Fundamentals of metal casting - Nguyen Ngoc Ha (Part 2)

f solid metal 6.2. Solidification Shrinkage • Occurs in nearly all metals because the solid phase has a higher density than the liquid phase • Thus, solidification causes a reduction in volume per unit weight of metal • Exception: cast iron with high C content – Graphitization during final stages of freezing causes expansion that counteracts volumetric decrease associated with phase change Solidification Contraction or Expansion 6.3. Hot Spots Hot Spots and Shrinkage Cavities Hot Spots and Shrinkage Cavities 6.4. Risers and Hot Spots Why Riser? • The shrinkage occurs in three stages, 1. When temperature of liquid metal drops from pouring to zero temperature 2. When the metal changes from liquid to solid state, and 3. When the temperature of solid phase drops from freezing to room temperature • The shrinkage for stage 3 is compensated by providing shrinkage allowance on pattern, while the shrinkage during stages 1 and 2 are compensated by providing risers. • The riser should solidify in the last otherwise liquid metal will start flowing from casting to riser. It should promote directional solidification. The shape, size and location of the risers are important considerations in casting design Progressive Solidification in Riser 6.5. Shrinkage Allowance • Patternmakers correct for solidification shrinkage and thermal contraction by making the mold cavity oversized • Amount by which mold is made larger relative to final casting size is called pattern shrinkage allowance • Casting dimensions are expressed linearly, so allowances are applied accordingly 6.6. 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 Achieving Directional Solidification • Directional solidification is achieved using Chvorinov's Rule to design the casting, its orientation in the mold, and the riser system that feeds it – Locate sections of the casting with lower V/A ratios away from riser, so freezing occurs first in these regions, and the liquid metal supply for the rest of the casting remains open – Chills - internal or external heat sinks that cause rapid freezing in certain regions of the casting Achieving Directional Solidification 6.7. Chills • Chills Pieces of material placed in the mold to speed up heat transfer in thicker areas of the part to prevent shrinkage porosity • Internal chills are left within the cast part; external chills are removed External Chills • (a) External chill to encourage rapid freezing of the molten metal in a thin section of the casting; and (b) the likely result if the external chill were not used External Chills Internal Chills Types of Internal and External Chills used in Casting Various types of (a) internal and (b) external chills (dark areas at corners) used in castings to eliminate porosity caused by shrinkage. Chills are placed in regions where there is a larger volume of metal, as shown in (c). 6.8. Riser Design • Riser is waste metal that is separated from the casting and re-melted to make more castings • To minimize waste in the unit operation, it is desirable for the volume of metal in the riser to be a minimum • Since the shape of the riser is normally designed to maximize the V/A ratio, this allows riser volume to be reduced to the minimum possible value Riser • Provide additional material to fill in as shrinkage occurs • Live risers receive hot metal directly entering the mold • Dead risers are filled by hot metal that has already passed through the mold • Blind risers are closed to the atmosphere • Open risers penetrate through the mold cavity • Side risers feed the part through the runner / gate system • Top risers are attached directly to the part Location of Risers and Open and Closed Risers • Top riser has the advantage of additional pressure head and smaller feeding distance over the side riser. • Blind risers are generally bigger in size because of additional area of heat conduction Location of Risers and Open and Closed Risers 7. STRESSES IN CASTINGS Hot Tears in Castings Examples of hot tears in castings. These defects occur because the casting cannot shrink freely during cooling, owing to constraints in various portions of the molds and cores. Exothermic (heat- producing) compounds may be used (as exothermic padding) to control cooling at critical sections to avoid hot tearing Hot Tears in Castings 8. CASTING DEFECTS •Defects may occur due to one or more of the following reasons: – Fault in design of casting pattern – Fault in design on mold and core – Fault in design of gating system and riser – Improper choice of moulding sand – Improper metal composition – Inadequate melting temperature and rate of pouring 8.1. Classification of casting defects Casting defects Surface Defect Internal Defect Visible defects Blow Scar Blister Drop Scab Penetration Buckle Blow holes Porosity Pin holes Inclusions Dross Wash Rat tail Swell Misrun Cold shut Hot tear Shrinkage/Shift 8.2. Surface Defects These are due to poor design and quality of sand molds and general cause is poor ramming Blow is relatively large cavity produced by gases which displace molten metal from convex surface. Scar is shallow blow generally occurring on a flat surface. A scar covered with a thin layer of metal is called blister. These are due to improper permeability or venting. Sometimes excessive gas forming constituents in moulding sand 8.2. Surface Defects • A scab when an up heaved sand gets separated from the mould surface and the molten metal flows between the displaced sand and the mold. • Penetration occurs when the molten metal flows between the sand particles in the mould. These defects are due to inadequate strength of the mold and high temperature of the molten metal adds on it. 8.2. Surface Defects • Drop is an irregularly-shaped projection on the cope surface caused by dropping of sand. • Buckle is a vee-shaped depression on the surface of a flat casting caused by expansion of a thin layer of sand at the mould face. A proper amount of volatile additives in moulding material could eliminate this defect by providing room for expansion. 8.3. Internal Defects The internal defects found in the castings are mainly due to trapped gases and dirty metal. Gases get trapped due to hard ramming or improper venting. These defects also occur when excessive moisture or excessive gas forming materials are used for mould making. Blow holes are large spherical shaped gas bubbles, while porosity indicates a large number of uniformly distributed tiny holes. Pin holes are tiny blow holes appearing just below the casting surface. Inclusions are the non-metallic particles in the metal matrix, Lighter impurities appearing the casting surface are dross. 8.4. Visible Defects 8.4. Visible Defects • Insufficient mould strength, insufficient metal, low pouring temperature, and bad design of casting are some of the common causes. • Wash is a low projection near the gate caused by erosion of sand by the flowing metal. Rat tail is a long, shallow, angular depression caused by expansion of the sand. Swell is the deformation of vertical mould surface due to hydrostatic pressure caused by moisture in the sand. 8.4. Visible Defects • Misrun and cold shut are caused by insufficient superheat provided to the liquid metal. • Hot tear is the crack in the casting caused by high residual stresses. • Shrinkage is essentially solidification contraction and occurs due to improper use of Riser. • Shift is due to misalignment of two parts of the mould or incorrect core location. 9. DESIGN FOR CASTS AND MOLDED PARTS 9.1. Visualize the Casting • It is difficult to follow section changes and shapes from blueprint. • Create a model to scale or full size to help designer to: – See how cores must be designed, placed or omitted – Determine how to mold the casting – Detect casting weaknesses (shrinks / cracks) – Determine where to place gates and risers – Answer questions affecting soundness, cost and delivery 9.2. Design for Soundness Most metals, alloys, and plastics shrink when they solidify Design components so that all parts increase in dimension progressively to areas where feeder heads (risers) can be placed to offset shrinkage • Disguise areas of shrinkage when unavoidable - The mold and pattern should be made larger than the casting by the amount of shrinkage 9.2. Design for Soundness - Shrinkage of casting varies not only with material but also with shape, thickness, casting temperature, mold temperature and mold strength - Thicker areas will cool slower than thinner areas. Areas of transition between thick and thin (ribs, walls, embosses, etc) will be prone to sink marks. Different tool shops and different materials will require a certain rib-to- wall thickness ratio. 9.2. Design for Soundness • The table below shows an average amount of shrinkage for important cast metals Design Rules: Disguising Sink Marks 9.3. Avoid Sharp Angles & Corners • When two or more sections conjoin, mechanical weakness is induced at the junction interrupting free cooling (most common defect in casting design). – Replace sharp angles with radii and minimize heat and stress concentration – In cored parts avoid designs without cooling surfaces – A rounded junction offers a more uniform distribution of strength Design Rules:Corners & Radii Good Corner Design Incorrect Corner Design Incorrect Corner Design Incorrect Corner Design • Generous radius • Uniform wall thickness • Smooth flow transition • Very sharp radii • High stress concentration • Sharp flow transition • Inside / outside radius mismatch • Non-uniform wall thickness • Non-uniform flow transition • Outside corner and inside radius • Non-uniform wall thickness • Non-uniform flow transition • Shrinkage stress / voids / sinks Sink 9.4. Minimize the Number of Sections  A well designed casting brings the minimum number of sections together at one point. • Staggering sections (where possible) • Minimizes hot spot effects • Eliminates weakness • Reduces distortion • Where staggering sections is not possible use a cored hole through the center of the junction. • Helps to speed solidification • Helps to avoid hot spots 9.5. Employ Uniform Sections • Thicker walls will solidify more slowly. – This means they will feed solidifying thinner walls. – Results in shrinkage voids in the thicker walls • Goal is to design uniform sections that solidify evenly. – If this is not possible, all heavy sections should be accessible to feeding from risers. Design Rules: Wall Uniformity Original Part Design • Very thick wall sections • Non-uniform wall thickness • Sharp inside and outside radii Improved Part Design • Thinner wall sections • More uniform wall thickness • Inside and outside radii (when possible) 9.6. Correctly Proportion Inner Walls • Inner sections of castings cool much slower than outer sections. – Causes variations in strength properties • A good rule of thumb is to reduce inner sections to 90% of outer wall thickness. • Avoid rapid section changes – Results in porosity problems similar to what is seen with sharp angles. Design Rules: Wall Uniformity Part gated from “thin to thick” hinders packing of thicker sections and can create flow problems. Gating from “thick to thin” when possible to improve flow and allow thicker sections to be packed. Internal runner to assist / improve the ability to pack the thick section when gating from “thin to thick” is necessary. Correctly Proportion Inner Walls • Whenever complex cores must be used, design for uniformity of section to avoid local heavy masses of metal. • The inside diameter of cylinders and bushings should exceed the wall thickness of castings. – When the I.D. is less than the wall it is better to cast the section as a solid. – Holes can be produced by cheaper and safer methods than with extremely thin cores 9.7. Fillet All Sharp Angles • Fillets (rounded corners) have three functional purposes: – To reduce the stress concentration in a casting in service – To eliminate cracks, tears and draws at re-entry angles – To make corners more moldable by eliminating hot spots – Improves flow of material • The number of different size fillet radii used in a pattern should be the minimized • Large fillets may be used with radii equaling or exceeding the casting section. – Commonly used to fulfill engineering stress requirements – Reduces stress concentration • Note: Fillets that are too large are undesirable – the radius of the fillet should not exceed half the thickness of the section joined. 9.7. Fillet All Sharp Angles • Large fillets may be used with radii equaling or exceeding the casting section. – Commonly used to fulfill engineering stress requirements – Reduces stress concentration • Note: Fillets that are too large are undesirable – the radius of the fillet should not exceed half the thickness of the section joined. • Tips to avoid a section size that is too large at an “L”, “V” or “Y” junction. • For an “L” junction : – Round an outside corner to match the fillet on the inside wall. (If this is not possible the designer must make a decision as to which is more important: Engineering design or possible casting defect) • For a “V” or “Y” junction: – Always design so that a generous radius eliminates localization of heat. Design Rules: Fillets & Corners Design Rules: Fillets & Corners Design Rules: Fillets & Corners 9.8.Avoid Abrupt Section Changes • The difference in relative thickness of adjoining sections should not exceed a ratio of 2:1. • With a ratio less than 2:1 the change in thickness may take on the form of a fillet. • Where this is not possible consider a design with detachable parts. • With a ratio greater than 2:1 the recommended shift for the change in thickness should take on the form of a wedge. – Note: wedge-shaped changes in wall thickness should not taper more than 1 in 4. • Where a combination of light and heavy sections is unavoidable, use fillets and tapered sections to temper the shifts. Design Rules: Section Changes Wall Thickness Transitions Tapered Transition Gradual TransitionStepped Transition Core out thicker areas where possible Poor Design Better Best 9.9. Maximize Design of Ribs & Brackets • Ribs are only preferable when the casting wall cannot be made strong or stiff enough on its own. • Ribs have two functions: – They increase stiffness – They help to reduce weight • Common mistakes that make ribs ineffective: – Too shallow – Too widely spaced Design Rules:Rib Dimensions • The thickness of the ribs should be approximately 80% of the adjoining thickness and should be rounded at the edge. • The design preference is for ribs to be deeper than they are thick. • Ribs should solidify before the casting section they adjoin. • The space between ribs should be designed such that localized accumulation of metal is prevented. • Preferably the ribs connect the attachment to the loading point. Design Rules:Rib Dimensions Design Rules:Rib Dimensions Correct Proportions Radius (fillet) Sink Mark Shrinkage VoidsExcessive Radius Part Wall Rib Excessive Rib Wall Thickness 9.9. Maximize Design of Ribs & Brackets • Generally, ribs in compression offer a greater safety factor than ribs in tension. • Exception: Castings with thin ribs in compression may require design changes to provide necessary stiffening and avoid buckling. • Thin ribs should be avoided when joined to a heavy section or they may lead to high stresses and cracking 9.9. Maximize Design of Ribs & Brackets • Avoid cross ribs & ribbing on both sides of a casting – Cross ribbing creates hot spots and makes feeding difficult – Alternative is to design cross-coupled ribs in a staggered “T” form. • Avoid complex ribbing – Complicates molding, hinders uniform solidification and creates hot spots • Ribs meeting at acute angles may cause molding difficulties, increase costs and aggravate the risk of casting defects • “Honeycombing” often will provide increased strength and stiffness without creating hot spots Design Rules: Rib Manufacturability Design Rules: Rib Design Maximize Design of Ribs & Brackets • Brackets carrying offset loads introduce bending moments, localized and in the body of the casting. • Tips to avoid this problem: – Taper “L” shaped brackets and make the length of contact with the main casting as ample as possible. – Brackets may frequently be cast separately and then attached, simplifying the molding. Maximize Design of Ribs & Brackets • A ribbed bracket will offer a stiffness advantage, but avoid heat concentration by providing cored openings in webs and ribs. – The openings should be as large as possible – The openings should be consistent with strength and stiffness • Avoid rectangular-shaped cored holes in ribs or webs. – Use oval-shaped holes with the longest dimension in the direction of the stresses Recommended Configurations May complicate die construction External ribs may cause poor distribution of stresses May complicate die construction Sharp corners, small radii H ≤ T H > T core out underside Good distribution of stresses Sharp corners, small radii, little draft Generous draft and fillets, angular transitions Ribs inside, good distribution of metals for all purposes. Avoid Using Bosses, Lugs & Pads • Bosses and pads can have adverse effects on castings: – They increase metal thickness – They create hot spots – They can cause open grain or draws • If they must be incorporated into a design you should blend them into the casting by tapering or flattening the fillets. Reducing Heavy Masses & Die Simplification A a c B b d Reducing Heavy Masses & Die Simplification A B C a dc b Reducing Heavy Masses & Die Simplification A B 9.10. Avoid Using Bosses, Lugs & Pads • The thickness of bosses and pads should be less than the thickness of the casting section they adjoin but thick enough to permit machining without touching the casting wall. • Exception: Where a casting section is light the following should be used as a guide: 9.10. Avoid Using Bosses, Lugs & Pads • Bosses should not be used in casting design when the surface to support bolts may be obtained by milling or countersinking. • A continuous rib instead of a series of bosses will permit shifting hole location. • Where there are several lugs and bosses on one surface, they should be joined to facilitate machining. – A panel of uniform thickness will simplify machining – Make the walls of a boss at uniform thickness to the casting walls Design Rules: Boss Design • Poor Boss Designs: – Result in the potential for sink marks and voids. – Sinks / Voids / Cooling stresses • Improved Boss Designs: – Bosses attached to the walls using ribs – Thick sections cored out – Gussets reinforce free standing bosses Design Rules: Boss Design