Effect of Holding Time Before Solidification on Double-Oxide Film Defects and Mechanical Properties of Aluminum Alloys

Transcription

1 Effect of Holding Time Before Solidification on Double-Oxide Film Defects and Mechanical Properties of Aluminum Alloys MAHMOUD AHMED EL-SAYED, HANADI A.G. SALEM, ABDELRAZEK YOUSSEF KANDEIL, and W.D. GRIFFITHS Double-oxide films (bifilms) have been held responsible for the variability in mechanical properties of aluminum castings. It has been suggested that the air entrapped inside a bifilm can react with the surrounding melt, leading to its consumption, which might improve the mechanical properties of the castings. In this work the effect of holding the melt before solidification on the distribution of mechanical properties, and by implication on entrained double oxide films, was investigated for several different aluminum alloys. The Weibull moduli of plate castings were determined under tensile conditions, and their fracture surfaces were examined for evidence of oxide films. The results suggested the occurrence of two competing mechanisms during the holding treatment: (1) the consumption of air inside the bifilms by reaction with the surrounding molten metal that may lead to improvements in mechanical properties and (2) the accompanying diffusion of hydrogen into the bifilms, which would be expected to have a deleterious effect on properties. DOI: /s Ó The Minerals, Metals & Materials Society and ASM International 2011 I. INTRODUCTION ONE of the most important casting defects affecting the reproducibility of mechanical properties of aluminum castings is the double oxide film defect, created because of surface turbulence of the liquid metal, common during metal transfer and pouring in the shape-casting process. When the liquid metal surface is exposed to air, a surface oxide film forms. As a result of surface disturbance, the liquid metal surface can be folded over onto itself, which causes the oxidized surfaces of the folded-over metal to come together but not to fuse. A layer of the local atmosphere is also trapped creating a double oxide film defect or bifilm which can be entrained into the bulk metal, as shown in Figure 1. [1,2] Such entrained double-oxide film defects represent one of the easiest possible initiating features for cracks because their unbonded inner surfaces can be separated with little force. Also, gas dissolved in the liquid metal can precipitate inside the bifilm, initiating porosity. [3] In addition, double-oxide films are favorable sites for the MAHMOUD AHMED EL-SAYED, Assistant Lecturer, is with the Department of Industrial and Management Engineering, Arab Academy for Science and Technology and Maritime Transport, Abu Qir, Alexandria 21599, Egypt. HANADI A.G. SALEM, Professor, is with the Department of Mechanical Engineering, American University in Cairo, New Cairo 11833, Egypt. ABDELRAZEK YOUSSEF KANDEIL, Dean, is with the Arab Academy for Science and Technology. W.D. GRIFFITHS, Senior Lecturer, is with the School of Metallurgy and Materials, College of Engineering and Physical Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom. Contact W.D.Griffiths@bham.ac.uk Manuscript submitted May 31, Article published online September 20, nucleation and growth of intermetallic compounds. These effects not only reduce the elongation, tensile strength, and fatigue properties of aluminum alloy castings but also increase their variability. Nyahumwa et al. [4] suggested that because of the transformation of the oxide layer from c-al 2 O 3 to a-al 2 O 3 (a process thought to take several hours), cracks can be introduced into the oxide, which allows the liquid aluminum to come into contact with, and react with, the gas inside the oxide film defect (presumably mainly oxygen and nitrogen). This mechanism could result in the consumption of the atmosphere inside the bifilm and possibly contribute to its deactivation. The rate of consumption of the internal atmosphere was examined by Raiszadeh and Griffiths, [5] who trapped an air bubble inside an Al melt and monitored its change in volume with time using real-time X-ray radiography. Their results showed that the oxygen in the trapped air should be consumed first to form Al 2 O 3, and then the nitrogen would react to form AlN. These reactions started immediately (with no need for an initiating phase transformation). Also, if the initial hydrogen content of the melt was higher than the equilibrium associated with the ambient atmosphere, the trapped air bubble increased its volume, which supported the idea that double-oxide film defects could act as initiation sites for hydrogen porosity during the solidification of Al castings. The reaction rates of the trapped air with the melt were used to build a semiempirical mathematical model to predict the duration of the atmosphere inside a double-oxide film defect, which suggested that the consumption of oxygen and nitrogen would not take more than approximately 3 minutes (an estimate heavily dependent on initial assumptions about the bifilm volume and surface area) VOLUME 42B, DECEMBER 2011 METALLURGICAL AND MATERIALS TRANSACTIONS B

2 a strain rate of 1 mm min 1. The tensile results were evaluated using a Weibull statistical analysis approach to assess the influence of the holding treatment on the variability of the mechanical properties of the castings. Finally, scanning electron microscopy (SEM) with energy-dispersive X-ray (EDX) analysis was used to investigate the fracture surfaces of the test bars. Fig. 1 The formation of a double oxide film defect: (a) The surface turbulence leads to a breaking wave on the metal surface, and (b) the two unwetted sides of the oxide films contact each other leading to the submerging of the bifilm into the bulk liquid metal. The aim of this work was to study the effect of the holding time of the melt before solidification on entrained double-oxide films and the corresponding change in mechanical properties of aluminum alloy castings. Understanding these issues could lead to the development of techniques by which the effect of double oxide film defects in aluminum castings might be reduced or eliminated. II. EXPERIMENTAL PROCEDURE The experimental procedure involved the production of castings (by the investment casting technique), which contained oxide films of different ages (0, 10, and 20 minutes). Three different aluminum alloys were considered in this work: commercial purity Al, Al- 7 wt pct Si-0.3 wt pct Mg (2L99 alloy) and Al-5 wt pct Mg alloy, so as to involve different oxide films that might have different behaviors (Al 2 O 3, MgAl 2 O 4 spinel, and MgO, respectively). In each experiment, approximately 10 kg alloy was melted and held at approximately 1073 K (800 C) under a vacuum of 80 mbar for 1 hour. This procedure was intended to remove previously introduced oxide films from the melt. [6] The liquid metal was then prepared in such a way as to promote surface turbulence and splashing, by being both poured from a height into preheated ceramic shell molds and being stirred in an induction furnace (using a power setting of 7.5 kw and frequency of 2350 Hz, for 1 minute). This was intended to cause the creation and entrainment of new doubleoxide film defects and their introduction into the melt. One casting was then allowed to solidify immediately, whereas two other castings were maintained in the liquid state by placing the filled ceramic shell molds in a furnace for 10 and 20 minutes, respectively, before removal and solidification. During holding, the hydrogen content of the melts was evaluated using a Hyscan H measuring device (Scanivalve Corporation, San Diego, CA). After solidification, each casting was machined into 15 tensile test bars with the shape and dimensions shown in Figure 2 and tested using a Zwick 1484 tensile testing machine (Zwick Roell AG, Ulm, Germany), with III. RESULTS Table I shows the results of the Weibull analysis for both ultimate tensile strength (UTS) and percentage elongation values obtained from the different aluminum alloy castings, together with the position parameter and R 2 values of the linear fits to the Weibull plots, and also the results from the hydrogen measurements. Figure 3 illustrates the effect of the holding time before solidification on Weibull moduli of the UTS (Figure 3(a)), Weibull moduli of the pct elongation (Figure 3(b)) and the amount of H in solution in the liquid metal (Figure 3(c)). In all three alloys, both the UTS and pct elongation Weibull moduli were a maximum in the case of the casting held for 10 minutes in the liquid state before solidification, although in the case of the pure aluminum and Al-7Si-0.3Mg alloys, the differences in Weibull moduli were slight. In the case of the Al-5Mg alloy, the increase in Weibull moduli was most marked. The Table I and Figure 3(b) also show that the hydrogen content of all the alloys increased consistently with holding time, but the Al-5Mg alloy possessed a greater hydrogen content because of the greater solubility of hydrogen in this alloy. [7] Figure 4(a) shows an SEM image inside a pore on the fracture surface of a tensile test specimen from the Al-5Mg alloy. Many oxide fragments were visible inside the pore, and an EDX analysis of the fragments indicated the presence of MgO, which suggests that the origin of this pore lay with a double-oxide film defect. Figure 4(b) shows an SEM image with the corresponding EDX analysis from the fracture surface of a specimen of Al-7Si-0.3Mg alloy, in which an iron intermetallic is associated with a spinel substrate, suggesting a nucleation relationship. These images are an indication of the role played by double-oxide films in creating other defects such as porosity and intermetallics. Figure 5 shows whisker-like oxides found within pores of castings held for 20 minutes in the liquid state before solidification (in these cases commercially pure Al and Al-5Mg alloys). The pores were also associated with oxide films. The delicacy of these features suggests they formed during solidification, rather than earlier, such as within an oxide film defect floating within the liquid metal. Although the interconnections are too small to influence mechanical properties, their presence may be informative about conditions inside the pores during their formation. IV. DISCUSSION As shown in Table I, the Weibull moduli of the commercial purity Al alloy and Al-7Si-0.3Mg alloy METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 42B, DECEMBER

3 Fig. 2 Sketch of the casting and the tensile test specimens taken. Table I. Results of the Weibull Analysis for the Test Bars of Different Al Alloys that Contained Oxide Films of age 0, 10, and 20 Minutes Commercial Purity Al Alloy Al-7Si-0.3Mg Alloy Al-5Mg Alloy 0 min 10 min 20 min 0 min 10 min 20 min 0 min 10 min 20 min H (cm 3 /100 g) UTS (MPa) Weibull modulus Position parameter (MPa) R Pct elongation Weibull modulus Position parameter (pct El) R consistently exceeded 30, a value of Weibull modulus often associated with a casting made with a welldesigned running system, with reproducible properties. [8] Also, oxide films, as demonstrated by EDX analysis in the SEM, were associated mostly with pores (as shown in Figure 4(a)), rather than lying on the fracture surfaces. This could be an indication of the effect of the holding treatment on the elimination of oxide films. Other consequences of the entrainment of doubleoxide films were illustrated in Figure 4. Not only do they act as cracks in the solidified casting, but also H dissolved in the liquid metal can precipitate inside the bifilm gap initiating porosity. In addition, double-oxide films can be favorable sites for the nucleation and growth of a wide variety of intermetallics. This might minimize the effect of the holding treatment on the mechanical properties of the castings, especially in the case of the commercial purity Al alloy and Al-7Si-0.3Mg alloy. In this work it was found that for all three alloys (despite their different oxides), the Weibull moduli representing the UTS and pct elongation increased to a peak at holding times of 10 minutes, although the H content of each alloy rose progressively with holding time, as illustrated in Figure 3. The moduli decreased significantly when the holding period was increased to 20 minutes. This finding is suggestive of competing mechanisms affecting the distribution of mechanical properties. The first mechanism may be related to the consumption of air inside the bifilms because of its reaction with the surrounding molten metal, whereas another mechanism may be related to the hydrogen content of the liquid metal picked up from the atmosphere (and hence, the porosity size and number in the casting) VOLUME 42B, DECEMBER 2011 METALLURGICAL AND MATERIALS TRANSACTIONS B

4 Fig. 3 Plot of the holding time vs (a) Weibull modulus of UTS, (b) Weibull modulus of pct elongation, and (c) H content of the melt. Previous experiments suggested that the atmosphere inside double-oxide film defects could be consumed within a few minutes. [5] Some bonding of the two layers of the bifilm might then take place, or some reduction in the size and hence impact of the oxide films might occur, which could improve the mechanical properties of the castings. This mechanism would tend to increase Weibull moduli with time. In contrast, the H content increased as the liquid metal was held for longer periods, which would lead to increased effects of porosity and a decrease in overall mechanical properties, as well as a decrease in the Weibull modulus with time. The Weibull moduli for the castings allowed to solidify immediately thus represented the scatter of mechanical properties derived from castings that contained the lowest hydrogen contents, but that perhaps also contained oxide films then in the process of losing their initial atmospheres. The morphology of these defects seems to have been most harmful to the mechanical properties of the Al-5Mg alloy. The castings held for 10 minutes in the liquid state might, therefore, have possessed the best Weibull modulus (narrowest distribution of properties), because at this time the defects may have lost all or most of their initial internal atmospheres and may have absorbed little hydrogen from the surrounding melt. Under these circumstances, the oxide film defects may have been reduced in volume and had a morphology that was least harmful to the properties of the castings. The mechanical properties of all castings subsequently declined with holding up to 20 minutes as the H content of the melt increased, which would increase the H content and the volume of the oxide film defects as well, increasing their deleterious effects. In support of this, the whisker-like structures within pores (shown in Figure 5 and demonstrated by EDX to be oxides), occurred mostly at holding periods of 20 minutes. These structures are suggestive of ceramic structures grown from a vapor phase. This indicates the formation of oxide structures within porosity that contains an atmosphere, which suggests that at 20 minutes holding time, double oxide film defects may have still contained an atmosphere which may be separating their internal surfaces and preventing any bonding. The oxide film defects at this time could therefore still have deleterious effects on mechanical properties. METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 42B, DECEMBER

5 Fig. 4 Consequences of entrainment of bifilms inside Al castings: (a) oxide-associated porosity and (b) oxide-related intermetallic phase. Fig. 5 Interconnections between adjacent oxide layers in castings containing 20-min-old oxide films: (a) commercially pure Al and (b) Al-5Mg alloy VOLUME 42B, DECEMBER 2011 METALLURGICAL AND MATERIALS TRANSACTIONS B

6 The Al-5Mg alloy exhibited the largest differences between the Weibull moduli between the different holding periods. This may be associated with the high solubility limit for H in this alloy and also the permeability of its oxide film, MgO, which might lead to a more rapid reaction of O and N with the surrounding melt, as well as enhance diffusion of H into the oxide film defect interior. These results may therefore be an indication of the significance of the H content in controlling the scatter of mechanical properties, in addition to the effect of oxide film defects. V. CONCLUSIONS 1. SEM examination of the fracture surfaces revealed oxide films, which demonstrated a role for such defects in influencing the failure of Al castings. 2. SEM examination also showed that the interior surfaces of porosity associated with double-oxide film defects can develop whisker-shaped interconnections, which are apparent in the castings held for 20 minutes before solidification. The interconnections would not have a significant effect on mechanical properties but could perhaps be indicative of chemical reactions resulting in deposition of ceramic whiskers, which in turn suggests an atmosphere present in the pores during solidification despite the (relatively) long holding periods. 3. The mechanical properties of the castings were most improved after a holding period for the liquid alloys of 10 minutes, perhaps because of the consumption of air inside the bifilms and a reduction in their size and effect on casting properties. 4. Increasing the holding period to 20 minutes increased the H content of the alloys. This may have resulted in a greater effect of porosity, which would reverse the initial enhancement in the distribution of mechanical properties because of air consumption, leading to a decrease in the Weibull moduli to values that were close to those of the castings that were solidified immediately after pouring. 5. The holding treatment may reduce the effect of double-oxide film defects in Al melts but would not prevent them from playing a role in the formation of other defects, such as serving as sites for the formation of hydrogen porosity or intermetallic phases. ACKNOWLEDGMENTS The authors thank Mr. Adrian Caden (of the University of Birmingham), Eng. Hanadi Hussein, Eng. Khalid Iraqi, and Eng. Ramy Wasfi (of the American University in Cairo) for their technical support during the laboratory work. Also, the authors wish to acknowledge the sponsorship of this study by the Arab Academy for Science and Technology, Alexandria, Egypt. REFERENCES 1. J. Campbell: Castings, 2nd ed., Butterworth-Heinemann, Oxford, UK, J. Campbell: Mater. Sci. Technol., 2006, vol. 22, pp W.D. Griffiths and R. Raiszadeh: J. Mater. Sci., 2009, vol. 44, pp C. Nyahumwa, N.R. Green, and J. Campbell: Metall. Mater. Trans. A, 2001, vol. 32A, pp R. Raiszadeh and W.D. Griffiths: Metall. Mater. Trans. B, 2008, vol. 39B, pp R. Raiszadeh and W.D. Griffiths: J. Alloys Compd., 2010, vol. 491, pp P.N. Anyalebechi: Scripta Metall. Mater., 1995, vol. 33, pp C. Nyahumwa, N.R. Green, and J. Campbell: AFS Trans., 1998, vol. 58, pp METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 42B, DECEMBER