Scanning Electron Microscopy of active surface of sand mould used in metal casting

Transcription

1 Scanning Electron Microscopy of active surface of sand mould used in metal casting (Study cum Research on Micro defects and their relationship with casting defects) by Dr. Pervaiz Habibullah, Ph.D Abstract Since long ago, in Foundry Sand Testing and Control, silica sand used for sand moulding is observed in binocular at low magnification (20-50x). In the present paper, we have observed the surface of the sand mould coated with graphite and other impregnating agents, such as, polystyrene soln in CCl 4 and in trichloroethylene, by SEM at high magnification and have observed the micro fissures and micro voids (created by aspiration) in the impregnated layer. Mould washes are generally refractory based and do not react chemically with the mould surface, therefore physical adsorption, if occurs, will be by van der Waals forces. The liquid-solid adhesion increases with the ability of the adhesive (in our case it is mould wash ) to wet the solid (mould surface) reaching a maximum, when wetting angle is 0 o and wetting is complete. In a case when solid vapour surface tension is greater then liq vapour surface tension, at these conditions, fissures will occur within the adhesive. Soon as the tumbling hot molten metal touches the active surface of the mould cavity, each of the pores and micro fissures (as revealed by SEM images) of the mould surface acts as epicenter for evolution of gas. When the pores, micro fissures and micro voids are saturated with the gas coming from the mould, a supplementary gas pressure (Pg) in the superficial layer of active mould surface is developed. If Pg increases than counter pressure (sum of metallostatic pressure (Pm) external pressure (Pext) and pressure necessary to overcome the capillary forces (Pc = 2σ Cosθ / r, where σ is surface tension of the liq. metal, θ is contact angle and r is radius of the pore) and when inequality +Pg > Pm + Pc + Pext prevails, mould gas traps in the liq. metal in early stages of solidification and segregates ahead of the solidifying front and normal processes of nucleation and growths of pores from the gas, in the solidifying front occur. Impregnating solutions are beneficial in reducing the metal penetration defect because of their relatively high diffusivity in the mould surface and their role in increasing adhesivity of the active surface of the mould by generating adhesive forces on the mould surface. These forces are an outcome of movements of molecules which are mobile at the surface of adhesive, during adsorption. Future Research may be carried out on mechanism of appearance of micro fissures in the sand mould surface, washes and impregnents and their images at different stages of appearance and growth may be photographed using modern techniques. Keywords SEM images of sand mould surface, micro fissures and voids in layers of mould coating, micro defect role in creating casting defects.

2 Scanning Electron Microscopy of active surface of sand mould used in metal casting (Study cum Research on Micro defects and their relationship with casting defects) by Dr. Pervaiz Habibullah, Ph.D 1. Introduction Several types of sands are used for moulding. Some of the main sand casting processes used in ferrous and non ferrous foundries are: greensand, dry sand, core sand, cement bonded sand, shell mould sand and some others. Silica (SiO 2 ) is the principal constituent of the sand moulds. Granular particles of silica sand, principally comprises 50-95% of the total material of a moulding mixture. They differ as per their physico chemical properties e.g. average grain size, their shape and distribution, their chemical composition and their refractoriness & thermal stability. Since long ago, in foundry sand testing and control, silica sand used for sand moulding is observed in binocular at low magnification (20-50x). This technique has been utilized for knowing the shape of the silica grains. As per AFS Foundry Sand Handbook, the sand grains may be rounded angular or sub-angular (see fig.1.1). The moulding sands used in foundries in general are the mixture of these types of grains. In the same token, the images of grains of silica sand from different localities of Romania are shown in fig Some of the high quartz silica sands have been marketed commercially for future mould and core making e.g. GIBA has marketed Badger sands (GMB) whose images are given in fig When GMB sands are compared with other customary sands of European markets (images given in fig. 1.3), it has been found that GMB sands have even surface, smooth grains, better reuse ability and offer lower cost. These are twice in mechanical strength as compared to the German quartz sands. (Comp SiO 2 = 99.70,Al 2 O 3 = 0.12, CaO = 0.12, FeO =0.04, K 2 O = 0.02, Na 2 O = 0.01, MgO <0.01, Ti 2 O 3 <0.01) [7]. Fig. 1.5 (a) depicts the typical greensand grains that are part of the green sand mould. The chemical analysis shows that the greensand is mainly silica SiO 2 base sand. Fig. 5(b) shows the typical grain structure of the dry sand core that is in contact with the casting surface. The chemical analysis shows that the dry sand is silica (SiO 2 ) sand [6]. Images of reclaimed sands are given in fig. 1.6 (a to d)

3 Sand grains as per specifications of AFS Fig. 1.1 Sand grain shapes (a) rounded sand grains (b) angular sand grains (c) compounded sand grains (d) sub angular sand grains (From AFS Foundry Sand Handbook,7 th Ed. 1963)

4 Different silica sands from Romania* Fig. 1.2 Sand grains** as observed in a binocular (Mag. 20 x ) ** (a) Sand from Aghiresh (b) Sand from Valeni de Munte (c) Sand from Caraorman, (d) Sand from Fagatul iriei (e) Sand from France used in Romania * author has been experimenting on these sands during his Ph.D. at UPB, Bucharest Romania.

5 Fig.1. 3 European silica sands (a) Ultra structure (seen b binocular) Fig. 1.4 GMB Silica sand (b) SEM image (a) Ultra structure (b) SEM image *Badger sands (GMB)

6 Fig.1.5 SEM Image and EDX spectrum of greensand and dry sand [6] (a) SEM Image and EDX spectrum of green sand separating from the cope and the drag (b) SEM Image and EDX spectrum of dry sand grains separating from the external surface of dry sand core [6]

7 Fig. 1.6 Sand reclaimed from moulding material (at x20) (a) Aghiresh sand (from Romania) with sod silicate (b) same sand with clay (c) same sand with thermo reactive bonding material (d) same sand with furanic resin.

8 2. Experimental In the present paper we have observed the surface of the sand mould coated with graphite and other impregnating agents, such as, polystyrene solution in carbon tetrachloride and polystyrene solution in trichloroethylene, by scanning electron microscope at high magnification. 2.1 Sand samples Three sand samples consisting of natural bonding sand and 5% molasses were prepared and dried in the oven. Each of these was, then impregnated. Sample 1 Fine graphite powder emulsion in kerosene oil and baked in the oven. Sample 2 Polystyrene in CCl 4 solution and. Sample 3 Polystyrene in trichloroethylene solution Impregnated surface each of the sand sample was observed with scanning electron microscope. 2.2 SEM Images Graphite powder in kerosene oil coating Fig. 2.1 (a) Naturally bonding sand + 5% molasses, surface of the sand sample coated with graphite emulsion and baked SEM image (at 50x mag.) Layer of graphite is visible. White particles are impurities Fig. 2.1 (b) Naturally bonding sand + 5% molasses, surface of the sand sample coated with graphite emulsion and baked (SEM image at 500x mag., patches of graphite are visible Fig. 2.1 (c) Naturally bonding sand + 5% molasses, surface of the sand sample coated with graphite emulsion and baked (SEM image at 1000x mag., voids among the irregular particles of graphite are more visible) Fig. 2.1 (d) Naturally bonding sand + 5% molasses, surface of the sand sample coated with graphite emulsion and baked (SEM image at 2000x mag., irregular types of voids formed by aspiration of the mould wash are visible)

9 Scanning Electron Microscopy of sand samples coated with different mould washes or impregnating agents Graphite powder in kerosene oil Fig. 2.1 (a) Naturally bonding sand + 5% molasses, surface of the sand sample coated with graphite emulsion and baked SEM image (at 50x mag.) White particles are impurities Fig. 2.1 (b) Naturally bonding sand + 5% molasses, surface of the sand sample coated with graphite emulsion and baked (SEM image at 500x mag., patches of graphite are visible)

10 Fig. 2.1 (c) Naturally bonding sand + 5% molasses, surface of the sand sample coated with graphite emulsion and baked (SEM image at 1000x mag., voids among the irregular particles of graphite are more visible) Fig. 2.1 (d) Naturally bonding sand + 5% molasses, surface of the sand sample coated with graphite emulsion and baked (SEM image at 2000x mag., irregular types of voids formed by aspiration of the mould wash are visible)

11 Polystyrene dissolved in CCl4 Fig. 2.2 (a) Naturally bonding sand + 5% molasses, impregnated with polystyrene dissolved in CCl 4 (SEM image at 50x mag., particles of the sand are bound with each other by the impregnent) Fig. 2.2 (b) Naturally bonding sand + 5% molasses, impregnated with polystyrene dissolved in CCl 4 (SEM image at 100x mag., silica grains are completely enveloped by the impregnating solution). Fine fissures appear in the layer of impregnent. Fig. 2.2 (c) Naturally bonding sand + 5% molasses, impregnated with polystyrene dissolved in CCl 4 (SEM image at 500x mag., sand grains are completely enveloped by the impregnating solution). Fine fissures and voids are visible in the layer of impregnent. Fig. 2.2 (d) Naturally bonding sand + 5% molasses, impregnated with polystyrene dissolved in CCl 4 (SEM image at 1000x mag., silica grains are completely enveloped by the impregnating solution). Fine fissures and voids are visible at larger magnification

12 Polystyrene dissolved in CCl 4 Fig. 2.2 (a) Naturally bonding sand + 5% molasses, impregnated with polystyrene dissolved in CCl 4 (SEM image at 50x mag., particles of the sand are bound with each other by the impregnent Fig. 2.2 (b) Naturally bonding sand + 5% molasses, impregnated with polystyrene dissolved in CCl 4 (SEM image at 100x mag., silica grains are completely enveloped by the impregnating solution. Fine fissures appear in the layer of impregnent

13 Fig. 2.2 (c) Naturally bonding sand + 5% molasses, impregnated with polystyrene dissolved in CCl 4 (SEM image at 500x mag., sand grains are completely enveloped by the impregnating solution. Fine fissures and voids have appeard in the layer of impregnent. Fig. 2.2 (d) Naturally bonding sand + 5% molasses, impregnated with polystyrene dissolved in CCl 4 (SEM image at 1000x mag., silica grains are completely enveloped by the impregnating solution). Fine fissures and voids are visible at larger magnification

14 2.2.3 Polystyrene dissolved in trichloroethylene Fig. 2.3 (a) Naturally bonding sand + 5% molasses, impregnated with polystyrene dissolved in trichloroethylene (SEM image at 50x mag., particles of the sand are completely enveloped by the impregnating solution). Fine fissures have appeared in the layer of impregnent Fig. 2.3 (b) Naturally bonding sand + 5% molasses, impregnated with polystyrene dissolved in trichloroethylene (SEM image at 100x mag., particles of the sand are enveloped by impregnent). Fine fissures and voids are visible Fig. 2.3 (c) Naturally bonding sand + 5% molasses, impregnated with polystyrene dissolved in trichloroethylene (SEM image at 500x mag. Crater in the layer of impregnent) Fig. 2.3 (d) Naturally bonding sand + 5% molasses, impregnated with polystyrene dissolved in trichloroethylene (SEM image at 1000x mag.) Fissures in the layer of impregnents are visible

15 Polystyrene dissolved in trichloroethylene Fig. 2.3 (a) Naturally bonding sand + 5% molasses, impregnated with polystyrene dissolved in trichloroethylene (SEM image at 50x mag., particles of the sand are completely enveloped by the impregnent solution. Fine fissures have appeared in the layer of impregnent Fig. 2.3 (b) Naturally bonding sand + 5% molasses, impregnated with polystyrene dissolved in trichloroethylene (SEM image at 100x mag., particles of the sand are enveloped by impregnent). Fine fissures and voids are visible

16 Fig. 2.3 (c) Naturally bonding sand + 5% molasses, impregnated with polystyrene dissolved in trichloroethylene (SEM image at 500x mag. Crater in the layer of impregnent) Fig. 2.3 (d) Naturally bonding sand + 5% molasses, impregnated with polystyrene dissolved in trichloroethylene (SEM image at 1000x mag.) Fissures in the layer of imprenents are visible

17 3. Discussion on adhessivity & adsorption of impregnating solutions in sand mould surface and micro defects of impregnating layer The sand moulds used in metal casting are protected by applying the mould washes or impregnating their active surface with different absorbents solutions. Chances of penetration of liquid metal in pores of mould and other defects are reduced by application of mould washes. A mould wash should necessarily fulfill the following conditions: [1] -- It should form a uniform and fine layer on the surface of cores and moulds (it should increase adhesivity of mould surface) -- It should adhere strongly to the surface of the mould without cracking and exfoil (it should adsorb properly in the mould surface). -- It should not sediment or degrade during the time of making of mould upto the pouring of liq. metal Adhesion and adhesives [9] The ability of mould washes to spread and thoroughly wet the mould surfaces is highly important in mould mechanics and mould technology. Ref. fig. 3.1, the work to break away the adhesive (which may be considered as a viscous liquid) from the solid is the work required to create a liquid-vapour and a solid-vapour interface from an equivalent area of liquid-solid interface, i.e. it is the work to totally de-wet the solid surface. The work (W) to cause breakage at the interface, per unit area, is given by: W = γ lv + γ sv - γ ls (1) where relative magnitudes of three surface tensions (energies): liquid-solid γ ls, liquid-vapour γ lv and solid-vapour γ sv The three surface tensions operate simultaneously and they will be in equilibrium when γ sv - γ ls = γ lv cos θ (2) and therefore W = γ lv (1+ cosθ). (3) Thus, the liquid-solid adhesion increases with the ability of the adhesive (in our case it is mould wash) to wet the solid, (in our case it is mould surface) reaching a maximum, when θ = 0 o and wetting is complete, given by: W = 2γ lv (4) For this to be the case γ sv > γ lv eq. (2) and under these conditions fissures will occur within the adhesive (in our case it is mould wash) since the energy necessary to form two liquidvapour interfaces is less than that to form a liquid-vapour and a solid-vapour interface. Fig. 3.1 Surface forces acting at periphery of a droplet of an adhesive

18 3.2 Adsorption [9] The broken surface bonds will readily attract to themselves any foreign atoms or molecules that have a slight affinity for the surface material. This effect is known as adsorption, and by satisfying or partially satisfying the unsaturated surface bonds it serves to lower surface energy. Adsorption is a dynamic process, i.e. molecules are constantly aligning on and taking off from the surface. Different molecules adsorb with varying degrees of intensity, depending on the nature of the bond that is able to form at the interface, and the strength of the bond and may be expressed in the terms of φ a, the energy of adsorption. As in the case of interactomic bonds, a negative value of φ a is taken to indicate positive adsorption, i.e. the molecules are attracted to the interface, and the surface energy (tension) is lowered thereby. A positive value of φ a indicates a repulsive interaction and the molecules avoid the surface. Typical plots of φ a against the distance of the adsorbed layer from the surface are given in Fig. 3.2 If the molecule being adsorbed is non-polar and does not react chemically with the surface, adsorption, if it occurs, will be by Van der Waals bonds, and the minimum value of φ a is small (curve 2 in fig.3.2). If on the other hand the molecule is strongly polar, (as is the case with water or ammonia), the electrostatic forces between the surface and the charged portion of the molecule give rise to stronger bonding. If a chemical reaction occurs as part of the bonding mechanism. The bonding is still stronger (curve 1, fig. 3.2) and affect is referred to as chemisorption. Energy of adsorption φa Fig.3.2 Energies of adsorption for different adsorption mechanisms: curve 1, chemisorption; curve2, physical adsorption The behaviour of water is of particular importance in this context (particularly in our case of coating or impregnating water based washes on the sand mould). Because of its ability to form hydrogen bonds with neighboring molecules, water adsorbs rapidly and strongly on most solid surfaces. Despite the tenacity with which such a layer is held (clay does not lose all its adsorbed water until heated to 330 o C), the interaction is thought to be as physical adsorption. It is, in a sense, a halfway stage to solution or alternatively to the taking up of crystalline water of hydration. Bonding is strong enough to maintain a surface layer perhaps several molecules thick, but the affinity is not sufficient for the molecules to penetrate into the interstices of the structure. The physical nature of such a film cannot be thought of as a fluid in the accepted sense of the term even when more then 1 molecule thick, as in the case of clays. Yet the molecules are mobile in this situation. They will not desorbs readily, but they can diffuse along the surface. Such movements have been observed over the vast internal surface area of cement gels, and are primarily responsible for the slow creep of concrete under stress. The ability of water

19 molecules to penetrate solid-solid interfaces in clays and build up thick adsorbed layers results in the swelling of clays. Polystyrene Polystyrene is a thermoplastic. Pure solid polystyrene is colorless, hard plastic with limited flexibility. The chemical makeup of polystyrene is a long chain hydrocarbon with every other carbon connected to a phenyl group (the name given to the aromatic ring benzene, when bonded to complex carbon constituents). Polystyrene s chemical formula is (C 8 H 8 ) n. The force of attraction in polystyrene is mainly due to short range van der Waals attractions between chains. Because it is an aromatic hydrocarbon, it burns with an orange-yellow flame, giving off soot. Complete oxidation of polystyrene produces only carbon dioxide and water vapour. Polystyrene is classified as highly flammable or easily ignited. The ability of the system to be readily deformed above its glass transition temperature allows polystyrene (and thermoplastic polymers in general) to be readily softened and molded with the addition of heat. In modern ferrous foundries, the polystyrene solution in organic solvents such as CCl 4, trichloroethylene and others, are used to impregnate the active surface of the mould. Polystyrene dissolved in organic solvents, impregnated on the mould surface readily adsorbs in the superficial layer of the active surface of the mould cavity. When hot metal is poured in the mould, impregnated with such solutions, the polystyrene which has already penetrated in the pores of the mould, being highly flammable, is readily ignited and changes into cellulous and sticky type of mass, reducing the radii of intergranular pores of the mould surface consequently avoiding the risk of penetration of liq. metal in the mould surface. 4. Conclusion 1. Scanning Electron Microscopy (SEM) of micro defects of active surface of the sand mould coated with mould washes or impregnated with solutions and their relation with casting defect, can be summarized as follows: i) When active mould surface is coated or impregnated, the adhessivity of the mould surface increases due to physical adsorption of mould wash or impregnating solution in the mould surface. Mould washes are generally refractory based and do not react chemically with the mould surface, therefore physical adsorption, if occurs will be by van der Waals forces. ii) The behaviour of water is of particular importance in this context (particularly in our case of coating or impregnating water based washes on the sand mould). Because of its ability to form hydrogen bonds with neighboring molecules, water adsorbs rapidly and strongly on most solid surfaces. Despite the tenacity with which such a layer is held, the interaction is thought to be as physical adsorption. Bonding is strong enough to maintain a surface layer perhaps several molecules thick, but the affinity is not sufficient for the molecules to penetrate into the interstices of the structure. iii) The liquid-solid adhesion increases with the ability of the adhesive (in our case it is mould wash) to wet the solid, (in our case it is mould surface) reaching a maximum, when wetting angle is 0 o and wetting is complete. In a case when solid vapour surface tension is greater then liq vapour surface tension, under these conditions, fissures will occur within the adhesive (see text). iv) Soon as the tumbling molten metal touches the active surface of the mould cavity, each of the pore or micro fissure (as revealed by SEM images) of the mould surface acts as epicenter for evolution of gas. When the pores the micro fissures and micro voids are saturated with the gas coming from the mould, a supplementary gas pressure (Pg) at liq. metal interface is developed. When the supplementary pressure (Pg) in the superficial layer of active mould surface increases than counter pressure (sum of metallostatic pressure (Pm) external pressure (Pext) and pressure necessary to overcome the capillary forces (Pc = 2σ Cosθ/r, where σ is surface tension of the liq. metal, θ is contact angle and r is radius of the pore) (i.e. Pg > Pm + Pc + Pext) and when this inequality prevails, mould gas traps in the liq. metal in early stages of solidification and

20 segregates ahead of the solidifying front and normal processes of nucleation and growths of pores from the gas in the solidifying front occur. Fig. 4 (a c) show SEM images of gas porosity. (Some other SEM images of hydrogen blowholes, interior of the blowholes and blowhole-steel interface are given in the album of author s book entitled: Morphology of Exogenous Blowholes in Steel Castings ) [12] (a) (b) (c) Fig. 4 (a) SEM image of the typical defect (void) and surrounding areas. (b) SEM Image of the inside area of the defect (void) depicted (c) SEM image of the outside area of the defect (void) depicted [6].

21 v) In case of sand mould ferrous casting, the apparent contact angle between sand grains and melt decreases with formation of FeO at the interface, chemical penetration occurs at this stage. Liq. metal penetration is governed by capillary forces and head height: When head height pressure is greater then the surface tension resistive force, liq. metal can penetrate into the mould. Chances of metal penetration and sand burn-on are reduced by coating the mould surface with mould wash or impregnating it, with impregnating solution. These are beneficial in reducing the penetration defect because of their relatively high diffusivity in the mould surface and their role in increasing adhesivity of the active surface of the mould by generating adhesive forces on the mould surface. These forces are an outcome of movements of molecules which are mobile at the surface of adhesive, during adsorption. vi) In a casting experiment, conducted with the sand mould impregnated with a polystyrene sol. in CCl 4, (in two layers) it is noted that the surface of the casting is enveloped in a thin, bright, cellulous removable layer. When casting was subjected to sand blasting, this layer was removed easily. The sample of the layer was observed in the microscope. Contrary to the properties of polystyrene, (which shows stickiness on its ignition), it was found highly fragile and porous. 2. The susceptibility of mould and core surface towards fissuring depends upon many factors. Some of these measures based on practical experiences are as follows [11]: -- addition of high content of clay >2.5% produces fissures. Clay addition upto 1.5% does not create fissures. -- ratio clay: zirconia should be below 0.03%. At a ratio 0.04%, all washes are susceptible to fissuring. -- bentonite contracts on drying, loosing water of crystallization, therefore, its correct amount may be added which should be appropriately the function of dilation of refractory material, otherwise, it will exfoliate or crack. -- weak ramming of mould and very low mechanical resistance of moulding material increase the risk of appearance of cracks and exfoliation. -- a wash or paste with low relative density (having little refractory material) and with greater viscosity (excessive amount of bonding material) give rise to the fine fissures. -- addition of excessive amount of dextrin or linseed oil creates fine fissures and give rise to the centers of high stress on the mould surface which brings about exfoliation. -- all the washes made by bentonite suffer fissuring on heating by radiation. -- mixing of bentonite with sodium salts largely attenuates tendency of fissuring. -- depth of penetration of liq. alloy in pores of mould decreases with thickness of the layer of the mould wash. 3. Future Research may be carried out on mechanism of appearance of micro fissures in the sand mould surface, washes and impregnents and their images at different stages of appearance and growth may be photographed using modern techniques. 5. Acknowledgement Author is grateful to the SEM section of PITMAEM, Pakistan Council of Scientific and Research, Lahore Centre, Pakistan for scanning electron microscopy of the sand samples.

22 Bibliography 1. Buzila. S-Projectarea si executarea formelor, Ed. Didactica si pedagogica Bucuresti 1976,p Habibullah P, New Aspects of Mechanics of Sand Moulds used for Metal Casting, 3. Habibullah P, Miracle of Imposing Vacuum ( depression ) on Sand Moulds, pub Salman Art Press, Lahore. Ed. 1, Habibullah P A Survey of Silica Sands of Pakistan and researches on their better utilization in ferrous and non ferrous foundries 34 th IGC Australia, Sept and 60 th IFC, Bangalore, March Habibullah P, Researches on Superficial impregnation of vacuum mould Journal of Engg. and Ap. Scs. V.7 No.2 July-Dec Foseco, SEM-Invaluable and practical tool for casting defect analysis before DOE 7. Frank Iden et al. Zeitschrift GIESSEREI 98; 05/2011; s.26; Die Haftungsmechanismen von Cold-Box-Bindermitteln auf der Formstoffoberflache (The future of mould and core making J.M.Illston andp.l.j. Domone Construction materials 3 rd Ed.p Buzila. S-Projectarea si executarea formelor, Ed. Didactica si pedagogica Bucuresti 1976,p Buzila. S-Projectarea si executarea formelor, Ed. Didactica si pedagogica Bucuresti 1976,p Habibullah P, Morphology of Exogenous Blowholes in Steel Castings pub. Salman Art Press, Lahore, Pakistan, Ed. 1, 2011 (album of blowholes in iron and steel castings)