Structure, Defects and Properties of Finished Castings 1. Oxide film, bubble damage and shrinkage problem

Transcription

1 MME 6203, Lecture 02 Structure, Defects and Properties of Finished Castings 1. Oxide film, bubble damage and shrinkage problem Today s Topics... Summary of casting defects Oxide film and bubble trail defects Shrinkage porosity 1

2 1. Summary of Casting Defects Oxide films and bubble trails Segregation, inclusion and gas porosity Shrinkage cavity Hot tear and cold crack Residual stress 2. Oxide Film and Bubble Trail Some liquid metals may be really like liquid metals. e.g., pure liquid gold, C-Mn steels at a late stage of melting These, however, are rare. many liquid metals are actually so full of sundry solid phase floating about, that they actually closely resemble slurries than liquids. e.g., inclusions from charge, refractory lining; reaction products, etc. Many of the strength related properties of liquid metal can only be explained by assuming that the melt is full of defects. classical physical metallurgy or solidification science (which consider metals as merely pure metals) unable to explain important properties of cast materials such as the effects of DAS, and existence of pores and their area density 2

3 2.1 Reactions of the melt with its environment [a] Pick up of moisture from damp refractories M + H 2 O = MO + H 2 H 2 = 2[H] [b] Reaction involving hydrocarbon fuels CH 4 + 2O 2 = CO 2 + 2H 2 O M + H 2 O = MO + H 2 H 2 = 2[H] Reactions products 1. dissolve rapidly in the metal, and diffuse away into its interior 2. those unable to diffuse escape into the atmosphere 3. remain on the surface as films [H] 2 = k p H2 the equilibrium gas pressure of a melt applies both to the external and internal environments of the melt. 3

4 Two film-forming reactions: 1. Formation of oxide film by decomposition of moisture M + H 2 O = MO + 2[H] 2. Formation of graphite film by decomposition of hydrocarbon C x H y = xc + y[h] Oxide films usually start as simple amorphous layers, which quickly convert to crystalline products as they thicken, and later often develop into a bewildering complexity of different phases and structures. Some films remain thin, some grow thick. Some are strong, some are weak. Some grow slowly, others quickly. Some are heterogeneous and complex in the structure, being lumpy mixtures of different phases. A film is not harmful when it remains on top of the surface in case of aluminium, the surface film protects the liquid from catastrophic oxidation (as in the case with Mg) The problem with a surface film only occurs when it becomes a submerged film In conditions for the formation of a transient film, if the surface happens to be entrained by folding over, although the film is continuously dissolving, it may survive sufficiently long to create a legacy of permanent problems. These could include the initiation of porosity, tearing or cracking, prior to its complete disappearance. Entrained films form the major defect in cast materials. 4

5 2.2 Entrainment For many common liquids, the surface of which is a solid, but invisible film If perfectly clean water is poured, or is subject to a breaking wave, the newly created liquid surfaces fall back together again, and so impinge and mutually assimilate. the body of the liquid re-forms seamlessly. If the liquid metal surface happens to fold (by the action of a breaking wave, or by droplets forming and falling back into the melt), the surface oxide film becomes entrained in the bulk liquid. The entrainment process is a folding action that necessarily folds over the film dry side to dry side. The submerged surface films are therefore necessarily always double. Also, of course, because of the negligible bonding across the dry opposed interfaces, the defect now necessarily resembles and acts as a crack. The cracks have a relatively long life, and can survive long enough to be frozen into the casting. they have a key role in the creation of other defects during the process of freezing, and ultimately, degradation of the properties of the final casting. 5

6 Ways of mixing of surface film into the bulk: 1. Melt charge materials 2. Pouring 3. Surface flooding 4. Surface turbulence 5. Confluence weld 6. Bubble trails Pouring Surface Flooding (a) (b) (c) 6

7 Surface Turbulence Reynolds number : Re = Vrd / n Re < 2000, smooth, laminar, turbulent-free flow Re > 2000, turbulent flow Measures bulk turbulence Weber number : We = V 2 rr / g We = , free from surface turbulence We = 100, surface turbulence becomes problematic We = , creates atomization!! Measures surface turbulence V = velocity of melt d = linear dimension of flow path r = radius of curvature of film r = density of melt n = viscosity of melt g = surface tension Confluence Weld (a) (b) (c) 7

8 Bubble Trails 2.3 Entrainment defects If the entrained surface is a solid film the resulting defect is a crack. It may be only a few nanometres thick, and so be invisible to most inspection techniques. In the case of the folding-in of a solid film on the surface of the liquid the defect will be called a bifilm (i.e., a double film defect). Figure 2.4 Entrainment defects: (a) a new biflm; (b) bubbles entrained as an integral part of bifilm; (c) liquid flux trapped in a biflm; (d) surface debris entrained with the biflm; (e) sand inclusions entrained in the bifilm; (f) an entrained old film containing integral debris 8

9 Entrainment creates bifilms that: 1. may never come together properly and so constitute air bubbles immediately; 2. alternatively, they may be opened (to become thin cracks, or opened so far as to become bubbles) by a number of mechanisms: (a) (b) (c) (d) precipitation of gas from solution creating gas porosity; hydrostatic strain, creating shrinkage porosity; uniaxial strain, creating hot tears or cold cracks; in-service stress, causing failure in service. Figure 2.40 (a to d) Stages of unfurling and inflation of bifilms 9

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11 2.4 Significance of bifilm General problem due to submerged bifilms Submerged bifilms are always associated with air or other gas, trapped on the non-wetted dry surface of the film, or trapped between the folded film. The gaseous films floated around the liquid constitute cracks in the liquid and, after freezing, constitute cracks in the finished products. The gas-coated film acts as excellent nucleating sites for the subsequent growth of bubbles or shrinkage cavity. Higher-melting-point heavy phases may be precipitated on to the floating oxides, which form defects with large, coarse crystals of heavy intermetallic phase, together with entrained oxide film and associated porosity. 11

12 Machining Problem Oxides are much harder than the metal itself, causing dragging out during machining, leaving unsightly grooves. The cutting edge of tool is often chipped or blunted by encounters with such problems. Leak Tightness For thin-sectioned castings (< 5 mm), film defects can be extended from wall to wall across the mould cavity, and so connect the casting surfaces with a leak path. Bubble defects are specially troublesome with respect to leak tightness, since they necessarily start at one casting surface and connect to the surface above. A leak path is almost guaranteed. Fluidity The fluidity of clean melt is always higher than that of dirty melt, and can be cast at a lower temperature. The cumulative benefits are valuable. (oxide free casting high properties; low porosity high properties; low pouring temperature finer grain size, high properties Mechanical Properties Castings made using processes which reduce surface turbulence have been found to have uniform good mechanical properties with low scatter. They also show excellent fatigue resistance. 12

13 Al-4.5Cu fractured surfaces (a) Oxide covered (0.3 % elongation) (b) Ductile fracture (3.0 % elongation) 3. Shrinkage Defects 3.1 General Shrinkage Behaviour Liquid contracts on freezing because of rearrangement of atoms from open randomly-packed structure to a regular densely-packed structure. FCC and HCP solids contract more during solidification. What happens to a poorly fed casting? 13

14 A sphere has been fed via an ingate of negligible size and the source of feed metal is cut off after a solid shell of thickness X is produced. R 0 Liquid R X Feed Liquid dx What will happen during solidification of the next onion-layer of thickness dx? Solid Solidification model for an unfed sphere Either a pore will form or the liquid will expand a little to compensate the volume difference. If no favourable nucleus available for pore formation, the liquid has to accommodate this by expansion, creating a state of tension or negative pressure and sucking the solid shell inwards. Whether the driving force for pore formation wins over the driving force for feeding will depend on whether nuclei for pore formation exist. If not (i.e. the metal is clean), then pore will not be able to nucleate and feeding is forced to continue until the casting is completely frozen. If favourable nuclei are present, then pores will be created at an early stage before the development of any significant hydrostatic pressure, with the result that little feeding will occur and the casting will develop its full percentage of porosity as defined by the physics of phase change. 14

15 In most practical cases, the situation is somewhere between these two extremes, with castings displaying some internal porosity, together with some liquid feeding. In such cases feeding has continued under increasing pressure differences, until the development of a critical internal stress at which some particular nuclei, or surface puncture, can be activated at one or more points in the casting. Feeding is then stopped at such locality, and pore growth starts. When pores appear early freezing contraction Steel 3 vol.% Aluminium 7 vol.% diameter of shrinkage cavity formed in 100 mm dia sphere casting 31 mm 41 mm Theses considerable cavities require a dedicated effort to ensure that they do not appear in castings. There are occasions when castings having defects of only 1 or 2 mm in size are scrapped!! For vast majority of cast materials, therefore, shrinkage porosity is the most common and most important defect in castings. 15

16 3.2 Feeding the Six Rules 1. Heat transfer requirement 2. Volume requirement 3. Junction requirement 4. Feed path requirement 5. Pressure differential requirement 6. Pressure requirement 3.3 Feeding Mechanisms The gradual formation and growth of dendritic mass of solids during solidification presents increasing difficulties for the passage of feeding liquids. During solidification, the pressure inside liquid also falls, causing and increased pressure difference between the inside and outside of the casting. Such negative pressure difference is undesirable in casting because it causes problems by providing the driving force for the initiation and growth of volume defects such as porosity. 16

17 There appears to be at least five mechanisms by which such pressure difference (and the hydrostatic tension caused by it) can be reduced in solidifying material. 1. Liquid feeding 2. Mass feeding 3. Interdendritic feeding 4. Burst feeding 5. Solid feeding Schematic representation of the five feeding mechanisms in a solidifying casting Liquid feeding Generally precedes other forms of feeding. For skin-freezing materials, this is the only method of feeding. Occurs at the early stage of solidification Wide feed path due to low liquid viscosity, and the pressure difference required to continue feeding is negligibly small (~ 1 Pa). When about 99 % solidification is completed, the pressure could reach up to about 100 Pa only (1 atm 10 5 Pa). For all practical purposes, therefore, the hydrostatic stresses created during liquid feeding never causes a problem!! 17

18 Inadequate feeding only resulted when inadequate-sized feeder is used. Feeding terminates early and air is drawn into the casting. Two forms of porosity resulted: Skin-freezing alloys smooth shrinkage pipe, extending from the feeder into the casting as a long funnel-shaped hole. Long-freezing-range alloys feeding occurs through interdendritic channels porosity resembles a mass of spongy, interconnecting shrinkage pipes. Mass feeding Movement of slurry of solidified metal and residual liquid. This movement is arrested when the volume fraction of solid reaches anywhere between 0 and 50 % depending on Pressure differential that driving the liquid Amount of free dendrites in the liquid Role of mass feeding is of minor importance since the critical stages of feeding which most influence defects occur later after mass feeding comes to a stop. Mass feeding period can be extended by grain refinement and formation of more equiaxed grains. 18

19 Interdendritic feeding Feeding of residual liquid through mushy zone. The pressure gradient required for interdendritic feeding of a cylindrical area of pasty zone. DP 2 2 a l L d 32 h 1- a 4 2 R D 2 h = viscosity of liquid d = dendrite arm spacing a = solidification shrinkage R = radius of capillary L = length of past zone D = diam. of pasty zone l = heat-flow constant (rate of freezing) The pressure difference is the most sensitive to the size of flow channel, R. DP becomes extremely high as R becomes small. In the absence of suitable nuclei for pore formation, the high hydrostatic pressure is somewhat compensated by the inward collapse of the solid. Effect of the presence of eutectic Eutectics (short-freezing range alloys) solidify in the planer mode. In presence of eutectic, the interdendritic flow paths do not taper to zero, but finish abruptly trimmed. 19

20 Burst feeding As solidification progresses, both hydrostatic stress inside liquid and strength of feeding barrier increase in a poorly fed region of casting, but at different rates. If stress grows at a faster rate, failure of casting is expected. If the barrier is only a partial barrier, failure may not occur. Instead, feeding occurs in a burst when a sudden yield of feeding barrier occurs due to hydrostatic tension. The internal stress will be reduced to allow the casting to remain free from shrinkage porosity. If the barrier is substantial, it may never burst, causing the resulting stress to rise and eventually exceeds the pore nucleation threshold. The stress is then released by forming the shrinkage cavity. Solid feeding At the later stage of solidification, certain sections of casting may become isolated from feed liquid. Further solidification in this isolated region would cause a high hydrostatic stress in the remaining liquid, high enough to cause the surrounding solidified shell to deform inwards by plastic or creep flow. P = 2 Y ln (R 0 /R) Y = Yield stress of solid R 0 = Radius of spherical casting R = Internal liquid radius Stress in liquid developed depends upon the plastic flow of solid that, in turn, is a function of yield stress and geometry of casting. For iron sphere of 20 mm in diameter, P can be reached up to -200 to -400 atm when the casting is % solid. In the last liquid drops, this can reach up to atm! 20

21 3.4 Initiation of Shrinkage Porosity In absence of gas, and if feeding is adequate, no porosity will be formed. For large and/or complex castings, one or more regions of casting are not well fed and the liquid contains dissolved gasses. The internal hydrostatic stress reaches to a level when internal pore can form. If solid feeding occurs, internal pore will not occur, but the solidification shrinkage will appear at the surface of the casting. 21

22 Internal porosity by surface initiation If the liquid is connected to the outside surface, then the liquid can be sucked in, causing the porosity to form connected to the surface. The sucking of liquid from surface also draws air which flows along the interdendritic channel causing air feeding (as opposed to liquid feeding) into the casting. The porosity formed in this way is indistinguishable from microporosity. In thin-section castings, little or no feeding is necessary and sucking of surface liquid is negligible. For intermediate thickness, surface initiated pores can occur because of interdendritic feeding problems. This pore-forming mechanism is common in long freezing range alloys at a later stage of solidification, initiated often from a hot spot. Sometimes these pores can connect internally two opposite surfaces of thin casting causing these alloy castings unsuitable for pressure-tight applications. A high enough positive internal pressure is necessary at all locations to prevent initiation of this type of surface-connected internal porosity. 22

23 Internal porosity by nucleation Short-freezing range alloys do not exhibit surface-connected porosity. A sound, solid skin is formed at the early stage of solidification. Feeding is not a problem at this stage. At the end of freezing, pores are nucleated in the interior of liquid due to poor feeding which have no connection with the outside surface of casting. After nucleation, further solidification results growth of these pores. These centreline porosity are concentrated near the centre of the casting and do not impair the leak-tightness of the casting. In presence of surface-activated foreign particles, nucleating of these pores is not a problem. In absence of such particles, nucleation only occurs when the internal pressure accumulates and reaches to a certain threshold value, P f, called the fracture pressure. The gas pressure, P g, inside the liquid will join the negative pressure, P s, to push the liquid away for the nucleation of the pore. P f = P s + P g 2g/r = P i P e = DP 2g/r* = P g (-P s ) = P f (condition for pore formation) (for pore of critical size) 23

24 For well-fed castings with dissolved gases, P s = 0. Freezing will proceed along the line ADCE. Gas pore will form heterogeneously at E on nucleus 1. Gas pressure goes back towards D. For poorly fed casting having no dissolved gases, P g = 0. Internal pressure falls along the line AF. At F, fracture pressure for heterogeneous nucleation on nucleus 1 is met, and a shrinkage cavity forms. The hydrostatic tension is released and pressure returns to A. In practice, both gas and shrinkage will be present to some degree, and the freezing will progressed along the line ABCD. Both porosity and cavity will form. External porosity If internal porosity is not formed, then the surface will sink due to solid feeding. Adequate positive internal pressure would reduce or eliminate solid feeding and the casting would be sound and maintain its shape. Too high an internal pressure would reverse the movement of the surface and make the casting swell. Examples: GCI, castings having high head of metal. 24

25 3.4 Growth of Shrinkage Porosity The internal pores nucleated within a stressed liquid grow explosively fast at the beginning. The subsequent growth of pore occurs leisurely and is controlled by the rate of solidification i.e., the rate of heat extraction. For surface-initiated pores, the rate is slow since the initial stress is lower and the puncture of surface will occur relatively slowly as the surface collapses plastically into the forming hole. When pore formed as shrinkage pipe, the growth is progressive and controlled by the rate of heat extraction from the casting. 3.5 Final Form of Shrinkage Porosity Shrinkage cavity or pipe During liquid feeding, the solidification front progresses gradually towards the centre of the casting. The liquid level in the feeder falls gradually, thus generating a smooth conical funnel shaped shrinkage pipe. The secondary shrinkage cavities are only an extension of the primary pipe. When shrinkage problem occurs in an isolated region inside the casting: For short-freezing-range alloys, shrinkage pipe would occur. The shape of this pipe is similar to those occur in case of liquid feeding. For long-freezing range alloys, layer porosity is formed in an isolated region inside the casting. 25

26 Layer porosity Usually observed in all types of casting alloys. Conditions favourable for nucleation of layer porosity: long pasty zone poor temperature gradient alloys with high thermal conductivity moulds with low rate of heat extraction Initially layer porosity is believed to be caused by the same mechanism that causes hot tear. Presently, its formation can be explain by the equations that governs the interdendritic feeding mechanism: DP 16 a dr N 1 a dt 1 R 3 2 x Lx a l L d DP 32 h 1- a 4 2 R D The hydrostatic tension increases parabolically with distance x though the pasty zone of length L. The stress continues to increase parabolically with advancing solidification. Local stress exceeds the threshold value and a pore will form. The pore will spread immediately along the isobaric surface and form a layer. Local stress will dissipate instantly. The maximum stress will be at the centre of the remaining liquid and amounting about 1/4th of the original. Stress once again will increase with time and second layer of porosity will form. Further nucleation and growth events will produce successive layers until the whole casting is solidified. The final state consists of layers of porosity with considerable interlinking. 26

27 So it is clear that centreline porosity, layer porosity, and dispersed porosity transform imperceptibly from one to the other. Also, as the gas content of the alloy is increased, the shrinkage porosity changed gradually from layer porosity to dispersed pinhole porosity. In real castings, the nature of porosity is mixed in nature, allowing a complete spectrum of possibilities from pure shrinkage layer type to pure gas-dispersed type. Next Class MME 6203, Lecture 03 Structure, Defects and Properties of Finished Castings 2. Linear contraction, hot tear and cold crack and residual stresses 27