Metals Flashcards
Name the casting methods
Lost wax Investment
Sand
Low pressure die (gravity)
High pressure die
What is the difference between ferrous and non-ferrous metals?
Ferrous contain iron, non-ferrous do not
What are the main 4 examples of ferrous metals?
Mild steel (low carbon)
Medium carbon steel
High carbon steel
Cast iron
Properties of mild steel
Ductile
High tensile strength
Tough
Malleable
Poor resistance to corrosion
Least amount of carbon
Medium carbon steel properties
Harder than low carbon
Less ductile, malleable and tough
Higher carbon content
Cast iron properties
Harder outer skin
Brittle core
Good under compression
Mild steel uses
Nuts
Bolts
Washers
Screws
Car bodies
Fences
Medium carbon steel uses
Springs
Gardening tools
Railways.
Cast iron uses
Machine parts
Cookware
Problem with ferrous metals
Tendency to rust due to iron content
Non ferrous metal examples
Aluminium
Zinc
Tin
Copper
Properties of aluminium
Lightweight
TIG welded
Conductor
Malleable
Ductile
Uses of aluminium
Drinks cans
Aircraft bodies
Bike frames
Baking foil
Zinc properties
Low melting point
Good corrosion resistance
Zinc uses
Galvanising steel for protection
Tin properties
Ductile
Malleable
Low melting point
Corrosion resistant
Tin uses
Solder
Food cans coating
(Rarely used in pure form)
Copper properties
Ductile
Malleable
Tough
Corrosion resistant
Good conductivity
Soldered or brazed
Copper uses
Electrical wire
Water pipes
Water tanks
Central heating pipe
What is an alloy? How are they made?
Metals made of two or more metals, or combining two or more elements, one of which must be a metal. In order to optimise advantages of metals mechanical and physical properties.
Alloys are produced in either a furnace or in a casting process
Metals are melted together into specific quantities and then poured into bars for future products
Examples of alloys
Stainless steel
Duralumin
Brass
Properties of stainless steel
Tough
Hard
Corrosion resistant
18% chrome, 8% nickel
High melting point
Uses of stainless steel
Cutlery
Kitchenware
Sinks
Duralumin properties
4% copper, 1% magnesium and manganese
High tensile strength (same as mild steel)
More lightweight, ductile
Becomes harder when worked
Duralumin uses
Aircraft parts
Vehicle parts
Brass properties
Corrosion resistant
Good conductivity
Low melting point (930 degrees)
Casts well
Brass uses
Boat fittings
Door furniture
Taps
Ornaments
What is the aim of heat treatment?
To alter the metals properties via heating and cooling its grain structure
Examples of heat treatment
Hardening
Tempering
Case hardening
Annealing
Normalising
What is hardening?
High and medium carbon steel is heated to a temp above recrystallisation. It is then held here for a given time to create a consistent temp. Then it is rapidly cooled in water, oil, air or salt water.
Outcomes of quench hardening
Increased hardness so products resists wear and fracture on surface
Does not change core properties
What is tempering?
Typically follows a hardening process. Metal is heated to a temp just below critical temp for a certain time depending on the steel. It is then slowly air cooled. Temperature controls the reduction in hardness (higher the temp, more hardness removed).
Outcomes of hardness
Increased machinability
Reduces brittleness as well as hardness
Increases ductility and toughness
What is case hardening?
Case hardening hardens the surface of steels. Performed by carburising or quenching.
What is carburising?
Case hardening process which changes chemical composition of surface of low carbon steel so it can absorb more carbon, increasing surface hardness.
Steel place in ceramic box packed with carbon
Heated to 940 degrees
Carbon atoms diffuse into materials structure
Depth of carbon layer determined by time in box
Product is heated to around 760 degrees and quenched.
Annealing
Only metals with lower melting point like cast iron or steel. Relieves stress from work hardened metal.
Heated to high temp where internal stresses are relieved
Heated above recrystallisation temperature but below melting point
New grains formed and cooling allows them to develop
Outcomes of annealing
Easier to work
Less brittle
More ductile
What is elasticity
Ability to be deformed and then return to original shape when force removed
What is plasticity?
Ability to be permanently deformed and retain deformed shape
What is malleability?
Ability to withstand deformation by compression without cracking. It increases with temperature
What is ductility?
Ability to be drawn out under tension, reducing the cross-sectional area without cracking (eg stretching a material into a wire)
What is hardness ?
Ability to resist abrasive wear such as scratching, surface indentation or cutting
What is toughness?
ability to absorb impact without fracture
What is durability?
Ability to withstand general wear over a period of time
What is biodegradability?
Ability to naturally decompose by actions of bacteria or other living organisms
Electrical conductor
Allows flow of electricity
Provides little resistance to flow of charge
Electrical insulator
Does not allow flow of charge
Thermal conductor
Allows transfer of heat energy through material
Better conductors allow heat to transfer faster
Thermal insulator
Prevents heat energy transfer
Types of strength
Bending
Compressive
Tensile
Shear
Torsional
Tensile strength meaning
Ability to resist stretching or pulling forces
Shear strength meaning
Ability to resist slinging forces on parallel plane
Torsional strength meaning
Ability to resist rotational forces from applied torque of tension
Bending strength meaning
Ability to resist forces that may bend the material
Compressive strength meaning
Ability to withstand being crushed or shortened by pushing force
Types of casting
Sand
Investment
Low/high pressure die casting
Draw a diagram of sand casting
Draw a diagram for investment casting
How does investment casting work?
The process-
1. A wax pattern is created (can be done in the form of a tree) from a master mould made from steel or aluminium
2. This is then dipped into the refractory clay slurry
3. This is fired in a kiln to bake the clay hard
4. Once final coat has hardened the wax is burned out
5. Molten mould then poured into cavity and solidified
6. Refractory mould then broken including the runner and any connecting channels
Why use investment casting
Great surface finish
Intricate shapes can be made
Wax can be reused
Strength of products
Why shouldn’t you use investment casting
Time consuming waiting for coast to harden
Expensive start up cost
Complex machinery
Difficult to make parts with hollow centre
Uses of investment casting
Collectible figures
Jewellery
Construction
How does sand casting work
The process-
1.Place the mould pattern (wood) (made using CAD) into a steel box called a ‘drag’
2. Packs and around the pattern and level it
3. Flip the drag over and and a second box called the ‘cope’ is clamped on top
4. Two sprue pins are then placed into the cope to create the runner and the riser
5. Pack sand into the cope around the sprue pins and other half of pattern
6. Separate the cope from the drag and remove the pattern and sprue pins. This should leave a cavity and channels. Cut connecting channels into the sand to connect the channels to the cavity and then connect cope and drag again
7. Pour molten metal through runner until the riser is filling telling us the cavity is full
8. Allow to cool and then remove the sand and cut off the runner and riser using a hacksaw.
Why use sand casting
Be used for any quantity of products (affordability)
Relatively cheap
Use high melting point metals unlike die casting
Sand can be reused
Why not sand casting
Rough surface finish due to grainy sand
Labour intensive
Less accurate than other methods
Moulds require constant checking and changing
Moulds are one use
Uses of sand casting
Wood working clamps
Motor housings
Drain covers
Post box
How does low pressure die casting work
- Die split in half and each side heated to high temperature to prevent metal cooling to quickly. A lubricant is also applied to maintain this temp
- Two half’s clamped together tightly using high pressure.
- Molten metal poured in and gravity pulls it down into the entire mould. Metal poured into runner and the riser tells us when it is full
- It is then left to cool and solidify fur a sufficient tie before removing the mould
- Ejector pins remove the cast from the machine
- Gates, runners, sprues and flash are all removed to creat finished product
What metals can be die casted and why
Non ferrous metals due to lower melting point as most moulds are made from tool steel. If ferrous were used they would deform the mould as melting points are too similar
How does case hardening work
Tongs and other PPE are used for safety
Metal heated until cherry red
Dipped into carbon powder
Once soaked the process repeats
Metal then reheated and quenched in water to rapidly cool
Cons of gravity die casting
Slow as reliant on gravity
Create a low volume as a results of this
Very expensive to setup so only really used for mass produced items or large batches
Pros of gravity die casting
Creates parts with thinner sections than sand casting but thicker than pressure die casting
Produces parts with excellent mechanical properties suitable for heat treatment
Less machinery than pressure die casting
Relatively smooth surface finish
Less porosity than pressure as no turbulence
How does pressure die casting work?
- Molten metal stored in chamber or crucible.
- A pneumatic/hydraulic plunger forces a shot of the metal into the mould
The high pressure forces the mould to fill allowing fine details to be moulded - Once the metal has solidified ejector pins remove the mould from the cavity
4.
Pros of high pressure die casting
Fast process enabling high volumes to be produced
Can be automated as no need for runners and risers
Fine details can be moulded
Can make thin walled parts
Cons of high pressure die casting
Expensive to set up
Turbulence from high pressure can cause porosity in components
Expensive dies
What are the moulds in die casting made from?
Tool steel
Uses of die casting
Alloy wheels
Toy cars
Door knobs
Handles
How is plaster of Paris different to sand
Uses plaster of Paris instead of sand and can only use non-ferrous metals
What is carburising?
Steel placed in ceramic box packed with carbon
Heated to 950 degrees
Carbon diffuses into steel building up surface content of carbon
Depth of layer determined by time (longer exposure means thicker layer)
Quenched to cool
What is quenching?
Hot metal quenched in water to cool it rapidly and seal hard surface case without affecting inner properties
