Lecture 10 Flashcards
what does SLM or LPBF stand for?
selective laser melting
laser powder bed fusion
what are the purposes of the shielding gas glow in the machine? what gas is used?
- maintain a low oxygen level, which prevents oxidation, contamination and corrosion (very important for Ti, a very reactive material)
- ensures high material quality and process stability (removes fumes created by melting)
- Argon is usually used (inert gas)
what are some process parameters that you can change? which pattern is the best for bulky parts?
- laser power
- scanning speed
- layer thickness
- hatch spacing and pattern (chessboard, stripes, xy, …)
- melt pool width and depth
Chessboard is better for bulky parts, because the heat is distributed better.
what are three main defects that can happen, and what are the laser power and scanning velocity ranges?
1) keyhole:
- slow velocity but high power
- too much pressure in the melt pool -> bubbles that explodes and melt pool collapses trapping the gas -> keyhole
2) poor bonding:
- fast but low laser power
3) balling:
- too fast and too much power
how can scanning strategies be used with NiTi for example? Give an application of NiTi
by changing the laser power, you can influence the evaporation of the Ni component in the alloy -> influence the properties of the final material.
Print an implant in NiTi, deform it into a practical shape to introduce it into the body -> body temperature makes it go back to its original shape
why is magnesium interesting for biomedical applications? One issue with it?
You can define its microstructure -> controlled degradation rate, it won’t stay in the body forever and no second surgery is needed to remove the implant.
During degradation -> hydrogen byproduct, which changes the surrounding pH and can cause necrosis
what are advantages of LPBF titanium?
- open interconnected framework for optimal cell migration and proliferation
- bone grafting is not necessary
- bone marrow application is optional
what is the big advantage of LPBF in patient specific implants?
porous scaffolds have a bone-compatible elastic modulus design (avoid stress shielding), and can promote bone ingrowth, as well as prospects for implant vascularisation.
3 reasons why supports are needed. Which surface do the supports have to touch?
- fixation to building platform
- heat conduction to platform
- carrying the heat caused bending stresses
Not touch the surfaces that will be in direct contact with the bone (they need to have good surface finish)
name all the steps in post processing (7)
- heat treatment (annealing to homogenise the stresses)
- platform and supports removal
- deburring (abrasive)
- sandblasting (to get homogeneous surface roughness)
- (vibratory grinding)
- dry electropolishing if very low roughness needed
- drilling screw holes
what is one problem that can be seen after sandblasting? what to do in this case?
- remaining pores on implant surface become visible
- preferred places for particles or bacteria
- have to be removed by grinding / deburring
what is hot isostatic pressing and what does it do? when is it done?
uses heat and pressure to reduce the size of defects and the pores -> improves strength, ductility and fatigue life. Time consuming and expensive, so only for load bearing implants.
what equilibrium has to be found with the roughness in medical applications?
cells like very rough surfaces, and it is better for integration. But you don’t want particles falling off the implant.
