Billet quality directly controls the quality of extrus - PDF document

l Billet quality directly controls the quality of extrusions and affects productivity, in term profitability. l The quality of billets& cost are the two important considerations for extrusion producers.l Day today advancement in billet process, technology & equipment plays critical role. Principals of Billet Making Basic Functional Billet Making Process Diagram QA Lab Analysis Perspective 32 October 2011 Billet Cutting Homogenising Cast Log There are many steps involved between the inputs of raw material to the final casting process:The direct chill (DC) continuous casting process developed in 1933 by W.T.Enoor. DC casting has several advantages compared with other techniques such as tilt mould casting.. l Has minimum Metal Segregation l Can produce large ingots/ billets l Flexibility to cast varying speedl Minimum cracking in hard alloys l Molten metal slowly and uniformly with relatively low temperature to avoid many problems. Current Aluminium Billets Casting Technology The molten aluminium is poured into a shallow, watercooled mould normally of round, cross section shape. When metal begins to freeze in the mould,the false bottom in the mould is lowered at a controlled speed, and water is sprayed on the surface of the f the mould. The sensors and programmed logic, the automated systems can now control a multitude of casting parameters. l The water cooled mould, used in hot top casting machine, is complex assembly and not easy to change. l Different Alloys / Sizes, requires different moulds. l The modular machine design (Wagstaff ~ Developed decade ago) permits casting at a different speeds and billet sizes in the same machine. Casting Practices / Casting VariablesThe principal variables that influence the production performance are: l Pouring Temperature : In the furnace should be kept as low as possible to prevent gas pickup and formation of oxide. Metal should reach 0 to the casting unit at 28~30 C above liquid temperature, of the alloy being cast. (Liquid's temperature is different for different alloys) lthe alloy and billet size. This is the most important variable in the DC cast process. l Type of Mould : Lightweight mould with good machinability and good thermal conductivity i. e. (aluminium alloy 6061/5052are suitable). These moulds are so designed that the cooling water first comes in contact with the mould wall and then passes on to the surface of billet as it comes out of mould.l Metal Head : Is the distance from bottom of mould to the liquid surface and is usually kept at a depth of at least 2” l Rate of Water Flow : Approximate 432 but/lb of heat must be transferred the metal. Temperature and rate of cascading water flow through mould wall must be adjusted so that water will wet the entire surface of billet being cast.Charging MaterialVirgin ingots, alloying elements, master alloys & in house process scraps are the major components charged in the furnace.Tight control of hardener chemistry is essential.Alloying elements with High melting points (Silicon ~Manganese ~Nickel ~Copper and Chromium) are added in the form of master alloys (As small ingots / bars) as hardeners. While these are added to molten metal ensure that metal is not over heated.Alloying elements with low melting points (Magnesium & Zinc) usually added in pure state and in bar form to molten aluminium.In house scrap (from previous operations) or segregated purchased scrap can be added to be cost competitive in market. To maintain billet/ extrusion quality, proper balancing of scrap and molten aluminium is required while adding the same.Melt & HoldingSingle furnace is possibleBut for large operations single- hearth (refractory lined) two furnaces with special coating on inner wall of melting furnace preferred. Advantages : l Better heat transfer, shorter time to uniform temperature. Is determined by Casting Speed : ldue to degradation. l Smooth and non-porous wall surfaces, less contamination due to minimized residue.After aluminium and alloying elements are melted together in suitable furnace, the melt is transferred to the holding furnace.The tilting furnace with fully open front single door facilitates stirring, skimming and cleaning of hearth. Melting furnace with electromagnetic field stirring works with principles : l Low frequency electromagnetic field is applied to micro physical field in casting process. l When electromagnetic field is applied in sump pool, it generates convection mixing near the solidification front at the centre of the billet. Vigorous forces convection induced by the electromagnetic field detaches the dendrite fragments from the solidification front, which increases the nucleation rate and number, promotes the formation of the fine equated grain structure of the billet. Advantages are : l Melt temperature is 5~10 degree lower than conventional melting furnace which reduces liquid region in the billet head near the mould called as sump. l Homogeneous alloying elements.l Promotes the formation of the fine equated grain structure.Fluxing DegasingFluxing is a chemical treatment of molten aluminium.The Chemical compounds are usually inorganic salt mixtures. Fluxing also includes the treatment of aluminium melts by inert or reactive gases to remove solid or gaseous impurities. In line gas- fluxing with cylindrical rotorremoves 61~66% of hydrogenand decrease inclusions more than 65%.Rotary flux injector supplied by STATS (Canada)The principal types of fluxes are:l Cover Fluxes l Cleaning Fluxes l Drossing Fluxes l Refining Fluxes Reduce contamination from bricks Solid and liquid Impurities may appear in the form of liquid and solid inclusions that persist through melt solidification, into casting billets. Fluxing of melt facilitates the accumulation and segregation of undesirable constitutes from the melt. Gaseous Impurities : Hydrogen gas is the only gas with appreciable solubility in both liquid( app 1.4 ppm) and solid ox. 0.12ppm)Degasing is big concern & major in subsequent billet casting.Potential Sources of Hydrogen are:l Furnace atmospherel Charge materiall Fuell Fluxesl Metal & Mould Reactionl External Mould In solid aluminium hydrogen is detrimental causing porosity in casting & chances of blister formation during extrusion also in subsequent heat treatment of extruded shape.Methods used are :l Gas purgingl Tablet Type Flux Degasingl Mechanical Mixer Refinement of Metal :The metal flowing from the holding furnace to DC casting machine is first refined (In Mixer) with inert gas ARGON or mixture of ARGON + CHLORIN. l It removes dissolved hydrogen and non-metallic impurities by means of spinning nozzle.l Exhaust system is essential for chlorination process. Chorine is the most effective gas for degassing. Grain RefiningThe goal is to get a fine equated grain structure. Regardless of casting mould (Graphite or Composite), determination of liquidated zone becomes difficult. The depth of liquidation varies within the band of large grains with coarser, feathered grain structure. Advantages are: Minimizes l Shrinkagel Hot crackingl Hydrogen porosity.Improves l Mechanical propertiesl Response to thermal treatmentl Electromechanical process l Resistance to surface tearing. Reasons of different types of grain structure l Alloy chemistryl Solidification ratel Addition of grain refiner Widely used grain refiners are master alloys of ll Ti Bi wire Titanium + Boron l Company can decide product excellence standards l To keep bulk customers happy may link it with specific customer quality plan Quantitative method of measuring the quality of process and material. Filtration SystemFiltration is the heart of quality of the billet. Filtration is an on-line process that occurs just before the molten metal enters the DC cast machine.Types of solid –phase inclusions in the molten aluminium:l Oxides l Spinalsl Boridesl Carbides ll Nitridesl Refractory inclusionsTypes of Filters l Metal l Fiber glass screenl Rotary degassingl Bed filtersl Bonded particle filtersl Cartridge filters l Deep bed ceramic foam filters Intermetallic Perspective 34 October 2011 l Ease of Usel Economicsl Space Constraintsl Auxiliary Metal Treatment Capabilitiesl Desired Filtration Efficiencyl End Product Applications Selection of filter depends upon ll Refractorinessl Thermal Shock Resistancel Corrosion Resistance Strength Selection of filter material is importantMust satisfy Vertical Casting V/s Horizontal CastingVertical DC casting machine developed in 1933 by W.T.Enoor and is in vogue.In 1989, one of the Japanese developed high speed horizontal continuous casting machine. Speed is nearly 3 times more than conventional casting machine. l Chemistryl Dimensional tolerancesl Billet Surface qualityl Shell Zonel Billet homogenizing Homogenisation of BilletsAs cast billets has a lower workability for following reasons: l Grain boundary and dendritic cell segregation, low melting point eutectics and brittle intermediate compounds l Supersaturated solutions of finely dispersed precipitates of alloying components increase the high temperature flow stress. Steps to be followed Reasons Certain alloying elements,including manganese, iron, and zirconium either finely dispersed or precipitates retard crystallization. It affects colour anodizing of extrusion of Al Mg Si (Mn) As regards Almgsi alloys extrusions During cooling after casting reduces the hardenability of Mg2Si in Almg2si extrusions impairs the finish of bright finish alloys Grain boundary segregation Variation in concentration of dissolved alloying elements result in a streaked texture after anodizing Heterogeneous grain and cell boundary precipitation gives structural marking. Reduce the quality of the finish of bright finish section. Steps to be followed Reasons Heating billet logs with a particular rate. Diffusion of respective alloy element present is enhanced. Holding constant temperature for a certaintime. Any concentration gradients within the alloy are diminished. Keep higher temperature Faster homogenizing obtained Higher temperature should not exceed lowest melting point of element present in alloy. To avoid localised melting Results in localised melting. This could cause microstructural damage (excessive void formation / segregation /blistering/ cracking) cannot be repaired subsequently. Cooling with proper cooling rate. Uniform cooling rate is desired. Get good microstructure /improve properties /improve productivity. Faster cooling rate not desired Shows opposite results, Variable temperature in the homogenizing furnace not desired Can cause excessive/ under homogenization can affect microstructure / physical properties. Steps in Homogenizing Horizontal continuous billet casting with continuous homogenizing. The billet moves continuously through the furnace by means of a walking beam system. In this system every billet receives exactly the same temperature and time for heat treatment compare to batch type furnace.Billet ScalpingMeans machining of the liquidated skin of the billet generally for harder alloys and also for the wider profile of softer alloys where CCD (circumscribed circle diameter) of the die is very close to container bore.For indirect extrusionprocess where there is no relative displacement between billet and container, the machined billets are being used.Casting DefectsThe objective of the DC casting process is to produce quality billets of:l Uniform Chemistryl Fine metal grain formationl Strength Problems or defects associated with DC casting billets are:l Segregationl Bleedingl Cold shuttingl Porosityl Grain growth Cracking or Splitting Difference in the contraction from the skin to the centre of the billet due to temperature gradient from skin to the centre. In the solidification process, metal starts freezing from the outer skin to the centre of the mould because outer skin is cooled by water flow. Because of difference in contraction from the skin to the centre of the cast due to gradient difference in temperature thermal stresses develop. Internal stress causes cracks only when it exceeds the tensile stress limit of the alloy being cast. Surface Segregation Billet structure, is one of the origins of streak defects in extrusion.Surface segregation or shell zone present in the billet microstructure during early stage of solidification during DC casting and located in the billet peripheral zone. High concentration of lower melting temp. constitutes is forced by the pressure of metal above it to fill up the gab left in solidified metal found at the outer surface (normally 6%) due to shrinkage.Liquidation occurs in the combination with freezing and partial remelting of the molten aluminium with cold mould surface. Inverse Segregation Low Melting Temp BleedingHigh Casting speed defect Flow of molten metal along solidified outer billet surface. Causes because of casting at higher speed causes hot liquid metal at the centre. Relatively slow casting speed with rapid cooling causes as wrinkle as deep 3.2~6.4mm on the outer billet surface.Moderate speed avoids liquidation, where the elements with low melting point ooze to the outside surface of billet. Cold Shutting Slow Casting Speed Followed by Rapid Cooling Defect It is generally in the columnar grain along the circumferences of the billet. Grain Growth