Lecture 4 - 316L Flashcards
What are the physical metallurgical processes during L-PBF?
melting and solidification
remelting and re-solidification
layer by layer welding
repeated heating and cooling
Do thermal gradients and cooling rates correlate?
yes
Rank the processes from highest to lowest cooling rates and thermal gradients.
L-PBF, L-DED, WAAM, conventional casting. directional solidification has a higher thermal gradient but lower cooling rate than conventional casting
What is the equilibrium cooling rate?
~0 (diffusion-mediated)
What is the supersaturation cooling rate?
10^2 K/s - 10^6 K/s (diffusion-mediated and diffusion-limited)
What is the polymorphic cooling rate?
~10^8 K/s (diffusionless)
What is the glass cooling rate?
~10^10 K/s (diffusionless)
What determines the solidification mode?
Temperature Gradient over growth rate (G/R)
What determines the fineness of solidification?
temperature gradient times growth rate (G*R)
What are the five solidification modes from most to least?
planar, cellular, dendritic (columnar, equiaxed)
What does a low cooling rate produce?
coarse structure
What does a high cooling rate produce?
fine structure
What are four applications for 316L stainless steel?
medical, kitchenware, appliances, tools.
What is 316?
maximum C content 0.08%
What is 316L?
maximum C content 0.03%
What is 316H?
high carbon 316 (0.03%-0.08%)
What is 316LN?
316L with nitrogen hardened
What does build orientation affect?
residual stress
microstructure/property
surface roughness
porosity
What are strengthening factors in L-PBF 316L?
impurities, pores, small angle GB’s, local misorientations, chemical inhomogeneity, residual stresses
What are strengthening mechanisms in metals?
dislocation hardening
grain boundary strengthening
solid solution strengthening
precipitation strengthening
multiple phase strengthening
What equation governs dislocation hardening?
taylor equation
What equation governs grain boundary strengthening?
hall-petch equation
What three types of boundaries are found in L-PBF 316L?
solidification cell boundaries (k1)
high angle grain boundaries (k2)
low angle grain boundaries (k3)
What is solid solution strengthening?
strengthening by solute additions
What is dislocation hardening?
higher density dislocations and higher yield strength
What is precipitation strengthening?
strengthening by fine 2nd phase precipitates
What is multiple phase strengthening?
strengthening with a mismatch in moduli or lattice structures
Does multiple phase strengthening likely exist in L-PBF 316L?
no
What principle governs precipitate strengthening?
Orowan looping
When does dislocation bowing occur?
When the volume of particle phase increases above some critical value or when the interface boundary is incoherent
Which elements have the best strengthening effect on steels and why?
C, N (smallest atomic radius)
What are residual stresses?
stresses that remain in a solid material after the original cause of stress has been removed
What is Type I residual stress?
a macro stress on the parts scale, main cause of part distortion
What is Type II residual stress?
micro stress, caused by anisotropic elastic property of grains
What is Type III residual stress?
micro stress, caused by composition or microstructure heterogeneity
What is Hart’s criterion applied to?
strain rate sensitive materials
Why is Mo and Cr added to 316L if they reduce the strength?
corrosion resistance
What happens to cellular walls when transition metals are added?
segregations occur along boundaries
What do corrosion studies focus on?
surface roughness, composition, feedstock, residual stress, pore effects
What are the different types of corrosion?
uniform attack, general corrosion, galvanic, pitting, crevice, cavitation, erosion, selective leaching, intergranular corrosion, stress corrosion cracking
What does selective leaching include?
dezincification of brass
What does intergranular corrosion include?
Al exfoliation and sensitized SS
What are the different types of surface finishes?
electro-polish, tumble polish, contour scan/re-melting
How does AM 316L compare to wrought samples?
higher breakdown potential
Why are steels so popular?
Easy to print, used to calibrate machines, cheaper powder cost
What properties are most important to consider?
mechanical properties and corrosion resistance
How does the melt pool depth compare to layer thickness?
usually 3-4 times the layer thickness
What regions are remelted during L-PBF?
Layers underneath and overlapping layers
What experiment considers only melting and solidification?
single track experiments
What is the thermal gradient and cooling rate for L-PBF?
10^6-10^7 K/m and K/s
Why does L-DED have a smaller thermal gradient and cooling rate than L-PBF?
larger laser beam, layer thickness much thicker
What are equiaxed microstructure?
no temperature gradient and growing in random and very direction
How can planar microstructures be controlled?
temperature gradient
What does a fine microstructure mean?
Stronger materials, high mechanical stress
What are the assumptions about phase?
single-phase only
When AM with 316L, what elements are difficult to control?
C, O, H, N
What do residual stresses cause?
Warping/change in shape
What is a fusion boundary?
change in composition due to fusion
What are high-angle grain boundaries aligned with?
scan direction
What is the amount of low-angle grain boundaries?
~50-60%
What is the angle for low-angle grain boundaries?
2-10 degrees
What is the local misorientation angle?
below 2 degrees, difficult to measure
How high is Si’s diffusivity?
Low
What happens to elements with low diffusivity?
They remain in the element
Does annealing remove impurities?
Not all
Are low angle grain boundaries stable?
yes
What are the size of impurities (N,H)?
< 1 nm
What are the size of elemental segregations?
< 1 nm
What are the size of nanopaticipates?
10-100 nm
What are the size of cellular sizes?
.1 - .9 micrometers
What are the size of local misorientations?
1-2 degrees/micrometers
What are size of grain widths?
2-50 micrometers
What are the size of dislocations?
~micrometers
What are the size of grain lengths?
up to 100 micrometers?
What are the size of fusion boundaries?
30-400 micrometers
What angle is the Hall-Petch relationship measured?
10-15 degree
Which types of boundaries have the highest slope?
solidification cell boundaries
How does Orowan looping prevent dislocation?
The particles block the dislocation
What elements decrease strength of steel?
Si and Mn
Where do Cr and Si tend to segregate?
Along boundary walls
What does a lattice strain deviation mean?
Presence of residual stresses
What does annealing do?
reduces differences in hardening rate and lattice strain deviation
Why should we anneal after 3D printing L-PBF even though yield strength goes down?
eliminate local residual stresses, improve creep, ductility, fatigue
What is good about AM 316L?
strength, ductility (fatigue is very bad)
Better work hardening gives you better ________?
ductility
What does twinning do?
gives additional boundaries to block dislocations, stronger work hardening
What is Cr used for?
salty water environment
What is Mo used for?
higher temp oxidation resistance
What surface surface has the worst roughness?
downskin (overhang)
What is the breakdown potential?
Surface breaks down (higher breakdown potential is better)
