Embedded Files
Students Learn About:
forming methods
– forging
– casting
– cutting
– joining
structure and properties of appropriate materials
– alloy steels such as stainless steel, titanium
– polymers
– ceramics
Students Learn To:
describe forming processes for materials used in biomedical engineering
discuss emerging technologies used in biomedical engineering materials
compare the macrostructure and properties of materials used in biomedical engineering
explain the properties and uses of appropriate biomedical engineering materials
- forming methods
forging
forging
Forging is the process of shaping a metal with forceful blows. It may be done with the metal hot (hot forging) or cold (cold forging or pressing). Hot forging is carried out above the recrystallisation temperature. The simplest type of forging is that which a blacksmith does against an anvil. Forging may draw out a metal while reducing its cross-sectional area (drawing), reduce its length while increasing its cross-sectional area (upsetting) or it may force the metal into dies to take the required shape, as in drop forging.
When a molten metal is poured into a mould it begins to solidify by the formation of small crystal nuclei. From these crystal nuclei dendrites begin to grow, dissipating heat energy as they solidify. Dendrites are three dimensional structures that resemble six pine trees all growing from a centre along the three principal axes.
The dendrites continue to grow until the lengthening arms meet other dendrites. When this occurs the dendrites can grow no further and it is here that grain boundaries form. Since the dendrites arms can get no longer the existing arms thicken and grow until the entire area of the dendrite becomes a single grain. If material is cooled rapidly there will be more crystal nuclei form and more dendrites, hence smaller grains. Slow cooling means less, nuclei, less grains and larger crystals. This is the way a metal would solidify in ideal circumstances, often cooling rates and heat dissipation may result in differently shaped grains.
cutting
cutting
Turning
Turning
Turning involves applying the sharp point of a cutting tool to a metal surface as it’s rapidly spun. This removes the top layer of metal to a pre-determined size.
Grinding
Grinding
Grinding involves a smooth metal surface and very high-quality metal. A grinding machine has an abrasive wheel that turns while making contact with the metal, wearing the surface down and helping separate it.
Drilling
Drilling
Drilling requires a drill bit applied directly to metal, then the typical combination of force and rotation that you’d see when a screw was being drilled into a wall. Drilling allows for very precise holes in metal where needed.
Welding
Welding
Welding or burning techniques involve application of high heat to the surface metal, which brings it up to a temperature at which it will soften and then break along a precise line.
Flame
Flame
A gas flame that reaches a temperature of above 3,500 degrees Celsius can also be used. This heats the metal to melting point, then directs a stream of oxygen onto the spot, causing the metal to burn and melt along a line.
Laser
Laser
Laser cutting is relatively similar – using a concentrated beam of light that can be reduced to a single point at a very high temperature. This allows exact, precise shapes to be cut out of metal, and lasers are often controlled by a computer to get maximum accuracy here.
Plasma
Plasma
A more recent technique is plasma cutting, which uses a plasma torch to pump either oxygen or inert gas out of the nozzle while simultaneously sending an electrical arc through the gas. This creates plasma hot enough to melt metal while moving quickly enough to blow metal molten away.
Water Jet
Water Jet
Water jets, or erosion technology, mimic the effect of water on metal in the natural environment – just at a much higher rate. This method doesn’t rely on any heat, and can therefore be used on metals that are sensitive to high heat.
joining
joining
describe forming processes for materials used in biomedical engineering
describe forming processes for materials used in biomedical engineering
Which machining / cutting processes seem most appropriate for biomedical devices
What are some pro and con's to equiaxed structures
- structure and properties of appropriate materials
stainless steels - as a class complete their definitions, pro's, con's (include photos)
stainless steels - as a class complete their definitions, pro's, con's (include photos)
Ferritic
Ferritic
Stainless Steel
Occurs at aroubd 500 - 700 degress
Has a BCC structure
Course grain structure
Common in automotive and nuclear, hihg resistance to stress corrosion cracking
Martensitic
Martensitic
Stainless Steel
Formed from Austenitic steel
Can be quenched or tempered
quenching provides increased tensile strength
Tempering provides increased hardness strength
Can be alloyed with chromium, carbon,
Transitions from FCC to BCC
Uses included knives, ball bearings, samurai sword
Highest yield strength when it has high percentage of carbon
Austenitic
Austenitic
Stainless Steel
Forms at 660 - 800 degrees
Has. a FCC structure
Favourable properties of ductility, formability, and corrosion resistance
Hardness and strength increased through cold working
Non Magnetic
Duplex
Duplex
Stainless Steel
Forms below 300 degrees
Has a dual phased structure
is a mixture of ferritic and Austenitic steel
Higher toughness and ductility than ferritic, not as great as austenitic
Resistant to corrosion due to chromium and nitrogen
Used in bridges due to high corrosion / marine environments
Precipitation Hardened
Precipitation Hardened
Stainless Steel
XXXX
A process in which heat increase a materials iverall toughness, yield strength and hardness
The grain strucutre in created uniformally, therefore strengthens the maerial
titanium alloys
titanium alloys
Titanium is a relatively lightweight metal; it provides excellent corrosion resistance, high strength to weight ratio and good high temperature properties. The corrosion resistance is provided by an adherent TiO2 film that is stable until 535°C. Titanium is allotropic and exists as an HCP structure at room temperature and changes to BCC above 882°C. Titanium is usually used in alloyed form and a variety of alloys exist. A common titanium alloys is titanium with 6% aluminium and 4% vanadium, these are used extensively in biomedical applications such as, investment cast knee and hip implants, tibial nails, screws, LVAD housings and pacemaker housings. They have excellent yield strengths around 966 MPa. Their usage is favourable due to a durable passive layer forming which prevents detrimental corrosion when in contact with bodily fluids.
In addition to biomedical engineering titanium alloys find extensive use in aeronautical engineering. For a period they looked to be the next big frame material for high end bicycles but carbon fibre composites are now preferred there. One of the challenges of titanium alloys is that they can be difficult to hot work above 535°C due to reactivity above this temperature; moreover they can be a challenging material to weld, While TIG processes can be used the metal must be shielded from air until it cools below 535°C.
cobolt-chromium-molybdenum alloys
cobolt-chromium-molybdenum alloys
Another alloy that is used in biomedical engineering is a cobalt alloy with additions of around 25-30% chromium and 5% molybdenum. These alloys have lower yield strength than the titanium alloys, but offer equivalent corrosion resistance and also good wear resistance. Recently concerns were raised about metal on metal abrasion causing cobalt and chromium particles to cause toxicity in the body.
polymers
polymers
Polymers find use in a variety of applications for biomedical engineers. Polycarbonate is used for device housings, while PVC finds use as blood bags. Polyamides are used as sutures and polyesters are used for vascular grafts. Importantly in hip and knee joints ultra high molecular weight polyethylene (UHMWPE); this is a denser version of polyethylene which offers excellent impact resistance and a low coefficient of friction.
ceramics
ceramics
Bioinert ceramics find use in biomedical engineering. Alumina (AlO3) is one of the more traditional bio-inert materials. It has been used for around 30 years, it is highly stable in the body and offers good compressive strength and high wear resistance, but has limited tensile strength and fracture toughness. Alumina has been used in dental work and also as the femoral head in hip replacements. Partially stabilised zirconia is also used. Here a metastable phase is present in the original structure, when a crack passes through the metastable phase changes to a more stable structure which actually takes up greater volume. This essentially tries to close the crack and inhibits further growth.
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