制造工程与技术(热加工)10章

Fig : Schematic illustration of a typical riser-gated casting. Risers serve as reservoirs, supplying molten metal to the casting as it shrinks during solidification.
• Mold Geometry • Skin thickness
Heat Transfer
• Shrinkage – causes dimensional changes and, sometimes cracking, is the result of the following: – Contraction prior to solidification – Contraction during phase changes – Contraction as temperature drops to ambient temperature
Note: Castability – describes the case with which a metal can be cast to obtain a part with good quality.
Fluidity of Molten Metal
Fig : A test method for fluidity using a spiral mold. The fluidity index is the length of the solidified metal in the spiral passage. The greater the length of the solidified metal, the greater the length of the solidified metal, the greater is its fluidity.
Reynolds Number is used to quantify this aspect
• 0 < Re < 2000 => laminar flow
• 2000 < Re < 20 000 =>mixture of laminar and turbulent flow • Re > 20 000 => severe turbulence
Fluid Flow
Basic casting system: – Fluid is pored though a pouring basin – Flows though the gating system into the mold cavity
Schematic of typical riser-gated casting.
Grain Structure for Pure Metals
• Two types of grains are formed for a pure metal – Fine equiaxed grains – Columnar • Rapid cooling at the walls produces fine equiaxed grains • Columnar grains grow opposite of the heat transfer throughout the mold following the chill zone
Principles of fluid flow – Bernoulli’s Theorem – Continuity Flow Characteristics: turbulence is an important consideration in gating systems.
Flow Characteristics
Fig : Solidified skin on a steel casting. The remaining molten metal is poured out at times indicated in the figure. Hollow ornamental and decorative objects are made by a process called slush casting, which based on this principle
Techniques for minimizing turbulence
• Avoid sudden changes in flow direction • Dross or slag can be eliminated by vacuum casting • Use of filters eliminates turbulent flow in the runner system
• Casting is one of the most versatile manufacturing processes
Advantages of Casting
• Low cost and quick • Easier to manufacture • Can produce intricate shapes and internal openings • Can produce parts in one piece • Best suited for composite components
Illustration of Cast Structures
Alloys
• Solidification in alloys begins when the temperature drops below the liquidus TL and is complete when it reaches the solidus, TS.
Gate – is the portion of the runner though which the molten metal enters the mold cavity Risers – serve as reservoirs to supply the molten metal necessary to prevent shrinkage.
Introduction
Important Considerations
– Flow of Molten Metal – Solidification and Cooling – Type of Mold Material
Solidification of Metals
Involves liquid metal turning back in to solid metal The process is different for Pure metals and alloys Can be divided into two steps: Formation of stable nuclei Growth of crystals
Alloys
• Within the TL and TS Temperature range, the alloy is like a slushy with columnar dendrites
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
Fluid Flow
Sprue – is a vertical channel though which the molten metal flows downward in the mold
Runners – channels that carry the molten metal from the sprue to the mold cavity
Fundamentals of Metal Casting
CHAPTER 10
Topics
• • • • • • Introduction Solidification of Metals Fluid Flow Fluidity of Metals Heat Transfer Defects
Overview of Casting
Heat Transfer
Important consideration in casting – Heat flow in the system • Complex • Depends of flow characteristics Solidification Time – A function of the volume of a casting and its surface area • Solidification time = C volume 2 surface area – Effects on solidification time
• Casting uses the idea that a liquid metal can take the shape of any vessel containing it. • When the metal cools it has taken the shape of its container