Solidification Processes

Transcription

1 CASTING PROCESSES I Lecture Notes by Zulkepli Muhamad Solidification Processes Starting work material is either a liquid or is in a highly plastic condition, and a part is created through solidification of the material Solidification processes can be classified according to engineering material processed: Metals Ceramics, specifically glasses Polymers and polymer matrix composites (PMCs)

2 Classification of solidification processes Classification of casting processes Metal casting processes Foundries Expandable mold Permanent mold Single-crystal growing Sand Shell Expandable pattern Plaster Ceramic investment Composite mold Slush Pressure Die Centrifugal Squeeze semisolid Single-crystal for microelectronics Single-crystal turbine blade Directional solidification

3 Casting Processes 1. Expendable mold processes uses an expendable mold which must be destroyed to remove casting Mold materials: sand, plaster, and similar materials, plus binders 2. Permanent mold processes uses a permanent mold which can be used many times to produce many castings Made of metal (or, less commonly, a ceramic refractory material Overview of Casting Technology Casting is usually performed in a foundry Foundry = factory equipped for making molds, melting and handling molten metal, performing the casting process, and cleaning the finished casting Workers who perform casting are called foundrymen

4 Casting Process in which molten metal flows by gravity or other force into a mold where it solidifies in the shape of the mold cavity The term casting also applies to the part made in the process Steps in casting seem simple: 1. Prepare mould/die 2. Melt the metal 3. Pouring melt into a mould/die 4. Let it solidify 5. Finishing processes Metal casting components

5 Metal casting components Metal casting components

6 Production steps Molding Pattern making Core Making Gating system Sand Mold Melting Pouring Casting Heat treatment Finishing Furnaces Solidification Shakeout Removal of risers and gates Cleaning & Inspection -additioanl heat treatment -defects -dimensions Why Castings?

7 Capabilities and Advantages of Casting Can create complex part geometries Can create both external and internal shapes Some casting processes are net shape; others are near net shape Can produce very large parts Some casting methods are suited to mass production Parts Made by Casting Big parts: engine blocks and heads for automotive vehicles, wood burning stoves, machine frames, railway wheels, pipes, church bells, big statues, and pump housings Small parts: dental crowns, jewelry, small statues, and frying pans All varieties of metals can be cast, ferrous and nonferrous

8 Sand Casting process 680 Kg Compressor Frame

9

10 Sand mould Sand mould

11 Sand Moulding Processes Sand Moulding Processes (cont.)

12 Sand Moulding Processes (cont.) Two forms of mold: (a) open mold, simply a container in the shape of the desired part; and (b) closed mold, in which the mold geometry is more complex and requires a gating system (passageway) leading into the cavity

13 The Pattern A full-sized model of the part, slightly enlarged to account for shrinkage and machining allowances in the casting Pattern materials: Wood - common material because it is easy to work, but it warps Metal - more expensive to make, but lasts much longer Plastic - compromise between wood and metal Types of patterns used in sand casting: (a) solid pattern (b) split pattern (c) match-plate pattern (d) cope and drag pattern

14

15 The Core Full-scale model of interior surfaces of part It is inserted into the mold cavity prior to pouring The molten metal flows and solidifies between the mold cavity and the core to form the casting's external and internal surfaces May require supports to hold it in position in the mold cavity during pouring, called chaplets Core held in place in the mold cavity by chaplets (b) possible chaplet design (c) casting with internal cavity

16 Sand Casting - Process variations Green sand casting: the most common and the cheapest. Associated problems are that the mould has low strength and high moisture content. Dry sand: core boxes are used instead of patterns. Expensive and time consuming. Skin-dried sand: the mould is dried to a certain depth. Used in the casting of steels. Sand Casting - Economic considerations Production rates of 1-60 pieces/hour, but dependent on size. Lead time ranges from days to several weeks depending on complexity and size of casting. Material utilization is low to moderate % of material lost in runners and risers. Both mould material and runners and risers may he recycled. Patterns are easy to make and set, and are reusable.

17 Sand Casting-Economic considerations Pattern material dependent on the number of castings required. Easy to change design during production. Economical for low production runs. Can be used for one-offs. Tooling costs are low. Equipment costs are low. Direct labour costs are moderate to high. Can be labour intensive. Finishing costs can be high. Cleaning and fettling are important before secondary processing. Process Parameters

18 Pouring Desirable Mold Properties and Characteristics Strength - to maintain shape and resist erosion Permeability - to allow hot air and gases to pass through voids in sand Thermal stability - to resist cracking on contact with molten metal Collapsibility - ability to give way and allow casting to shrink without cracking the casting Reusability - can sand from broken mold be reused to make other molds?

19 Sands The sands used may be one of the following categories; naturally bonded (bank) - less expensive synthetic (lake) - this sand can have a variety of controlled compositions. Types of sand include, Zircon (ZrSiO4) - low thermal expansion Olivine (Mg2SiO4) - low thermal expansion Iron Silicate (Fe2SiO4) - low thermal expansion Chromite (FeCr2O4) - high heat transfer Foundry Sands Silica (SiO 2 ) or silica mixed with other minerals Good refractory properties - capacity to endure high temperatures Small grain size yields better surface finish on the cast part Large grain size is more permeable, to allow escape of gases during pouring Irregular grain shapes tend to strengthen molds due to interlocking, compared to round grains Disadvantage: interlocking tends to reduce permeability

20 Types of Sand Mold Green-sand molds - mixture of sand, clay, and water; Green" means mold contains moisture at time of pouring Dry-sand mold - organic binders rather than clay and mold is baked to improve strength Skin-dried mold - drying mold cavity surface of a green-sand mold to a depth of 10 to 25 mm, using torches or heating lamps Other Expendable Mold Casting Processes Shell Molding Vacuum Molding Expanded Polystyrene Process Investment Casting Plaster Mold and Ceramic Mold Casting