Fundamentals of metal casting

n caused by oxide film • Inclusions: Insoluble particles can increase viscosity, reducing fluidity • Solidification pattern: Fluidity is inversely proportional to the freezing temperature range 1. FLUIDITY OF MOLTEN METAL 1. FLUIDITY OF MOLTEN METAL • Mold design: The design and size of the sprue, runners, and risers affect fluidity • Mold material and surface: Thermal conductivity and roughness decrease fluidity • Superheating: The temperature increment above the melting point increases fluidity • Pouring: Lower pouring rates decrease fluidity because of faster cooling • Heat transfer: Affects the viscosity of the metal Fluidity Test Fluidity Test 2. MOLD FILLING • Bernouli’s equation: • Reynold’s number: Re= d/v • Short filling time • Potential turbulence h + p/g + v2/2g = const 2. MOLD FILLING • For this step to be successful, metal must flow into all regions of the mold, most importantly the main cavity, before solidifying • Factors that determine success – Pouring temperature – Pouring rate – Turbulence • Pouring temperature should be sufficiently high in order to prevent the molten metal to start solidifying on its way to the cavity 2. MOLD FILLING Pouring rate should neither be high (may stuck the runner – should match viscosity of the metal) nor very low that may start solidifying on its way to the cavity Turbulence should be kept to a minimum in order to ensure smooth flow and to avoid mold damage and entrapment of foreign materials. Also, turbulence causes oxidation at the inner surface of cavity. This results in cavity damage and poor surface quality of casting. Engineering Analysis of filling 1.v: velocity of liquid metal at base of sprue in cm/sec; g: 981cm/sec.sec; h: height of sprue in cm 2.v1: velocity at section of area A1; v2: velocity at section of area A2 3.V: volume of mold cavity COOLING EFFECTS ON MOLD CAVITIES FILLED WITH MOLTEN METAL Challenges of Molten Metal • Hot metal readily forms oxides (dross or slag) – Can be carried into the mold – Can be controlled by pouring methods – Control of temperature and atmosphere can slow creation of slag Challenges of Molten Metal • Dissolved gases – Porosity – Can be controlled by: • Vacuum degassing • Gas flushing • “Killing” - Reacting trapped gas with material that will form buoyant compound that will float to surface – Oxygen removed from copper by adding phosphorous – Oxygen removed from steel by adding aluminum or silicon Challenges of Molten Metal Solubility of hydrogen in aluminum. Note the sharp decrease in solubility as the molten metal begins to solidify. Challenges of Molten Metal • Temperature Control – Temp too Low • Misruns • Cold shuts – Temp too High • Excessive mold wear • Higher reactivity of molten metal • Penetration defects (excessive flash or entrapped sand 3. SOLIDIFICATION 3. SOLIDIFICATION 3.1.Classification of solidification processes 3.2.Solidification Defects • Shrinkage - Contraction of a casting during solidification. • Microshrinkage - Small, frequently isolated pores between the dendrite arms formed by the shrinkage that accompanies solidification. • Gas porosity - Bubbles of gas trapped within a casting during solidification, caused by the lower solubility of the gas in the solid compared with that in the liquid. • Sievert’s law - The amount of a gas that dissolves in a metal is proportional to the partial pressure of the gas in the surroundings. 3.3. Solidification of Metals • Transformation of molten metal back into solid state • Solidification differs depending on whether the metal is – A pure element or – An alloy – An eutectic alloy 3.3.1. Solidification of Pure Metals (a) Temperature as a function of time for the solidification of pure metals. Note that the freezing takes place at a constant temperature. (b) Density as a function of time 3.3.2. Alloy Solidification • Most alloys freeze over a temperature range • Phase diagram for a copper-nickel alloy system and cooling curve for a 50%Ni-50%Cu composition 3.3.2. Alloy Solidification Schematic illustration of alloy solidification and temperature distribution in the solidifying metal. Note the formation of dendrites in the mushy zone. 3.3.3. Solidification of Eutectic Alloys • Eutectic alloys solidify similar to pure metals. • Eutectic point on phase diagram is a point at which the liquid, on cooling, completely converts into solid at one temp. No intermediate phase (L+S) exists. • Al-Si (11.6% Si) and Cast Iron (4.3% C) are relevant casting eutectic alloys. 3.4. Directional Solidification • To minimize effects of shrinkage, it is desirable for regions of the casting most distant from the liquid metal supply to freeze first and for solidification to progress from these regions toward the riser(s) – Thus, molten metal is continually available from risers to prevent shrinkage voids – The term directional solidification describes this aspect of freezing and methods by which it is controlled 4.THERMODYNAMIC OF SOLIDIFICATION 4.1. Solidification of a pure element 4.2. Dendrite crystallization Dendrite formatition • In alloys, such as Fe-C, freezing and solidificaion occurs overa wide range of temp. There is no fine line of demarcation exists between the solid and liquid metal. • Here, ‘start of freezing’ implies that grain formation while progressing towards the center does not solidify the metal completely but leaves behind the islands of liquid metals in between grains which freeze later and there is multidirectional tree like growth. 4.3. Effects of Cooling Rates • Slow cool rates results in course grain structures (102 K/s) • Faster cooling rates produce finer grain structures (104 K/s) • For even faster cooling rates, the structures are amorphous (106 – 108 K/s) • Grain size influences strength of a material • Smaller grains have higher ductility and strength • Smaller grains help prevent hot tearing and/or cracks in the casting Melt-Spinning (a) Schematic illustration of melt-spinning to produce thin strips of amorphous metal. (b) Photograph of nickel-alloy production through melt-spinning. Source: Siemens AG 5. KINETIC OF METAL SOLIDIFICATION 5.1. Solidification Time • Once the material cools down to freezing temperature, the solidification process for the pure metals does not require a decrease in temperature and a plateau is obtained in the cooling curves, called thermal arrest. The solidification time is total time required for the liquid metal to solidify. • Solidification time has been found to be directly proportional to volume and inversely proportional to surface area. 5.1. Solidification Time • Total solidification time TTS = time required for casting to solidify after pouring • TTS depends on size and shape of casting by relationship known as Chvorinov's Rule where TTS = total solidification time; V = volume of the casting; A = surface area of casting; n = exponent with typical value = 2; and Cm is mold constant.        n TS m V T C A Mold Constant in Chvorinov's Rule • Mold constant Cm depends on: – Mold material – Thermal properties of casting metal – Pouring temperature relative to melting point • Value of Cm for a given casting operation can be based on experimental data from previous operations carried out using same mold material, metal, and pouring temperature, even though the shape of the part may be quite different What Chvorinov's Rule Tells Us • Casting with a higher volume-to-surface area ratio cools and solidifies more slowly than one with a lower ratio – To feed molten metal to the main cavity, TTS for riser must be greater than TTS for main casting • Since mold constants of riser and casting will be equal, design the riser to have a larger volume-to-area ratio so that the main casting solidifies first – This minimizes the effects of shrinkage Solidification of Iron and Carbon Steels (a) Solidification patterns for gray cast iron in a 180-mm (7-in.) square casting. Note that after 11 minutes of cooling, dendrites reach each other, but the casting is still mushy throughout. It takes about two hours for this casting to solidify completely. (b) Solidification of carbon steels in sand and chill (metal) molds. Note the difference in solidification patterns as the carbon content increases. Source: After H. F. Bishop and W. S. Pellini Solidified Skin on a Steel Casting Solidified skin on a steel casting. The remaining molten metal is poured out at the times indicated in the figure. Hollow ornamental and decorative objects are made by a process called slush casting, which is based on this principle. Source: After H. F. Taylor, J. Wulff, and M. C. Flemings 5.2. Temperature Distribution during Metal Solidification Temperature distribution at the interface of the mold wall and the liquid metal during the solidification of metals in casting Comparison: Sand Mold vs Metal Mold Sand Mold Sand casting Metal Mold Die casting 5.3. Solidification of ingots and castings © 2 0 0 3 B ro o k s/ C o le , a d iv is io n o f T h o m so n L ea rn in g , In c. T h o m so n L ea rn in g ™ is a t ra d em ar k u se d h er ei n u n d er l ic en se . Development of the ingot structure of a casting during solidification: (a) Nucleation begins, (b) the chill zone forms, (c) preferred growth produces the columnar zone 3, and (d) additional nucleation creates the equiaxed zone 5.3. Solidification of ingots and castings - Because of the chilling action of the mold wall, a thin skin of solid metal is initially formed at interface immediately after pouring. - The skin formed initially has equi-axed, fine grained and randomly oriented structure. This is because of rapid cooling. - As freezing proceeds, the grains grow inwardly, away from heat flow direction, as needles or spine of solid metal. 5.3. Solidification of ingots and castings - On further growth of spine, lateral branches are formed, and as these branches grow further branches are formed at right angle to the first branches. This type of growth is called dendritic growth. - The dendritic grains are coarse, columnar and aligned towards the center of casting. 5.3. Solidification of ingots and castings - The dendrites begin to form with freezing. However, due to large temperature spread between solidus and liquidus, the earlier portion of dendritic grains extract higher % of elements from liquid solution than the portion of grain formed later. - As a result, the molten metal in the center of mold cavity depletes from the elements and hence forms a different structure (see Fig). Pure metal Fe-Ni Alloy Grain Structure in Casting Chill Zone – Rapid cooling near the surface creates many nucleation sites and many small, randomly oriented grains. Columnar Zone – Directionally oriented grains radiating inward from the surface of the part. Equiaxed Zone (not shown) – Randomly oriented, spherical crystals. (isotropic properties) Three Cast Structures of Solidified Metals •Schematic illustration of three cast structures of metals solidified in a square mold: (a) pure metals; (b) solid-solution alloys; and (c) the structure obtained by heterogeneous nucleation of grains, using nucleating agents. 5.4. Segregation Composition change during solidification 5.4. Segregation • The non-uniform distribution of impurities or alloying elements. The degree of segregation depends not only on the chemical composition of the alloy, but also on the rate of cooling, both of the ingot as a whole, and of each individual point within the mass. 5.4. Segregation • For example, near the surface, where the rate of cooling is rapid, the segregated impurities are trapped in the rapidly growing crystals. Fur- their inside the ingot, where the cooling is slower, the segregates will collect together and produce the so-called ghosts, or they may tend to rise to the surface and collect in the scrapped ingot head. In normal segregation, the constituents with the lowest melting points concentrate in the last portions to solidify, but in inverse segregation this is reversed. The segregation tends to form in bands sloping inwards to the top of the ingot (A segregate) and at the same time, due to shrinkage, it takes a V shape (V segregate) along the upper part of the ingot axis. Microsegregation