Manufacturing Processes. Casting. Associate Professor Su-Jin Kim. School of Mechanical Engineering Gyeongsang National University.

Transcription

1 Associate Professor Su-Jin Kim School of Mechanical Engineering Gyeongsang National University

2 Outline 1. Physics 2. Sand 3. Investment 4. Die 5. Cost/Quality

3 is a manufacturing process by which a liquid material is poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. First casting: 4000 B.C. Bronze, iron age, light metal age? Engine block 5min

4 is Versatile Many types of metals Rapid production Wide range of shapes and sizes Complex parts as an integral unit Disadvantage Poor dimensional accuracy and surface quality Sand, Investment, Lost form SME 6:16

5 Fluid flow of mold filling Gravity casting: pouring cup sprue well runner riser cavity open riser Bernoulli s equation h p g Velocity of the molten metal leaving the gate 2 v 2g Constant h = elevation p = pressure at elevation v = velocity of the liquid ρ = density of the fluid v c 2gh

6 Fluid flow of mold filling Reynolds number, Re Re vd v = velocity of the liquid D = diameter of the channel ρ = density n = viscosity of the liquid. Ordinary gating system: Re = 2,000 ~ 20,000 mixture of laminar and turbulent flow

7 Heat transfer Heat flow depends on casting material and the mold and process parameters. Temperature distribution in the mold-liquid metal interface is shown below.

8 Solidification time Solidification time is a function of the volume of a casting and surface area (Chvorinov s rule). Solidifica tion time C Volume Surface Area n C = constant n = 2 if Sand casting n = 1 if Die casting Effects of mold geometry and elapsed time on skin thickness and its shape are show.

9 Shrinkage Shrinkage in casting causes dimensional changes and sometimes cracking

10 Expendable Mold Processes 1. Sand casting 2. Shell mold casting 3. Plaster mold casting 4. Investment casting (lost wax casting) 5. Lost foam casting

11 Sand casting Use expendable sand molds Low cost Small quantity production Complex entire parts at one time Low production rate(1 20 pieces/hour-mold) Green (moist) sand has almost no part weight limit Dry sand has a part mass limit of 2,500 kg The sand is bonded together using clays, chemical binders, or oils.

12 Sand casting

13 Sand casting

14 Shell mold casting Fine sand mixed with a resin is heated by the pattern and hardened into a shell around the pattern Can produce castings with close dimensional tolerances, good surface finish and low cost.

15 Plaster( 석고 ) mold casting Plaster is mixed with water and poured over the pattern. Accurate, good surface and low melting point alloy

16 Investment casting (lost wax casting) A wax pattern is coated by ceramic slurry that hardens into the mold. One pattern creates one part, which increases production time and labor costs. Complex geometries, good surface and high melting point alloy. SME:

17 Investment casting (lost wax casting) Wax pattern Mold Pouring Coin: Aerospace:

18 Lost foam casting Pattern is made of polystyrene form which evaporates with molten metal to form a cavity for the casting.

19 Non-expendable Mold Process 1. Permanent-mold casting 2. Die casting 3. Centrifugal casting 4. Continuous casting

20 Permanent mold casting The molten metal is poured into a metal mold.

21 Die casting The molten metal is forced into the die cavity by high pressure. SME 90s

22 Die casting Can be automated for large production runs. Strength-to-weight ratio of die-cast parts increases with decreasing wall thickness. Good surface finish and dimensional accuracy. It is used for aluminium, magnesium and copper alloys.

23 Centrifugal casting Molten metal is solidify in rotating mold. It uses centrifugal force to form cylindrical parts. Centrifugal force = mv 2 /r Gravitational force = mg

24 Continuous casting Developed for casting metal slab or strip continuously at low cost. Continuous casting Strip casting hot rolling slab strip

25 process comparison Process Advantages Disadvantages Sand Almost any metal is cast; Small to large parts; Low tooling cost Low dimensional accuracy & surface quality Plaster Cast Investment Cast Die Good dimensional accuracy and surface finish ; Fine details including thinner walls; Large parts cost less to cast than by Investment process Good dimensional accuracy and surface finish; Complex shape, fine detail; Ferrous and non-ferrous metals Excellent dimensional tolerances and surface finish; Parts require a minimal post machining; High production rate More costly than Sand or Permanent Mold- Limited to nonferrous metals Costs are higher than Sand, Permanent Mold or Plaster process s High die cost; Part size limited; Limited to nonferrous metals

26 Design for Corner X Uniform section Draft direction

27 Cost of casting Sand casting Tooling and equipment costs are low Direct labor costs are high Material utilization is low Finishing costs can be high Investment casting Tooling costs are moderate depending on the complexity Equipment costs are low Direct labor costs are high Material costs are low Die casting Tooling and equipment costs are high Direct labor costs are low to moderate Material utilization is high

28 Quality of Sand casting Tolerance (0.7~2 mm) and defects are affected by shrinkage Material property is inherently poor Generally have a rough grainy surface Investment casting Tolerance (0.08~0.2 mm) Mechanical property and microstructure depends on the method Good to excellent surface detail possible due to fine slurry Die casting Tolerance (0.02~0.6 mm) Good mechanical property and microstructure due to high pressure Excellent surface detail

29 Rate of Sand casting Development time is 2~10 weeks Production rate is depending on the cooling time : t~(v/a) 2 Investment casting Development time is 5~16 weeks depending on the complexity Production rate is depending on the cooling time : t~(v/a) 2 Die casting Development time is 12~20 weeks Production rate is depending on the cooling time : t~(v/a) 1

30 Economics of Cost of equipment per casting decreases as number of parts cast increases. When demand is small cost per casting increases, more economical to manufacture.

31 References POSCO: :

32 Solidification of metals Pure metals have defined melting points and solidification takes place at a constant temperature. Alloys solidify over a range of temperatures.

33 Phase diagrams A phase diagram or equilibrium diagram shows the relationships among temperature, composition and phases present in an alloy system.

34 The iron-carbon system Iron-carbon binary system is represented by the ironiron carbide phase diagram. Ferrite, Austenite, Cementite

35 Microstructure Metals solidified in square mold a) Pure metals: equiaxed grain on skin, columner grain solidified in a square mold. b) Solid-solution alloys: equiaxed grain inner zone c) Heterogeious nucleation of grains

36 Microstructure When alloy cooled slowly dendrite develops. Three basic types of cast structures. Columnar dendritic Equiaxed dendritic Equiaxed nondendritic

37 Ceramic-mold casting A ceramic slurry is poured over the wooden pattern. After hardening, The ceramic mold is removed, dried, and baked in an oven. Ceramic mold is suitable for casting high melting point metals like as steels. Ceramic mold Wooden pattern Mold is baked

38 Vacuum casting Alternative to investment, shell-mold and green-sand casting. Can be automated and production costs are similar to green-sand casting.

39 Squeeze casting The combination of casting and forging.