Characterization of Al-Si-Cu-Mg alloys
3.1.1 MICROSTRUCTURAL ANALYSIS
Both C355 (Al-Si5-Cu1-Mg0.4) and A354 (Al-Si9-Cu1.5-Mg0.4) alloys are characterized by a typical hypoeutectic microstructure, consisting of α-Al dendrites surrounded by the eutectic structure. Representative optical micrographs of C355-HT and A354-HT alloys, with fine and coarse microstructures, are shown in Figure 3.1. At a macroscopic comparison, the two alloys present a different quantity of eutectic phase, due to the different Si content, about 5 and 9 wt.% for C355 and A354 alloys, respectively. At the same magnification, it is also possible to observe the different microstructural coarseness between fine (Figure 3.1a,c) and coarse (Figure 3.1b,d) SDAS samples.
Figure 3.1. Optical images at the same magnification of C355-HT fine (a) and coarse (b) SDAS specimens, A354-HT fine (c) and coarse (d) SDAS specimens.
As expected, neither in C355 alloy, nor in the A354, appreciable differences were observed by optical microscopy between heat-treated and overaged samples. Soaking at 210 °C for
a b
c d
250 μm
41 h influences the morphology of strengthening precipitates (as will be discussed in the next paragraphs) at the nanometric scale but it does not induce microstructural modifications that can be appreciated by OM. SDAS was calculated in all the tested alloys;
the results well corresponded to the expected, nominal ranges. Results are summarized in Table 3.1. Overaged samples were characterized by similar SDAS values than heat-treated ones, as expected.
Table 3.1. Measured SDAS values on C355 and A354 samples for tensile and fatigue testing Fine SDAS
Grain size of the alloys was evaluated by image analyses. As expected, both A354 and C355 fine SDAS samples were characterized by smaller grain size than the coarse ones, as summarized in Table 3.2. It is well known, in fact, that grain size is strongly related to solidification rate, as well as on the amount of refining elements, such as Ti and B. In this regard, it should be pointed out that, at a comparable SDAS range, A354 samples presented larger grain size (about two folds) in comparison to C355 alloy, as shown in Table 3.2.
Table 3.2. Grain size measured on fatigue samples of A354 and C355 alloys.
Alloy SDAS Grain Size
This difference should be related to the lower amount of Ti and B contained in A354 alloy, as shown in Table 2.1, but also to the effect of Si content on grain size. As reported in the literature, in fact, an increase of the Si content beyond 5 wt% may lead to grain coarsening[1–3]. Eutectic silicon was correctly modified, both in fine and coarse microstructures of the two alloys, characterized by fibrous eutectic Si particles.
RESULTS AND DISCUSSION –AL-SI-CU-MG ALLOYS
Nevertheless, some partially unmodified areas were found in coarse microstructures of both A354 and C355 alloys (Figure 3.2). As fine and coarse SDAS samples contain the same Sr quantity, it should be inferred that, as reported by different investigators on Al-Si alloys[4,5], solidification rate plays a role on eutectic modification. Even if it is known that the effect of solidification rate on eutectic modification in Sr-modified alloys is less relevant in comparison to unmodified alloys[5], Si particles are generally refined by an increase of the cooling rate, changing their morphology from large plates to fine fibers. Further, it is worth noting that unmodified coarse Si particles were mostly adjacent to α-Al dendrites. As observed by Sjölander et al.[6], this could be related to back diffusion of silicon during cooling from the dendrites to the pre-existing eutectic particles.
Figure 3.2. Optical images showing the morphology of eutectic Si particles in C355 alloy (a) fine and (b) coarse SDAS samples, and A354 alloy (c) fine and (d) coarse SDAS samples.
The core of microstructural analyses was focused on the observation and identification of large intermetallic particles formed during solidification. It is well known that not only alloying elements strongly influence composition and morphology of intermetallic particles, but also solidification rate plays an important role, especially on the size of intermetallic compounds[7]. SEM analyses revealed the effect of different solidification rates on intermetallics distribution in A354 and C355 alloys. SEM micrographs were elaborated and
a b
c d
50 μm
Figure 3.3. Contrast elaboration of SEM images of C355 (a) fine and (b) coarse SDAS samples, A354 (c) fine and (d) coarse SDAS samples showing differences in morphology, size and distribution of intermetallic phases induced by different solidification rates.
Image analysis on SEM micrographs was carried out to relate the global average area fraction covered by the intermetallic particles to the solidification rate. From analyses results reported in Figure 3.4, in both C355 and A354 alloys, fine and coarse microstructures presented similar intermetallic area fraction. Consequently, it was inferred that solidification rate does not influence the global area covered by intermetallic particles.
Further, from the comparison it is possible to observe that the average area fraction is slightly higher in the case of C355 alloy. The reason of this difference still remains unexplained. Anyway, due to its higher Cu content, the A354 alloy could contain a higher quantity of θ-Al2Cu particles in the as cast state, which are known to easily undergo dissolution after solution treatment. As a result, after solution treatment, this could result in a decrease of the total intermetallic area fraction.
a b
c d
RESULTS AND DISCUSSION –AL-SI-CU-MG ALLOYS
Figure 3.4. Average area fraction covered by intermetallic particles (IM) in heat treated (HT) and overaged (OA) fine and coarse A354 and C355 alloys.
Nevertheless, coarse microstructures were characterized by intermetallics with larger particle area, as depicted in Figure 3.5. Intermetallic particles, in fact, did not exceed 10 μm2 of area in fine SDAS samples, whereas they ranged between 70 μm2 and 100 μm2 in coarse SDAS alloys. This could be related to the longer available time for growth in coarse microstructure. This kind of relationship has been reported also in different studies on Al casting alloys[4,8]. Interesting results were obtained also from the evaluation of the average intermetallic aspect ratio (Figure 3.6), which is strictly related to the particle morphology. A significant difference was reported between fine and coarse SDAS samples, both for C355 and A354 alloys: the average aspect ratio of particles contained in coarse microstructure was about three-fold the one measured in fine microstructure. Quasi-globular or slightly elongated particles were observed in fine samples (AR ≈ 1,7-3), while more elongated and acicular particles (AR ≈ 5.8-7.1) were observed in the coarse ones.
0.0%
0.5%
1.0%
1.5%
2.0%
2.5%
3.0%
Fine HT Fine OA Coarse HT Coarse OA
IM Area fraction
A354 TensileC355 Tensile A354 Fatigue C355 Fatigue
Figure 3.5. Average intermetallics (IM) particle area in heat treated (HT) and over-aged (OA) fine and coarse A354 and C355 tensile samples.
Figure 3.6. Average aspect ratio of intermetallic particles (IM) in heat treated and over-aged fine and coarse A354 and C355 on tensile and fatigue samples.
Table 3.3 summarizes the main intermetallic phases found on both fine and coarse C355/A354 samples. As a general observation, in both alloys no relevant difference was detected in the intermetallics composition between heat-treated and over-aged samples, either in fine or coarse SDAS samples. The main effect of soaking at 210 °C for 41 h is reflected on submicron precipitates, thus observable only at higher magnification, as will be discussed in the next paragraphs.
0 30 60 90 120
Fine HT Fine OA Coarse HT Coarse OA
[µm2]
IM Particle area
A354 TensileC355 Tensile
0 2 4 6 8 10
Fine HT Fine OA Coarse HT Coarse OA