Conclusion and perspective - Cooling rate effects on grain size, eutectic modification and Fe-b

The objective of this study was to investigate some features of casting and solidification of Al-Si alloys which are extensively used, in particular in automotive industry.

Control of melt preparation before casting is routinely made by thermal analysis with standard cups and dedicated to ensuring appropriate microstructure in terms of grain size and eutectic modification. The analysis of the characteristic features of cooling curves obtained from thermal cup was here extended to solidification of castings with various cooling rates.

Multivariate statistical analysis provided insight in the correlation between the various possible parameters and their significance. Thermal analysis showed the influence of nucleation time, eutectic cooling rate and DAS on the final grain size. In addition, thermal analysis experiments also indicated that the modification of eutectic silicon is not only a function of modifier element but also a function of cooling rates, eutectic time and combination of eutectic recalescence x eutectic time.

Further study with DTA for various cooling rates established the relation of cooling rate with eutectic modification level. Extrapolation data from DTA analysis of commercial alloys (A356) showed the presence of eutectic temperature depression, which usually relates to the effectiveness of eutectic modification as the effect of strontium addition. However, observation showed that eutectic depression is not necessarily accompanied with modified eutectic silicon. This result shows that the eutectic modification might be combination of growth blocking and hindering nucleation by modifier element.

While added in Al-Si die cast alloys, iron appears in mould cast alloys because of recycling. Emphasis has been put here on the most detrimental iron-rich intermetallic, i.e the so-called beta Al9Fe2Si2 monoclinic phase. Cooling rate effect on beta precipitation in Al-6.5Si-1Fe alloy was performed with DTA. The result showed that cooling rate has a significant effect on beta phase morphology. Low cooling rate produced long and thick beta precipitates. 3D analysis showed the beta precipitates grow in lateral direction in plate-like appearance. Most of the plates were nucleated on the skin surface and grow toward the center.

In situ tomography experiment revealed the nature of beta phase growth where the lateral growth dominated the growth mechanism. The growth was marked by high growth rate which slowed down after a saturation level was reached. The slowing down does not lead to a sudden increase of the thickening rate, the high solid fraction might have limited the thickening growth.

At very low cooling rates, it has been observed that Fe-rich precipitates appear both as

different areas in the samples. Chemical analysis, µ -XRD and EBSD all showed the Chinese-script precipitates to be β-phase as are the plates. This finding stresses the need not to differentiate microstructure analysis of Al-Si alloys on only shape recognition microscopic observations.

Finally, microstructure analysis of DTA samples showed the formation of some rosettes especially at high cooling rates which correspond to liquid that becomes isolated within the (Al) matrix during the solidification process. Because being separated from the interdendritic areas, solidification of these pools needs independent nucleation phenomena of Si and Fe-rich intermetallics. This means for most of them a significant undercooling which leads to a very fine multi-phase solidification microstructure. EPMA evaluation of rosette’s composition clearly illustrated the high undercooling mentioned above. Two original observations were made:

1. -phase precipitation.

2. Blocky β-precipitates were found in several rosettes which completed their solidification with very fine (Al)-Si eutectic. This shows that β does not nucleate Si as it has been previously reported.

Thermal analysis has shown itself as a tool that can be used to predict grain size and eutectic modification. It also showed the capability to identify phase reaction during solidification process. For future work, it would be of interest to study further the capability and sensitivity of thermal analysis with different chemical compositions, grain refiners and modifiers.

A model for beta phase growth has been proposed which still requires detailed calculation and could be developed further with numerical simulation. Furthermore, it would be of interest to utilize in-situ tomography for studying modification of eutectic silicon, adding a heavier element such as Zn to the (Al) solid solution to enhance the phase contrast between Al and Si.

Appendix

Appendix 1 Chemical Composition

Appendix 2 Thermal Analysis Data

Sand mould

EFModulusTN minTN maxVmaxΔTNt1t2ΔtnTe minTe maxTe2ranget3t4t coalest eutectictfTdCRLCReMLGSDAS 11.5615.4615.70.450.323.9836.5212.54568.2570.1553.562.2459.14951.06435.16491.92927.083.421.110.142.11.0589 1.15611.5612.20.450.718.9241.822.88563.5567552.260269.94594.34251.02324.4575.426.522.070.262.21.0571 1611.3612.10.450.812.3229.0416.72563.4566.6550.261.9181.5372.46169.18190.96360.146.922.630.392.30.9564 0.8613613.70.450.77.720.0212.32564.4568.2548.864.9120.34289.78112.64169.44282.085.324.270.562.70.9253 0.6610.5610.80.450.35.510.565.06560.3563.7545.265.665.34142.7859.8477.44137.289.828.641.133.80.7644 0.4609.1003.524.40.88558.8560.5535.373.827.9465.5624.4237.6262.0413.0213.182.914.20.5834 21.5615.8616.90.21.16210745.00568.8570.85104481.830.920.141.81.0487 1.156126140.42438441.00565.6568.6552623967973534017544.031.530.171.80.9681 1615.2616.60.31.4286133.00566.8570.1550.765.93196632913446352.532.200.212.20.9171 0.8612.5613.90.41.4275124.00565568.2548.9652395392123005124.432.250.282.30.9355 0.6614.4615.70.51.3162913.00565.5569.3541.174.61182661021482503.334.300.652.50.8348 0.4615.6615.70.10.1682.00564.3568.4540.275.576193701171874.238.600.843.70.5931 31.5614.96150.450.143.1250.167.04567.2570.2553.161.9437.58879.56394.46441.98836.443.491.590.151.70.6687 1.15614.90028.3837.49.02566.2570552.762.2288.86608.3260.48319.44579.923.691.890.2320.6479 1614.66150.450.420.6826.846.16566.2569.4552.862.2213.4466.84192.72253.44446.164.293.130.3120.6567 0.8614.4614.80.450.412.9819.86.82564.8568.2551.163.7114.18237.6101.2123.42224.625.494.020.642.10.5453 0.6613.6613.80.450.26.169.463.30564567.8551.762.176.12188.169.96111.98181.945.896.670.862.50.5047 41.5617617.10.410.134.5641.526.96564.9566.8551.465.7518.64960.96484.08442.32926.47.411.270.1430.7288 1.15616.3616.60.410.322.5635.2812.72564.9566.4550.865.8368.68745.2346.12376.52722.647.811.460.2030.7078 1615.8616.20.410.418.7232.6413.92565.1566.6549.766.5214.56453.84195.84239.28435.127.612.550.343.60.6270 0.8615.1615.90.410.81224.7212.72565.2566.1548.467.5139.2292.32127.2153.12280.328.115.420.564.80.5652 0.6614.2005.287.922.64563.8565.554965.285.2190.879.92105.6185.528.718.120.8550.5450 0.4613.1613.20.410.12.43.841.44561.5564.9542.27125.9257.8423.5231.9255.449.3115.832.955.80.46 51.5617.1617.60.450.546.6462.716.06568569.3552.365.3522.94983.18476.3460.24936.543.810.800.1330.5687 1.15616.86170.450.232.5642.910.34566.8569.1551.565.5355.74698.06323.18342.32665.54.011.290.1930.5474 1616.96170.450.122.2227.55.28566.5569.1551.865.2225.06487.74202.84262.68465.524.011.710.323.30.5370 0.8616.4616.60.450.214.7419.144.40564.6566.7549.966.7129.14242.44114.4113.3227.76.412.580.594.70.5358 0.6616.5616.80.450.38.1411.663.52564.4566.7550.766.178.32163.2470.1884.92155.16.415.680.9750.4848 0.4615.4-1.3607.048.141.10560.7535.579.938.7269.5231.6830.862.4812.415.002.405.60.4032 61.5615.3615.50.550.290.54100.449.90568.3570.1553.562594.361026503.82431.64935.464.020.770.1320.5795 1.15615.20054.958.683.78567.3569.9552.163.1341.1647.1286.2306592.24.221.460.232.20.4484 16150039.640.681.08566.7569.3551.863.2255.78528.84216.18273.06489.244.821.780.282.50.4680 0.8614.5614.80.550.346.0851.485.40565.7567.5550.164.7200.5430.02154.42229.52383.946.621.400.3930.4066 0.6614.2015.1216.020.90564.5566.9549.964.385.32174.0670.288.74158.947.225.220.923.70.3854 81.5614.4615.10.450.723.7644.4420.68565.5567.8554.260.9426.8828.08403.04401.28804.325.772.320.151.80.9079 1.15614.2615.30.451.114.7438.523.76564.4567553.861.5236.06474.76221.32238.7460.026.572.230.322.10.7068 1612.96150.452.111.4436.7425.30564.2567.2552.762.3224.4473.44212.96249.044626.374.320.292.30.6663 0.8612.2613.70.451.57.4823.3215.84563.2566.1550.163.6113.96247.5106.48133.54240.027.474.640.642.70.6754 0.6613.2613.60.450.44.8411.446.60562.6565.9551.861.874.8172.4869.9697.68167.647.677.420.9330.6551 91.5619.1004952.003.00566.2568.9551.467.74488253993777763.151.020.182.50.6292 1.15616003741.004.00563.1566.3551653366302992945935.751.300.222.80.4877 1617.4002425.001.00563.5566.2549.567.92084201842123965.852.550.363.50.4375 0.8617.3617.40.10.11416.002.00563.4565.4540.576.91192371051182236.653.750.663.60.4355 0.6618-0.4088.000.00563.4564.3537.480.66814560771377.753.201.1240.3645 0.4616.1-0.4033.000.00561.1563533.382.827622435599.0514.102.804.90.3436 01.5617.4619005610751.00565.9567.4554.364.74147133582996574.791.250.202.60.9480 1.15615.8617.30.31.5397435.00564.2565.5550.766.62684812292134426.691.570.322.80.8670 1616617.90.41.9255126.00564.7566.3549.568.42124011871893765.892.270.372.70.8463 0.8616.2617.70.41.5142612.00564.4565542.175.61162291021132157.193.250.692.80.8252 11.15618.2618.40.10.231398.00564.8567.4552.366.12514552202044244.881.220.322.60.7065 1617617.60.20.6213211.00564.2566.7551.266.41984101772123895.582.050.382.50.6557 0.8616.1616.60.30.512208.00563.1564549.267.4113207101941958.283.700.722.60.6058 0.6616.76170.30.36104.00562.9564.4547.469.66714561781397.884.701.143.50.5542 0.4614.9-0.60231.00560.2562.1535.179.8275325265110.180.002.8340.5031

Quick cup

REFModulusTN minTN maxVmaxΔTNt1t2ΔtnTe minTe maxTe2ranget3t4t coalest eutectictfTdCRLCreMLGSDAS 10.605611.2611.40.450.210.5616.55.94562.6564.9550.960.5126.88307.56116.32180.682978.624.820.5240.7453 20.605612.2612.40.10.212197.00564.2567550.561.91363341241983225.634.900.4630.655 30.605614.1614.20.450.115.419.363.96565.5568.3551.962.3151.8343.2136.4191.4327.85.393.710.4130.5158 40.605615.40012.4813.440.96564.8564.9552.163.3150.24304.32137.76154.08291.849.314.330.5160.4758 50.605613.80022.2224.642.42564.3566.3551.562.3146.52330.44124.3183.92308.226.813.380.473.50.4756 60.6056150024.4625.020.56566.8568.9552.162.9152.64334.08128.18181.44309.625.223.150.422.90.4657 70.605613.20013.213.420.22566.4568.6550.662.6137.28335.94124.08198.66322.745.003.790.452.80.454 80.605613.9614.30.450.411.4418.266.82563.7566.9552.861.5124.96297.88113.52172.92286.446.674.550.523.40.7753 90.605615.5-0.2020200.00562.7565.3550.864.71643421441783226.752.300.414.60.4157 100.6056130017.521.74.20561.5563550.662.4195.3413.7177.8218.4396.29.192.330.324.30.9563 110.605614.76150.710.313.1621.428.26563565.6551.463.6145.32323.96132.16178.64310.86.684.730.464.10.5259

Metallic mould

EFModulusTN minTN maxVmaxΔTNt1t2ΔtnTe minTe maxTe2ranget3t4t coalest eutectictfTdCRLCreMLGSDAS 11.15612.9006.613.867.26559.5532.280.757.6480.0851.0422.4473.4814.028.642.1440.7035 1613.5006.167.261.10559.956053083.550.1673.044422.8866.8813.526.592.664.50.6734 0.8611.7005.55.720.22557.5558.4526.685.132.5650.1627.0617.644.6615.129.324.044.70.6329 0.6610.4003.964.180.22556.4557.2520.989.522.6636.7418.714.0832.7816.3210.916.464.40.5828 0.4609.8-2.7202.862.860.00555.1555.8521.987.910.5618.487.77.9215.6217.7215.9111.234.90.4420 0.3607.8001.321.320.00552.3553.9517.490.48.1416.286.828.1414.9619.6249.5515.1750.4319 31.15614.1614.30.450.24.411.887.48564.9566.3540.174.263.1493.2858.7430.1488.887.3910.911.973.30.2543 1616.2003.086.383.30566.4567.4538.877.444.4467.9841.3623.5464.96.292.603.80.2337 0.8614.4005.285.720.44565537.177.329.4841.5824.212.136.38.6910.004.195.10.2231 0.6614.2005.065.060.00563563.1543.27122.6635.4217.612.7630.3610.5911.825.775.20.2328 0.4612.7001.541.540.00561563.352983.711.8823.9810.3412.122.4410.398.955.20.3622 0.3609.2610.83.181.61.322.861.54559.7563.7529.281.69.6820.688.361119.369.9929.099.9660.3521 41.15615.6007.28.641.44561.7562.6536.579.164.891.257.626.48411.615.081.9660.5938 1615.8005.286.240.96561.8564.9539.476.445.8465.7640.5619.9260.489.318.542.8260.4637 0.8614.1004.084.560.48560.3533.680.528.0838.642410.5634.5613.915.734.4160.5331 0.6612.4-1.6602.162.160.00560.5507.5104.915.8435.0413.6819.232.8813.7124.586.0560.5327 0.4611.1-0.8302.42.40.00556.752685.112.9622.5610.569.620.1617.5158.3312.5560.5021 0.3608.7-7.501.21.20.00552.251692.77.9215.126.727.213.9222.0159.5812.8660.4620 51.15614.10016.9418.921.98560.6561.9538.975.276.12105.4659.1829.3488.5211.214.911.9140.4737 1615.2009.910.560.66561.5530.484.852.1479.8642.2427.7269.9611.6110.672.674.30.4235 0.86140010.7812.541.76559.8561.9536.377.734.5448.6223.7614.0837.8411.214.855.644.30.4330 0.6613.5007.037.480.45559531.681.927.0641.820.0314.7434.7714.1117.566.104.50.3830 0.4612.2-1.8103.083.080.00557.6523.488.812.5420.689.468.1417.615.5118.6411.074.70.3721 0.3613.8-0.902.192.190.00557.1558.9533.280.69.919.587.719.6817.3914.2120.9112.7850.3321 61.15615.20.5501.985.043.06565.5565.6534.680.665.52100.4463.5434.9298.468.521.9240.4448 1613.91.9801.4412.0610.62563.9565548.465.546.2668.2244.8221.9666.789.122.614.30.3542 0.6614.5614.60.550.11.081.80.72562562.2533.181.522.8637.4421.7814.5836.3611.925.714.50.4127 0.4607.8608.51.10.70.92.71.80560.2527.481.112.619.811.77.218.913.927.444.70.3024 0.3612.3-1.101.081.080.00552.1514.697.710.617.289.526.6816.222.0210.9350.2921 71.15617.3008.589.020.44567.854275.371.596.5862.9225.08885.82.231.544.30.3245 0.8613.605.287.261.98564.9540.173.532.5645.127.2812.5439.828.78.413.544.80.2931 0.6612.2612.30.450.14.185.281.10563.9537.275.124.235.220.021131.029.711.144.7050.3328 0.4 0.3609.7611.26.811.53.744.841.10559.5563.3531.379.910.1215.186.385.0611.4410.324.0918.195.20.3021 91.15617.3-0.10770.00561.8562.2540.876.558785120719.8517.802.684.20.3735 1617.5-0.20660.00560.8560.953582.5446238185611.157.153.044.30.3331 0.8615.9-0.30550.00559.2560.4543.572.4304225123711.6514.105.024.40.3329 0.6615.60.50550.00557.7559.8534.481.2213216112712.258.005.534.50.2924 0.4615.7-20220.00556.5558.3531.384.41118971613.7519.5011.204.60.2921 0.3613.6-7.10110.00552.2553.5527.686712651118.5527.3016.104.80.2920 01.15618.3-0.10550.00563.2545.37354714917668.999.302.554.40.4037 0.8614.5-0.10660.00556.7531.583304324133715.494.184.244.80.4628 0.6614.7-0.30440.00557.4560520.993.8223418123012.1915.305.884.70.3327 0.4614.4-10220.00556.1558.8524.589.912211091913.3926.409.454.90.3022 11.15617.1617.30.10.26148.00564538.478.959765317708.2812.902.834.90.2636 0.8616.7-0.30330.00557.5529.387.4304327134014.7813.904.1750.2528 0.6614.1-0.40440.00554.852391.1203016102617.4813.806.5050.2629 0.4614.9-2.60110.00556.7557.8527.287.712201181914.487.5360.2526 0.3615.3-3.60110.00556.5558.3531.384712651113.9814.9016.6060.2420

Appendix 3 Assessment on Mondolfo’s eutectic temperature of A3xx alloys

One of the most used methods to evaluate the reference temperature of the (Al)-Si eutectic was proposed by Apelian et al. [1] who derived an equation expressing the eutectic temperature as a function of alloy's composition from the phase diagram compiled by Mondolfo [2]. The so-called “Mondolfo's equation” is still widely accepted as seen in the recent work by Wang and Lu [3].

Mondolfo's equation

In his work, Mondolfo equation evaluated the effect of six elements (Cu, Fe, Mg, Mn, Ni and Zn) on the (Al)-Si binary eutectic set at w_e_,_Si=12.5 wt.% Si and T_e_,_Si=577°C. The equation considered that the effect of these elements should be additive, so that the effect of each one could be evaluated based on the related ternary phase diagram. As an example, Figure 1 shows schematically the projection of the liquidus surface of the Al-Fe-Si system (right part of the diagram) and the evolution of the (Al)-Si eutectic temperature (left part of the diagram), i.e. along the line e_si-E_Fe, where E_Feis the three phase invariant eutectic point.

Figure 1. Schematic Al corner of the ternary Al-Si-Fe system

Assuming linearity of the temperature change (Te) along the two-fold eutectic-line, one has:

where w⁽_Fe^Al⁾^^Siis the iron weight content along the (Al)-Si eutectic line.

For any alloy with composition (w_Si, w_Fe) such as the one represented with the cross in the right side of figure 1, Mondolfo assumes that solidification of the (Al) primary phase leads to a liquid enrichment in Fe in proportion of

Si Si , e w

w . This corresponds to the arrow in the graph.

Accordingly, the (Al)-Si eutectic temperature for this alloy is decreased with respect to Te,Si

by: reference temperature for the start of the (Al)-Si eutectic reaction of an alloy is given as:

 

where the sum is extended to all X alloying elements.

Ternary Al-Si-X phase diagrams where X is Fe, Mg, Mn and Ni, are very similar, i.e with a esi-EXline that is at nearly constant Si. Data relative to these diagrams and relevant to the present work are listed in Table 1, where wÊ_Si^,^X and wÊ_X^,^X are the Si and X content of the ternary invariant eutectic, wÊ_X^,^S is the X content in solid (Al) in equilibrium with the ternary eutectic liquid and T_E,Xis the ternary eutectic temperature. The partition coefficients between (Al) and the liquid, kX, have been evaluated with the ternary eutectic data and are also listed in the table. Mondolfo used ternary phase diagrams mostly according to the extensive work by Philips [4]. Updated data were presently selected for the systems with Cu [5], Mn [6], and Zn [7], with Mondolfo’s original data then listed between brackets in Table 1. The last two columns list the aXvalues according to Mondolfo and to the present work respectively.

Using the partition coefficients in table 1, one can calculate the solidification path of Al-Si-X ternary alloys according to lever rule and Scheil's model. This is illustrated in Figure 2 in the case of Ni where is seen that Scheil and lever rule solidification paths are nearly superimposed. Interestingly enough, it is also noted that Mondolfo’s evaluation of the solidification path which is illustrated with the arrow lies close to these calculations. These observations apply to the other three elements, Mg, Fe and Mn. However, it is also clear from the graph in Figure 2 that the method applies only to alloy having their composition in the triangle Al-eSi-EX. Thus, Modolfo's method should be used only for alloys with a maximum content in X such that they precipitate the (Al)-Si eutectic before any other eutectic phase.

This maximum,w^max_X , depends on the alloy's Si content and is given as:

Table 1. Ternary eutectic points data of selected Al-Si-X ternary systems. See the text for definitions; compositions in wt.% and temperature in Celsius. Mondolfo's values are between

brackets when they have been updated.

Mg 12.95 0.85 4.96 0.171 555 4.43 4.43 1.036 4.59

Fe 12 0.05 0.7 0.071 576 1.43 1.43 0.96 1.37

Figure 2. Solidification path in the ternary Al-Si-Ni phase system.

For alloys with low level in alloying elements such as A356 where the total content in elements other than Al and Si is less than 1%, the accuracy of Mondolfo's equation is quite good in predicting the (Al)-Si eutectic temperature [8]. However, the equation fails to predict correctly in aluminum-silicon alloys with high Cu alloying content such as A319. ]. The reason for this is evidenced when considering the Al-Si-Cu phase diagram in Fig. 2b drawn according to He et al. [5]. It is seen that the (Al)-Si eutectic line moves far away to the left of the line at 12.5 wt. %Si along with increasing copper content. This leads to copper contents as estimated by the method much higher than the one when the solidification path reaches the (Al)-Si eutectic line. The maximum overestimation of the copper content is obtained for the ternary eutectic point when the method would give 2.5 (i.e. 12.5/5.0) times too high copper content. This means that the a_Cucoefficient as listed in Table 1 as “original factor” should be

the value at the ternary eutectic point, the correction cannot be that dramatic in all practicality.

With the experimental data analyzed below, it has been found that a multiplication factor of 0.75 gives appropriate results, i.e. aCu was set to 1.65 after accounting for the effect of Si ternary eutectic content, see Table 1. The Al-Si-Zn phase diagram is somehow similar to the Al-Si-Cu one in that the ternary invariant eutectic is located far away from the binary (Al)-Si eutectic, and in fact is very close to the Zn-rich corner of the phase diagram. For this system and owing to the low Zn levels in A3xx alloys, it seemed wiser to use the reported isopleth section at 5.3 at.%Si [7,9] to estimate the effect of Zn on the two-fold (Al)-Si saturation line.

The corresponding data is shown in Table 1 where it is seen that the final estimate of aZn

differs greatly from the one assessed with the Apelian’s method.

Figure 3. Solidification path in the ternary Al-Si-Cu phase diagram.

As a result, to accommodate the line changes, the updated equation are incorporated the changes of silicon content along the eutectic line and writes as follow:

  

Mg Fe Cu Zn Mn Ni



measurement. Therefore, the equation should be limited to chemical composition as follow:

Si : 1-12.5, , Mg 4.9, Fe  0.7, Cu  5, Zn  5, Mn  0.4 and Ni  5.

Other equations

There are several approach proposed to calculate the eutectic temperature. A different point of view was made by Morinaka [10] who introduced an equation for magnesium content calculation in aluminium alloy. Nevertheless, this work is appeared as another simplified Mondolfo approach which only considered the magnesium effect to the ternary eutectic temperature. A simple approach has been used by Vijayaraghavan et al. [11] that only considered the copper content in the A319 alloys obtained from the ternary Al-Si-Cu phase diagram.

w T 577.81.6

Studies by Joenoes and Gruzleski [12] which focused on the magnesium effect are proposed a series of empiric calculation with a coefficient depending on Si content in the alloys

Alloy Equation Max. wt% Mg

Al-7Si Te= 579 –17.6 Mg 1.01 (7)

Al-13Si Te= 576 – 6.9Mg 0.98 (8)

A 413.2 Te= 574.6 – 8.7Mg 1.25 (9)

Hausler and Schneider [13] also proposed a polynomial equation for Al-11Si base alloy made from the experiment with Mg content up to 1 %

e wMg w

T 5775.8 24.7 (9)

While those previous proposed equation only consider one element to the calculation and limited to one type of alloy chemical composition, Sthuldreier et al. [14] already considered 3 major elements in Al-Si-X ternary systems, which are Mg, Cu and Fe to the equation.



Mg Fe Cu



e w w w

T 57711 1.8 2.5 (10) Similar approach also proposed by Drossel [15] which create the equation using regression analysis from their experiment data.

However, Drossel limits the equation application to the following chemical composition:

Si 9.3 Cu2.5 Mg 0.6 Fe 1.15

Mn 0.4 Zn 0.63 Ni 0.43 Ti 0.05

Such an approach, that does not consider the enrichment in i of the liquid during primary (Al) precipitation.

A different approach was used derived by Djurdjevic, Sokolowski and collaborator [16,17]. They described the (Al) liquidus line with second order polynomial for Al-x systems of eutectic and then define a silicon equivalent (Sieq) for each element x. They considered that other element as part of the Al-Si binary phase which increases the total silicon content.

Sieq= wSi+i(ai+biwi+ciwi2

) (12)

which is used to calculate the liquidus of an alloy.



⁶^.¹¹ ⁰^.⁰⁵⁷ ²



The author limits the equation application to the following chemical composition for liquidus calculation as follow Si  12.6, Cu 10, Mg  10 and Zn  10 [18]. Then the enrichment of the liquid is described as did Mondolfo to calculate the eutectic temperature which writes:

 

_

where we,sihas been set to 12.3.

Comparison

One of the difficulties with treating thermal analysis data is the influence of cooling rate upon the eutectic depression. Experiment have showed that higher cooling rate causing a higher undercooling, therefore at higher cooling rate a lower eutectic temperature is detected by the apparatus. Other than that, there were two method normally used to extract the eutectic temperature from the cooling curve, which is TE,GAl-Si

and TE,NucAl-Si

. Unfortunately, the common practice of using T_E,Nuc^Al-Si is appeared inappropriate for determining the eutectic temperature. The method has the tendency to record the eutectic temperature at higher value than the actual eutectic temperature, as can be seen from a comparison to the CALPHAD calculation in Table 2. This phenomenon also clearly showed on thermal analysis cooling curve data with high cooling rate and high undercooling.

A comparison with other equation on calculated eutectic temperature, thermal analysis data acquired from several literature [1,16,19,20-28] and CALPHAD (thermocalc – TCAL1 database) with scheil calculation [27] is shown in Table 2.

Table 2. Comparison of several empiric calculations with experiment data and thermocalc database (Al)-Si (TCAL1)

9.98 0.01 0.12 0.01 0.01 0.00 0.00 576.7 576.7 576.6 577.5 575.5 576.6 - 24

8.03 1.09 0.14 0.00 0.00 0.00 0.00 573.9 573.4 574.0 574.9 575.0 573.5 TE^Al_,Nuc^^Si 25

*bold at ref column indicated that intermetallic phase is form prior the eutectic silicon reaction

Figure 4. Graph showing the correlation between experiment data and calculated eutectic temperature (the dotted line is the bisector)

Figure 4 plotted the result between the empiric calculated and experiment eutectic temperature. The graph showed that the equation 1 (Mondolfo’s equation) is not suitable to calculate an Al-Si alloy with higher alloying element, where the deviation becoming larger as the eutectic temperature drop due to the effect of high cooper content. While the equation from ref 14 (Stuhldreier equation) although have a low average, yet the data appears highly scattered and predict the eutectic temperature above the measured data for high Cu content.

Equation from ref 17 (Silicon equivalent) shows a good consistency, however their calculation is shows above the correlation line for most of the calculated temperature.

Furthermore, the equation also appeared fail to predict for an alloy with low addition element (the calculation showed their prediction value is above the eutectic silicon temperature). As for the proposed equation, it is showed a good agreement to the experiment, with deviation approximately ±1°C.

Reference

1. D. Apelian, G.K. Sigworth, K.R. Whaler: AFS trans., 1984, vol.92, pp. 297-307.

2. L.F. Mondolfo, Aluminum alloys: structure and properties, Butterworth, London, 1976.

3. R.Y. Wang, W. Lu, Spheroidization of eutectic silicon in direct-electrolytic Al-Si Alloy, Metall. Mater. Trans. A, 44 (2013) 2799-2809.

4. H.W.L. Phillips, Annotated equilibrium diagrams of some aluminium alloy systems, Monograph No. 25, Inst. Met., London, 1959

5. C.Y. He, Y. Du, H.L. Chen, and H. Xu, Experimental investigation and thermodynamic modeling of the Al–Cu–Si system, CALPHAD 33 (2009) 200-210.

6. Y. Du, Z. Jin, B. Huang, W. Gong, H. Xu, Z. Yuan, J.C. Schuster, F. Weitzer and N.

Krendelsberger, A thermodynamic description of the Al-Mn-Si system over the entire 14

7. K. Suzuki, M. Kagayama, Y. Takeuchi, Eutectic phase equilibrium of Al-Si-Zn system and its applicability for lower temperature brazing, J. Jpn. Inst. Light Met. 43 (1993) 533-538. (in Japanese)

8. G.K. Sigworth: AFS trans., 1983, vol.93, pp. 7-16

9. M.H.G. Jacob and P.J. Spencer, A critical thermodynamic evaluation of the systems Si-Zn and Al-Si-Zn, CALPHAD 20 (1996) 307-320.

10. M. Morinaka: US Patent No.6,345,910, 12 February 2002

11. R. Vijayaraghavan, N.Palle, J.Boileau, J.Zindel, R.Beals, F.Bradley: Scr. Mater., 1996, vol.35,7 , pp. 861-867.

12. A.T. Joenoes, J.E. Gruzzleski,: Cast Met., 1991, 4,2, pp. 62-72.

13. L. Hausler, W.Schneider: J.of Light Metals, 2002, 2, pp. 17-26.

14. G. Stuhldreier, E. Mettingen, K.W. Stoffregen: Giesserei, 1981, vol.68 pp.404-409 15. G. Drossel, Der einfluss von schmelzebehandlungen auf die dichtheit von gusskoerpon

aus Al-Si Gusslegierungen. Giessereitechnik, 27,1 (1981) 7-12

16. M.B. Djurjevic: Military technical courier, 2012, vol.60,1, pp. 152-168.

17. M.B. Djurdjevic, W. T. Kierkus, G. E. Byczynski and J. H. Sokolowski: AFS Trans., 1998, vol.47, pp. 143-147.

18. F.C. Robles Hernandez, M.B. Djurdjevic, W.T. Kierkus, J.H. Sokolowski: Mater. Sci.

Eng., A, 2005, vol.396, pp. 271–276.

19.S.Gowri, F.H.Samuel, Effect of alloying elements on the solidification characteristics and microstructure of Al-Si-Cu-Mg-Fe 380 alloy, Metallurgical and Material Transactions A, Vol.25A, 1994, p.437-448

20. J. Charbonnier: AFS Trans., 1984, vol.92, pp. 907-921

21. S. Farahany, A. Ourdjini, M.H. Idrsi, S.G. Shabestari: Thermochim. Acta, 2013, 553, pp. 59-68

22. Y.M. Han, A.M. Samuel, F.H. Samuel, H.W. Doty: Int. J. Cast Met. Res, 2008, vol.21, 5, pp. 371-380.

23. S. Thompson, S.L. Cockroft, M.A. Wells: Mater. Sci. Technol., 20, 2004, p.194-200 24. S.D. McDonald, A.K. Dahle, J.A. Taylor, D.H. StJohn: Metall. Trans. A, 2004,

Vol.35A, pp. 1829-1873.

25. M.B. Djurdjevic, G. Huber, Z. Odanovic: J. Therm. Anal. Calorim., 2013, vol.111, 5, pp. 1365-1373

26. M.B. Djurdevic, Z. Odanovic, N. Talijan: JOM, 2011, vol.63, 11, pp. 1-7

27. M.B. Djurdjevic, H. Jiang, J. Sokolowski: Mater. Charact., 2001, vol.46,1, pp. 31-38 28. Thermo-Calc Software TCAL1 database version 1.0, http://www.thermocalc.com

(Accessed 28 March 2013)

Appendix 4 Optical Microstructure

A356 – sand and metal mould series

Sample code No. 9

Sand mould Metallic mould

M 1.5

M 1.15

M 1

M 0.8

M 0.6

M 0.4

M 0.3

Sample code no. 10

Sand mould Metallic mould

M 1.5

M 1.15

M 1

M 0.8

M 0.6

M 0.4

M 0.3

Sample code no.11

Sand mould Metallic mould

M 1.5

M 1.15

M 1

M 0.8

M 0.6

M 0.4

M 0.3

DTA- flake to fibrous transition

0.6°C/min

1°C/min

6°C/min

30°C/min

DTA – various cooling rates DTA – Cooling rate 0.02°C/min

DTA – Cooling rate 0.05°C/min

DTA – Cooling rate 0.1°C/min

DTA – Cooling rate 0.2°C/min

DTA – Cooling rate 1°C/min

DTA – Cooling rate 2°C/min

DTA – Cooling rate 5°C/min

DTA – Cooling rate 10°C/min

DTA – Cooling rate 40°C/min

Quantitative analysis

Observed Microstructure Features

Cooling rates (°C/min)

0.02 0.05 0.1 0.2 1 2 5 10 40

Script phase 4 14 9 6 8 4 - -

-Rosettes - - 1 2 1 3 6 7 24

Appendix 5. List of publications

1. D. Ferdian, B. Suharno, B. Duployer, C. Tenailleau, L. Salvo, J. Lacaze, Differential thermal analysis assessment of beta phase precipitation in Al-6.5Si-1Fe Alloy, Transactions of the Indian Institute of Metals, Volume 65, Issue 6, December 2012, pp 821-825.

2. D. Ferdian, J. Lacaze, I. Lizarralde, A. Niklas, A.I. Fernández-Calvo, Study of the effect of cooling rate on eutectic modification in A356 aluminium alloys, Material Science Forum, Vol. 765- Light Metals Technology 2013, pp. 130-134

3. D. Ferdian, J. Lacaze, Evaluation of (Al)-Si Eutectic Reference Temperature of A3XX alloys, Material Science Forum, Vol. 790-791 (2014) - Solidification and Gravity VI, pp. 367 - 372

4. J. Lacaze, D. Ferdian, I. Lizarralde, A. Niklas, S. Eguskiza, A.I. Fernández-Calvo, Improved grain size prediction in aluminium-silicon alloys by thermal analysis, 71^st World Foundry Congress, 19-21 May 2014, Bilbao – Spain.

T E C H N I C A L P A P E R TP 2634

Differential Thermal Analysis Assessment of Beta Phase Precipitation in Al-6.5Si-1Fe Alloy

D. Ferdian^•B. Suharno ^•B. Duployer^• C. Tenailleau^• L. Salvo^•J. Lacaze

Received: 29 June 2012 / Accepted: 11 September 2012 Ó Indian Institute of Metals 2012

Abstract Iron-bearing intermetallic phases formed dur-ing solidification of Al–Si castdur-ing alloys are known for having detrimental effect on their mechanical properties.

This is particularly the case of the b-Al₅FeSi phase which precipitates as thin and extended plates. Many researchers already studied the factors that could influence the forma-tion of this phase and in most cases it has been concluded that low-level additives (e.g. manganese) may lead to the replacement of the beta phase with other intermetallics that are less harmful because of being more compact.In this preliminary work, differential thermal analysis (DTA) was used to study the effect of cooling rate (0.2–40°C/min) on

In document Cooling rate effects on grain size, eutectic modification and Fe-bearing intermetallic precipitation in hypoeutectic Al-Si alloys (Page 155-200)