Effect of Cu Concentration

Two XTGS experiments with the similar cooling rates (~1.2°C/s) were performed in Al-4Cu and Al-12Cu. The measured hydrogen concentrations from Ransley samples were approximately 0.25 ml/100gSTP. The experimental conditions are listed in Table 6.1.

Table 6.1 XTGS parameters, experimental conditions and material properties Conditions & Properties X-ray Radiography Parameters Expt. No. G

Selected frames from the real time images of Al-4Cu and Al-12Cu solidifying under identical conditions are shown in Fig. 6.2. In both alloys, the liquid starts solidifying from the hypoeutectic region and develops a dendritic structure (gray) directionally while interacting with the growing pores (white). As the columnar dendrites grow upwards against temperature gradients in both alloys, H and Cu solutes were partitioned from the primary Al into the interdendritic liquid with a ratio of ~0.173.

This led to a high concentration of Cu between dendrites, making these regions darker than the surrounding dendrites due to the high mass attenuation coefficient of Cu (see Chapter 4). Qualitatively, the primary and secondary dendrites are clearly visible in the Al-12Cu where there is a large fraction of Cu enriched eutectic. In the Al-4Cu, the Cu depleted primaries are visible, but the secondary arms are very difficult to resolve from the background noise since the x-ray attenuation variation is low. The enrichment of hydrogen in the liquid due to the growth of solids (according to a segregation coefficient of 0.1) resulted in such a large chemical potential (hydrogen supersaturation) that nucleation of pores occurs. Once nucleated, pores tend to propagate gradually along dendrites upward to absorb hydrogen from the liquid. In practice, one of them (P4) shows significant growth direction (downwards) and speed (large spurt within 0.2 s) in the last stage of solidification (fs = 0.9). To quantify the kinetics of pore formation, the evolution of pores in each slice can be extracted and registered as a function of temperature.

Fig.6.3a, b show the full field of view images at the end of solidification obtained from x-ray imaging of both Al-4Cu and Al-12Cu samples. Figs.6.3c, d show how pores grow as temperature decreases and solutes (Cu & H) enrich in the interdendritic region. The nucleation temperature for Al-4Cu and Al-12Cu are 625±24°C and 603±8°C, corresponding to a solid fraction of 0.48~0.86 and 0.37~0.54 respectively.

500μm T1

T2 P1

P2

P3 P4

P5

P6

P4-I

P4-II

500μm 500μm 500μm T1

T2 P1

P2

P3 P4

P5

P6

P4-I

P4-II

Fig. 6.2 Selected frames from the real time images of the observed pores in the solidification of Al-4Cu and Al-12Cu alloys obtained from XTGS experiments. T1 and T2 are the dendritic tips, P1-6 are pores nucleated at different stages, and P4-I and P4-II are highlighted to show the pore growing downwards to feed the shrinkage within ~0.2 s.

When pores nucleate, the dendritic morphology is already well developed and they have to grow in the narrow spaces between dendrites, as seen in Fig. 6.2. However, there is more interdendritic liquid in the Al-12Cu alloy, allowing pores to expand freely before the eutectic formation, while they are more constrained by the surrounding dendrite arms in the Al-4Cu alloy. The growth of three pores for each of the Cu levels was quantified using image analysis. The analyzed projections of the pores are shown in Figs.6.4a, b. In Al-4Cu, the shape of the pores seems more tortuous than that in Al-12Cu because of the lower copper concentration leading to less interdendritic space for pore expansion. All of the pores tend to follow the columnar dendritic structure and develop a final shape which is approximately ellipsoidal in Al-12Cu regardless their initial morphology, which is similar to the previous observations in Al-10Cu and Al-20Cu [156]. Pores can, however, split and merge into a single one due to curvature at the S/L/G triple junction in Al-4Cu.

Fig. 6.3 In situ observation of porosity formation in (a) Al-4Cu and (b) Al-12Cu. The colour rendered pores according to their temperature showing their evolution during solidification in (c) Al-4Cu, and (d) Al-12Cu.

The graphs at the bottom of Figs.6.4a, b show the evolution of the maximum dimension of each pore, with the colours matching those in the images. For the Al-4Cu (Fig.6.4a) all three pores grow very slowly after nucleation. One pore (lower LHS in red) shows a sudden spurt of growth close to the eutectic temperature. The graph shows that it grows almost entirely in the downwards direction (higher fraction of solid, fs), suggesting it might be expanding to feed the volume contraction of the eutectic below. The pore on the RHS also grows upwards (towards reduced fs) instead of downwards like the larger pore on the LHS does in its final sudden growth. It

Fig. 6.4 The quantified kinetics of porosity formation in (a) Al-4wt.%Cu and (b) Al-12wt%Cu.

The growth curve of each pore in the graph on each side is related to the corresponding pore (a, b) with the same rendered colour showing how the pore nucleates and grows during solidification.

should be noted that in the many experiments performed, the most frequently observed growth was towards lower fs regions.

For the Al-12wt.%Cu, pores nucleate at a much lower temperature (ranging from 610-595°C) but grow more quickly than in Al-4wt%Cu (compare Figs. 6.4 a and 6.4 b).

All the pores expand in size primarily at the low fs end, expanding in spurts on the same scale of the secondary dendrite arm spacing (SDAS). Perhaps the pores are constrained by the SDAS, building up pressure until they can push through to the next gap. Interestingly, the overall projected size of the pores in both samples is qualitatively similar, although the attenuation contrast between pores and metal in the Al-4Cu is much less than in the Al-12Cu, to a greater degree than can be explained by the additional attenuation of the higher copper concentration alloy. Therefore, it is proposed that most of the pores follow continuous growth curves and are mainly controlled by hydrogen diffusion while the large spurt of pore size is due to solidification shrinkage.

As pores only grow in the liquid during solidification, the amount of excess liquid (eutectic at the end of solidification) is one of the factors which affect pore size and morphology. SEM/EDX experiments were performed on the XTGS samples. No intermetallic or oxide impurities were detected in either samples. The measured average Cu concentration was 4.53 and 12.42wt.% respectively. Different amount of eutectic were obtained (3.7% for Al-4Cu and 18.5% for Al-12Cu), as shown in Fig.6.5. In addition, secondary arms (with a spacing of ~40 μm) are clearly observed in Al-12Cu (Fig.6.5b) while only a limited amount of secondaries are visible in the Al-4Cu casting (Fig.6.5a). This leads to the tortuous pore shapes in XTGS castings.