3.3 RESULTS AND DISCUSSION
3.3.1 Properties of cast products
Microstructure
The microstructure of five cast ingots is observed under an optical microscope. Figure 3.1a &
b shows the microstructure of the cast ingots a) at 5× magnification and b) at 20×
magnification. From the microstructure, it can be seen that three different zones have been formed, namely i) A as cored dendritic zones, ii) B as inter-dendritic zones, and iii) C as the fringe zone between dendritic and inter-dendritic zones. The presence of continuous grain at the edges of cast products has also been observed. The compositions of the cast ingots in all three zones have been studied for minimum 10 different locations and the average values have been reported here. Figure 3.1c & d shows the percentage of Cu and Sn in A and B respectively. It has been found that Cu:Sn ratio in A is changing from 85.7:14.3 at the center of the arms to 84.1:15.9 at the end of the arms and in the B it is found to be 71.0:29.0. Figure 3.1e shows the percentage of Cu and Sn in C and Cu:Sn ratio is found to be 73.7:26.3. Figure 3.1f shows the electron-mapping image inside and outside of the dendritic arms. From Fig.
3.1f, it can be seen that Sn content is higher in B than A. This microstructure is in line with the microstructure of peritectic Cu-Sn alloys published by Scott (1991) [21]. The copper-tin alloy phase diagram published by Acharya (2001) [88] confirms the compositional results obtained from EDX analysis in all three zones. From the phase diagram published by Acharya (2001) [88] it has also been found that the dendritic zone is in the α phase, the inter-dendritic zone is in eutectoid the δ phase and the fringe zone is in the β phase. Again, many authors reported that the α phase has FCC (face-centered cubic) structure and the β phase has BCC
reported that that the δ phase has SC (simple cubic) structure and is responsible for the brittle nature of cast ingots.
Figure 3.1 (a) microstructure of the cast ingot at magnification of 5×; (b)
microstructure of the cast ingot at magnification of 20×; (c) average composition in the α dendritic zone; (d) average composition in the inter-dendritic zone; (e) composition
in the thin zone; (f) electron mapping image
The increasing concentration of Sn from the center to the end of dendritic arms can be explained from the solidification process of molten bell metal. Solidification of molten bell metal is a complex phenomenon of simultaneous heat transfer and continuous solute precipitation termed as a thermo-solutal process [201]. The edge of the cast products cools
much faster and does a catastrophic change of the equilibrium conditions of molten bell metal.
High heat transfer in conduction mode from the molten bell metal to the mould cools the edge much faster in comparison to the other parts of the molten bell metal exposed to the surroundings due to the lower rate of convectional heat loss. Again, due to the higher melting point of copper, copper rich solutions solidifies at a much faster rate and start to precipitate in edges as the continuous α grains. Therefore, a very less amount of Sn could dissolve in the early precipitated grains. Early precipitated edge side α grains then act as a nucleus to precipitate more α grains to form columnar dendritic arms and continues to grow towards the center of ingots. As the α dendritic arm grows, the solidification time of dendritic arms increases and hence, more tin starts to deposit in the arms towards the end of it. The growth rate of dendritic columnar arms and parallel rate of fall of temperature of the molten metal are the major parameters controlling the final compositions of dendritic arms. As the solidification process goes on, a fringe zone surrounding to the α dendritic arms starts to precipitate as the β phase with a little higher Sn content than the α phase. Early precipitation of the α dendritic arms and surrounding β phase results in Sn rich solutions in the inter- dendritic zones, that solidifies as δ phase. Hence, the microstructure of cast ingots shows non- equilibrium solidification characteristics and form three different phases, namely the α phase inside the dendritic arms, the δ phase in the inter-dendritic zones and the β phase as fringe outside of the dendritic arms. The cumulative effects of the presence of these phases on the mechanical performance of bell metal products are discussed in the later sections.
Hardness
Hardness is measured in minimum 10 locations for each sample and average values have been reported here. Micro-hardness and bulk hardness are measured in etched and non-etched samples respectively. The average bulk hardness in cast ingot is found to be 125 ± 5HV. The micro-hardness in the dendritic arms of cast ingots (see Fig. 3.1.b) is found to be 105 ± 5HV and in the inter-dendritic zones, it is found to be 135 ± 5HV. Audy and Audy (2008) [65]
have reported an average hardness value of 180 ± 21.7HV in the dendritic α phase and 369 ± 52.3HV in the inter-dendritic eutectoid (δ) phase from the investigation of casted church bells manufactured in different eras; which are significantly higher than the value found in the present study.
A higher hardness value in the inter-dendritic zone compared to the α dendritic arms is attributed to the presence of the δ phase in the inter-dendritic zone (see discussion 3.3.1.a).
Hence, it is inferred that a higher δ phase will result in higher hardness values of cast bell metal ingots. Therefore, the lower hardness value of cast ingot selected in the present study from the study reported by Audy and Audy (2008) [65] may be attributed to the presence of more δ phase in the cast ingots. In general, the quantity of different phases present in cast ingots varies due to different cooling rates. Hence, there is a possibility that Audy and Audy (2008) [65] had used a different cooling rate than what is considered in the present study to solidify the molten metals from melting temperature to room temperature. Therefore, hardness values are different from each other. However, detailed investigations are required to ascertain the root cause of these differences.
Fracture strength at room temperature
The fracture test of 10 samples cut from five different cast ingots has been carried out in the UTM at room temperature to find the stress-strain behaviour of bell metal. Figure 3.2 shows the typical stress-strain behaviour of cast bell metal ingots under tensile and compressive loads.
Figure 3.2 Stress-strain behaviour of cast bell metal at room temperature
From the test results shown in Fig. 3.2, it can be observed that cast bell metal fails in the brittle fashion at room temperature under both types of load. The average value of Rmax_c
(maximum compressive stress) has found to be 850 ± 10MPa and the average value of Rmax_t
(maximum tensile stress) is found to be 200 ± 5MPa. The average value of E (modulus of elasticity) is measured to be 17.5 ± 1GPa. The average value of R0.2_c (compressive yield stress at 0.2% offset) is calculated from the stress-strain curve under compressive load and it is found to be 675 ± 5MPa. Nadolski (2017) [66] has reported Rmax_t values of 229.78MPa in cast ingots; which is 30MPa higher than the present study.
Brittle mode of failure behaviour in stress-strain curves under both tensile and compressive types of loads suggests that the δ phase formed in the ingots controls the fracture strength of cast bell metals over the α dendritic and β fringe phases.