A R C HIVE S of
ISSN (1897-3310) Volume 11 Issue 4/2011
F OUND R Y E NGINE E R I N G Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences
109 – 122
22/4
Manufacturing technology of high-quality pressure castings S. Pietrowski*, B. Pisarek, R. Władysiak Department of Materials Engineering and Production Systems, Technical University of Lodz 1/15 Stefanowskiego Street 90-924 Lodz, Poland *Corresponding author. E-mail address:
[email protected] Received 18.07.2011; accepted in revised form 27.07.2011
Abstract The paper presents manufacturing technology of pressure castings made of Al-Si alloy without porosity or with low microporosity of castings. It has been shown that the greatest impact on the porosity of the castings and the concentration of hydrogen has had the charge to the melting furnace. Liquidation or occurrence of a small microporosity of castings provides refining with solid refiners, nitrogen and modification of liquid alloy after various operations of preparing process. The liquid alloy stored in holding furnace should be refined once every 2 h with nitrogen. Authors developed a computer program of Al-Si alloys inspection with using of TDA method. The developed technology was verified under production conditions. Keywords:: Innovative materials and casting technologies, Pressure die casting, Porosity, Refinement, M odification
1. Introduction Generally known is that the pressure castings made of Al-Si alloy show the presence of gas porosity and shrink porosity . Both types of porosity can be minimized or completely eliminated by proper preparation of physico-chemical state of liquid alloy and the right construction of casting and its mould. At this work possibilities of minimizing or total eliminating the gas porosity were presented. The porosity is caused by the hydrogen, oxides, borides, nitrides and spinels and with harmful elements: Li, Be, Ca as well as with excess of Na, Sr and M g, which are present in Al-Si alloy. The biggest influence on the porosity, however, has the hydrogen. In Aluminium alloys primarily hydrogen is dissolved (about 8090%), followed by oxygen and nitrogen. The final amount of gas in Al-Si alloys depends on their initial content, atmospheric humidity, the type of melting furnace and its coverings, and the concentration of non-metallic inclusions. Hydriding of Al-Si alloys by their contact with water vapour
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occurs by the reaction 3H2+2Al→Al2O 3+6H. Formed in this way hydrogen goes through mainly into the aluminium alloy and in a small amount is released into the atmosphere in the molecular form. The smallest concentration of hydrogen about 0.01 cm3/100g, characterizes aluminium alloys that are melted in crucible-type or channel-type inductive furnaces. Al-Si alloys melted in electric resistance furnaces and gas furnaces contain about ten to twenty times more of hydrogen [19]. For example the table 1 presents the influence of the temperature on hydrogen diffusion coefficient D H [1-9]. It results that a rise in temperature of liquid aluminium much is increasing the rate of the D H diffusion. After crossing of temperature 745 In Figure 1 were shown an influence of the concentration of silicon and temperature of liquid Al-Si alloy on the coefficient DH [1-4].
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Table 1. Influence of the aluminium temperature on hydrogen diffusion coefficient D H Temperature, Hydrogen diffusion coefficient, DH*10-3, cm2/s t, C 670 0.7586 687 0.8670 745 1.3650 840 2.4380 900 3.6310 1015 4.7860 1045 5.6230
It shows that increasing the silicon content in aluminium reduces the solubility of hydrogen. The increase of liquid alloy temperature increases the solubility of the hydrogen in liquid metal. Overall, the reduction of the diffusion coefficient of hydrogen D H, increases its solubility in Al-Si alloy. Aluminium oxide (Al2O3) occurring in aluminium alloys increases the hydrogen solubility. This influence is increasing along with increasing the amount and the disp ersion of Al2O3 precipitates. It is believed that it is caused by the "mismatch" of crystal lattice of Al2O3 and Al. Due to the fact that oxygen atoms are strong electron acceptors, the hydrogen accumulates in these ”mismatch” areas. It is believed that these areas are also the "nucleus" the formation of gas bubbles [1-5]. The presented data suggest that in order to minimize the amount and pore size or total elimination of die casting is needed refining of liquid alloy with gas refiners (argon, helium) or solid one. The most preferred option is a simultaneous refinement of gas and solid refiners.
2. Results
Fig. 1. Influence of Si concentration and the temperature of liquid Al-Si alloy on hydrogen diffusion coefficient D H It shows that the increase of the temperature increases the DH ratio of liquid aluminium alloy. It decreases with increasing concentration of silicon to about 8.0% and then increases with further increase of silicon. Causes of this occurrence remained unsolved so far. Change of the solubility of hydrogen in Al-Si alloys in the function concentrations of the Si and the temperature were described in Figure 2 [1;2].
Fig. 2. Influence of Si concentration and temperature in Al-Si alloys to change the solubility of hydrogen
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The most often in pressure foundries technological process of producing castings consists of the following operations: loading the charge materials to the melting furnace → melting of alloys → removal of slag from the surface of the liquid metal → pouring of Al-Si alloy into the transfer ladle→ transport of the ladle to the holding furnace at the casting pressure machine → pouring of liquid aluminium alloy into the holding furnace → pressure die casting. M ost of Al-Si alloys are melted in the gas or oil furnaces. Due to the smelting of Al-Si alloys in gas or oil furnaces it follows increased gassing of alloys, that require absolutely of their refining if you want to get pressure castings at least of average quality or slightly below of that quality. The following is a preparation technology of liquid Al-Si alloy and its control method ensures obtaining high-quality of pressure castings without porosity or with negligible microporosity. It concerns the Enterprise Innovation - Implementation WIFAM A PREXER LLC. in Lodz. The company produces castings with using 226 and 231 Al-Si alloys of which average chemical composition shown in Table 2. It shows that alloy s 226 and 231 have different levels of silicon and copper. Aluminium alloy 231 contains about 11.0% Si and 0.9% Cu, ie increased amount of Si, and Cu decreased in comparison with the 226 Al-Si alloy. Figure 3 shows the effect of the charge type into the melting gas furnace, on the porosity of the control sample without refining and after refining with use of solid refiner ECOSAL-Al113. Refiner was fed on a liquid surface of alloy in the melting furnace. From Figure 3 results that the type of furnace charge and refining significantly affect the porosity of the castings. Without refining, the lowest porosity P = 2.82% was obtained for a batch consisting of 100% of pigs silumin 226 or 231, and the highest P = 6.28% for the batch made from 100% scrap recycle. Refining with ECOSAL-Al113 in the melting furnace according to the decreased porosity P = 2.0% and P = 4.23%. A similar effect occurs in the change of hydrogen concentration as shown in Figure 4. The lowest concentration of hydrogen H2 = 0.42 ppm found in alloy unrefined and smelted from charge containing
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100% of metallurgic pigs. The largest amount of H 2 = 2.25 ppm obtained in alloy smelted in 100% recycle from scrap. After refining it amounts adequately H2 = 0.20 ppm and 1.15ppm. It follows from the obvious fact that the porosity of castings increases with increasing concentration of hydrogen in Al-Si alloy. For example, the minimum and maximum porosity of the
sample after refining with ECOSAL-Al113 is shown in Figure 5 (a, b). The control of Al-Si alloy with TDA method showed significant differences as the course of the crystallization curve for the minimum and maximum porosity, how it was shown in Figure 6 (a, b).
Table 2. The average chemical composition of 226 and 231 Al-Si alloys Chemical composition, %weight Type Si Fe Cu Mn Mg Zn Ti 226 9.65 0.72 2.45 0.22 0.20 0.82 0.04 231 11.35 0.65 0.90 0.18 0.17 0.45 0.05
7
P, %
5
Pb 0.07 0.05
Ni 0.07 0.03
a)
Al-Si alloy unrefined Al-Si alloy after refining
6
Sn 0.03 0.01
4 3 2 1 0
100% pig
30% scrap
50% scrap
70% scrap
100% scrap
Fig. 3. Effect of the charge type and refining with ECOSALAl113 in the melting furnace on the porosity of the test sample
b)
2.5 Al-Si alloy unrefined
H2 , ppm
2 Al-Si alloy after refining
1.5 1 0.5 0
100% pig
30% scrap
50% scrap
70% scrap
100% scrap
Fig. 4. Effect of the charge type and refining with ECOSALAl113 in the melting furnace on hydrogen concentration in the test samples
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Fig. 5 (a,b). M inimum (a) and maximum (b) porosity of the test sample cast from the melting furnace after refining with ECOSAL-Al113
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a)
b)
a) Characteristic points
b)
Characteristic points
Fig. 6 (a,b). TDA curves for Al-Si alloy with minimum (a) and a maximum (b) porosity
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It results from here that the derivative curve of Al-Si alloy with the minimum porosity has smooth course (a), and with a maximum porosity (b) the curve has a shape like a saw blade. M any thermal effects exist in eutectic crystallization area. Therefore, based on the crystallization curve can be qualitatively assess the porosity of the casting. After pouring of liquid alloy to the transport ladle, at the same time was made the procedure of refining with ECOSAL-Al113 and nitrogen device with a rotating head. The porosity of the sample and the hydrogen concentration before and after the procedure are shown in Figure 7 and 8.
7
a)
Al-Si alloy unrefined Al-Si alloy after refining
6 5 P, %
The Figures 7 and 8 shows that refining in ladle with solid refiner and nitrogen causes further reduction of the porosity of castings and the concentration of hydrogen. A further influence of the kind of the metal charge to the melting furnace on the value of these quantities is characteristic feature of this process. After these treatments the minimum and maximum porosity of the samples was shown in Figure 9 (a, b). Corresponding TDA curves are shown in Figure 10 (a, b).
4 3 2 1 0
100% pig
30% scrap
50% scrap
70% scrap
100% scrap
b)
Fig. 7. Effect of the charge type to the melting furnace and refining with ECOSAL-Al113 and nitrogen with used a rotating head device in the transfer ladle on the porosity of the test sample
2.5
Al-Si alloy unrefined
H2 , ppm
2
Al-Si alloy after refining
1.5 1 0.5 0
100% pig
30% scrap
50% scrap
70% scrap
Fig. 9 (a,b). M inimum (a) and maximum (b) porosity of samples after refining in the ladle
100% scrap
Fig. 8. Effect of charge type to the melting furnace and refining with ECOSAL-Al113 and nitrogen with used a rotating head device in the ladle on hydrogen concentration in Al-Si alloy
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a)
b)
b) Characteristic points a) Characteristic points
Fig. 10 (a, b). TDA curves for Al-Si alloy with a minimum (a) and a maximum (b) porosity after refining in the ladle
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Derivative curves confirm the presence of significant porosity in castings. After refining in transfer ladle, the aluminium alloy is pouring into the holding furnace at the station of pressure casting machine. During pouring a modification of Al-Si alloy was applied and after of pouring out a refining was performed with nitrogen. The changes of porosity of castings and of the hydrogen concentration in them are shown in Figure 11 and 12.
7 6
P, %
5
Results from them a further reducing of porosity and hydrogen concentration in casts, in addition to a large extent they are made conditional on the type of metal charge to the melting furnace. Examples of minimum and maximum porosity are shown in Figure 13 (a, b). a)
Al-Si alloy unrefined Al-Si alloy after refining
4 3 2 1 0
100% pig
30% scrap
50% scrap
70% scrap
100% scrap
Fig. 11. Effect of the charge type to the melting furnace and the modification and refining in the holding furnace of Al-Si alloy on the porosity of castings
b)
2.5 Al-Si alloy unrefined
2
H2 , ppm
Al-Si alloy after refining 1.5 1 0.5 0
100% pig
30% scrap
50% scrap
70% scrap
Fig. 13 (a, b). M inimum (a) and maximum (b) porosity of castings after modification and refining in the holding furnace of Al-Si alloy
100% scrap
Fig. 12. Effect of the charge type to the melting furnace and the modification and refining in the holding furnace Al-Si alloys on the hydrogen concentration in castings
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TDA curves for minimum and maximum porosity of the castings were shown in Figure 14 (a,b).
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a)
b)
a) Characteristic points
b) Characteristic points
Fig. 14 (a, b). TDA curves for Al-Si alloy with a minimum (a) and a maximum (b) porosity after modifying and refining in a holding furnace
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They show a slight disorder on the crystallization curves for zero porosity of castings. The study showed that, in order to minimize or eliminate porosity of aluminium alloy, it should be make of refining of the liquid alloy every 2 hours in a holding furnace with nitrogen.
d)
For example, Figure 15 (a-f) shows the microstructure and the porosity of the castings made without the presented technology. a)
e)
b)
f)
c)
Fig. 15 (a-f). M icrostructure and porosity of castings made of a unrefined and a unmodified Al-Si alloys; microstructure: α phase, AlFeSi, Al2Cu, eutectic α + β Example microstructure after refining and modifying the silumin castings is shown in Figure 16 (a-i).
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a)
d)
b)
e)
c)
f)
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i)
g)
Fig. 16 (a-i). Example microstructure of die casts after the next treatment of refinement and modification
h)
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The comparison of microstructure and of porosity of castings from figure 15 and 16 shows that the technology of preparation of liquid Al-Si alloys to receive pressure casting ensures the production of castings without porosity or low microporosity. Presented in individual operations TDA curves of silumins are a part of control system of their quality . Authors developed a control computer program of Al-Si alloys using the TDA method. It is used to control the correctness of procedures the refinement and modification in individual operations of preparation of liquid aluminium alloys. Below, for example, in Figure 17 (a-g) the monitor screen shots of silumins control in the holding furnace is shown.
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a)
b)
c)
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d)
e)
f)
g)
Fig. 17 (a-g). Screenshots of the monitor of the control system of silumins in a holding furnace (a), quality alloy identification or accuracy of analysis; commands and technological treatments (b-g); b) correct alloy; you can pour moulds; silumin compatible with the technology, c) silumin incompatible with the technology; repeat the procedure of refining; refined alloy in the furnace, d) silumin incompatible with the technology; repeat the procedure modification and refining; to enter the recommended **% ** amount of refiner and to refine the alloy in the furnace, e) very small amount of metal in the probe, not pour moulds, repeat the test in the new probe, f) pouring temperature is too low; do not pour moulds; heat up the pouring cup or heat up the alloy in the furnace, repeat the test in the new probe, g) lack of properly found characteristic points of the graph; determine the characteristic points manually, or repeat the test
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It results from presented data that the program determines properties of prepared silumin: Rm, A 5, HB, P, ppm H2 and is suggesting operations one should do which, if prepared alloy isn't in accordance with the technology.
3. Conclusions
From the tests of presented work result following conclusions: decisive influence on the porosity of the castings and the concentration of hydrogen in them is to charge material into the melting furnace, after various technological operations, the liquid alloy should be refined with solid refiner and nitrogen, and during transfer to the holding furnace extra modified, Al Si alloy in holding furnace should be refined with nitrogen every 2 hours, above treatments eliminate the porosity of casts or appearing at them of small microporosity, effect of treatments performed should be controlled by developed author’s computer program using the TDA method.
References [1] S. Pietrowski: Silumins, Wydawnictwo PŁ, Łódź, 2001. (in Polish) [2] D.F. Czarnega, O.M . Bjalik, D.F. Iwańczuk, G.M . Remizow: Gazy w cwietnych metałłach. M etallurgia, M oskwa, 1982. [3] B.P. Buryliew: O primienienii teorii reguliarnych roztworow w żidkim spławom kremnija s elementami II-V periodow. Izw. Wyz. Uczeb. Zawied., Czornaja M etałłurgia, nr 8, 1963, s. 35. [4] T.K. Kostina: Rastworimost wodora w żidkich spławach nikelija s kremniem. Cwietnaja M etałłurgia, nr 3, 1970, s. 22. [5] M .B. Altman, E.B. Głotow, R.M . Rjabinina: Rafinirowanie aljuminiewych spławow w wakumie. M etałłurgia, 1970. [6] Tłumaczenie z angielskiego. M etałłurgia, 1968. [7] R.M . Gabidullin, W.A. Zasypkin, W.D. Juszin, W.N. Titow: Aluminiewyje spławy. M etałłurgia 1968. [8] N.S. Postnikow, W.W. Czekasow: Progresiwnyje metody pławki i litja aluminiewych spławow. M etałłurgia, 1973. [9] L. Grand: L’affinage des alliages aluminiu-silcium hypersilices. Review Aluminium, nr 301, 1962, s. 967 [10] S. Pietrowski: High quality pressure castings of silumins, Innowacje w odlewnictwie ciśnieniowym. Instytut Odlewnictwa Kraków, 2009; 2010 (in Polish)
Acknowledgements The work was made as a part of the grant No. 6ZR7 2009C/07340 by agreement No. 04535/C. ZR-6/2010.
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