Widmanstätten) occur at aging temperatures of 150–300 °C, and the larger slab-shaped at 250 °C [
34]. These temperature ranges correspond well with the approximate temperature history that would have been experienced during air cooling of the forgings that were conducted at a target temperature of 300 °C. They also mention that the second phase sizes and morphologies which precipitate are highly dependent on the temperature and aging time, and in this study since air cooling was employed, both of these parameters would naturally spatially vary throughout the forged component and would be different in location #3 (
Figure 8a) and location #13 (
Figure 8b). Supporting this, is the observation that there is an apparent difference in the uniformity of the distribution in these continuous precipitates between the two locations. In location #3 (
Figure 8a) the precipitate distribution is somewhat striated in the middle of the micrograph, with a non-uniform volume fraction of the second phase, as compared with location #13 (
Figure 8b) which is much more uniform in nature across the entire image. Furthermore, from the higher magnification images, it can be observed that, in general, location #3 (
Figure 8c) has comparatively larger precipitate sizes relative to location #13 (
Figure 8d). A more refined precipitate structure with smaller spacing between particles leads to a higher propensity to resist dislocation movement when plastically deformed which is beneficial to the mechanical properties [
34]. Both effects of this increase in uniformity in precipitation distribution and decrease in size in location #13 (
Figure 8b) can contribute to an increase in strength, and fatigue life (as observed in the two samples tested at 160 MPa where there is a difference in life of about 6×) [
1,
34] The source of this variation in life has many contributing effects, the most significant of which are: firstly, the effect of the thermal process history on the microstructure and precipitate morphology discussed here; secondly, the equivalent strain imposed by the mechanical deformation during forging which will be subsequently discussed; and finally, the well documented stochastic nature of fatigue.
Figure 9 illustrates the relationship between the primary mechanical process parameter (equivalent plastic strain) and the resulting yield strength of AZ80 Mg after forging. The equivalent plastic strain is generally obtained from some form of numerical simulation of the forging process, the details of which are outside of the scope of this work but are presented for reference only to give a relative indication to the severity of the plastic deformation, and the amount of material modification that has occurred due to the process-related deformation. Some interesting observations can be made regarding the cast-forged materials which require between ε
eq ≈ 125–175% to achieve the elevated material property and transition from the as-cast domain of yield strength to the wrought domain of strength. This threshold of equivalent plastic strain (or mechanical deformation) for forging of as-cast AZ80 Mg develops the link between the process-related state variables and the resulting material properties, as previously there was poor correlation between this beneficial material modification and the forging temperature as observed in
Figure 6 and the unintuitive increase in both strength and ductility of some cast-forged studies be more similar to those of the wrought domain of AZ80 Mg material properties (seen in
Figure 3). Several researchers have found in hot deformation studies that as-cast AZ80 Mg requires a moderate to significant amount of deformation strain to recrystalize its dendritic microstructure, even if the process parameters are favourable to do so. Jiang et al. found in cast AZ80 Mg, that was compressively deformed at 350 °C to a strain of ε
Ture = 61% there still is ~9% of un dynamically-recrystallized (DRXed) microstructure, and the average un DRXed grain size continually decreases with increasing strain [
36]. Other studies, also found similar results, where even at strains of ε
Ture > 100%, the parent cast microstructure, still was not fully recrystalized [
26,
27,
37]. This correlation between increasing mechanical deformation and improvement in structural properties associated with a transformation from a cast-dendritic microstructure to a wrought microstructure resulting from the forging process is obviously only present when the starting base material is as-cast. The reliance on the forging process having a sufficient level of mechanical deformation to augment the material properties is much less important when the starting material is of the as-extruded varieties, as the microstructural modification is less intense [
16]. Hence, the modification in fatigue properties in the extruded then forged AZ80 Mg in the current study is predominantly due to the increase in strength from a modified texture, refined grain structure and evolved precipitate size/distribution/morphology. However, if the processing conditions during forging are not optimally selected, a dramatic impact can result in the strengthening mechanisms of each of these aforementioned contributing effects. More specifically, if the forging temperature is too high, then grain growth can occur which can result in lower strengths. If the temperature is too low, the pragmatic constraint of poor workability and inadequate die-fill will dominate. The processing window of the current work and the studies presented here is quite narrow and optimizing the forging parameters specifically to achieve the properties which are desirable for the service application is of utmost importance to achieve a holistic improvement in forging process design. Each of these effects and their impact on the property-performance relationship are highly coupled in nature; that is to say, they are commonly inter-related with one another and are frequently not mutually exclusive. Exploration of optimizing this property-performance relationship with a summary of the fatigue characterizations and modelling that was done in the current study will subsequently be discussed in more detail in
Section 3.3.