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MAT SCI 298 > Lecture 4 - 316L > Flashcards

Lecture 4 - 316L Flashcards

1
Q

What are the physical metallurgical processes during L-PBF?

A

melting and solidification
remelting and re-solidification
layer by layer welding
repeated heating and cooling

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2
Q

Do thermal gradients and cooling rates correlate?

A

yes

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3
Q

Rank the processes from highest to lowest cooling rates and thermal gradients.

A

L-PBF, L-DED, WAAM, conventional casting. directional solidification has a higher thermal gradient but lower cooling rate than conventional casting

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4
Q

What is the equilibrium cooling rate?

A

~0 (diffusion-mediated)

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5
Q

What is the supersaturation cooling rate?

A

10^2 K/s - 10^6 K/s (diffusion-mediated and diffusion-limited)

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6
Q

What is the polymorphic cooling rate?

A

~10^8 K/s (diffusionless)

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7
Q

What is the glass cooling rate?

A

~10^10 K/s (diffusionless)

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8
Q

What determines the solidification mode?

A

Temperature Gradient over growth rate (G/R)

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9
Q

What determines the fineness of solidification?

A

temperature gradient times growth rate (G*R)

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10
Q

What are the five solidification modes from most to least?

A

planar, cellular, dendritic (columnar, equiaxed)

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11
Q

What does a low cooling rate produce?

A

coarse structure

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12
Q

What does a high cooling rate produce?

A

fine structure

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13
Q

What are four applications for 316L stainless steel?

A

medical, kitchenware, appliances, tools.

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14
Q

What is 316?

A

maximum C content 0.08%

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15
Q

What is 316L?

A

maximum C content 0.03%

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16
Q

What is 316H?

A

high carbon 316 (0.03%-0.08%)

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17
Q

What is 316LN?

A

316L with nitrogen hardened

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18
Q

What does build orientation affect?

A

residual stress
microstructure/property
surface roughness
porosity

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19
Q

What are strengthening factors in L-PBF 316L?

A

impurities, pores, small angle GB’s, local misorientations, chemical inhomogeneity, residual stresses

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20
Q

What are strengthening mechanisms in metals?

A

dislocation hardening
grain boundary strengthening
solid solution strengthening
precipitation strengthening
multiple phase strengthening

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21
Q

What equation governs dislocation hardening?

A

taylor equation

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22
Q

What equation governs grain boundary strengthening?

A

hall-petch equation

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23
Q

What three types of boundaries are found in L-PBF 316L?

A

solidification cell boundaries (k1)
high angle grain boundaries (k2)
low angle grain boundaries (k3)

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24
Q

What is solid solution strengthening?

A

strengthening by solute additions

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25
Q

What is dislocation hardening?

A

higher density dislocations and higher yield strength

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26
Q

What is precipitation strengthening?

A

strengthening by fine 2nd phase precipitates

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27
Q

What is multiple phase strengthening?

A

strengthening with a mismatch in moduli or lattice structures

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28
Q

Does multiple phase strengthening likely exist in L-PBF 316L?

A

no

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29
Q

What principle governs precipitate strengthening?

A

Orowan looping

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30
Q

When does dislocation bowing occur?

A

When the volume of particle phase increases above some critical value or when the interface boundary is incoherent

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31
Q

Which elements have the best strengthening effect on steels and why?

A

C, N (smallest atomic radius)

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32
Q

What are residual stresses?

A

stresses that remain in a solid material after the original cause of stress has been removed

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33
Q

What is Type I residual stress?

A

a macro stress on the parts scale, main cause of part distortion

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34
Q

What is Type II residual stress?

A

micro stress, caused by anisotropic elastic property of grains

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35
Q

What is Type III residual stress?

A

micro stress, caused by composition or microstructure heterogeneity

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36
Q

What is Hart’s criterion applied to?

A

strain rate sensitive materials

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37
Q

Why is Mo and Cr added to 316L if they reduce the strength?

A

corrosion resistance

38
Q

What happens to cellular walls when transition metals are added?

A

segregations occur along boundaries

39
Q

What do corrosion studies focus on?

A

surface roughness, composition, feedstock, residual stress, pore effects

40
Q

What are the different types of corrosion?

A

uniform attack, general corrosion, galvanic, pitting, crevice, cavitation, erosion, selective leaching, intergranular corrosion, stress corrosion cracking

41
Q

What does selective leaching include?

A

dezincification of brass

42
Q

What does intergranular corrosion include?

A

Al exfoliation and sensitized SS

43
Q

What are the different types of surface finishes?

A

electro-polish, tumble polish, contour scan/re-melting

44
Q

How does AM 316L compare to wrought samples?

A

higher breakdown potential

45
Q

Why are steels so popular?

A

Easy to print, used to calibrate machines, cheaper powder cost

46
Q

What properties are most important to consider?

A

mechanical properties and corrosion resistance

47
Q

How does the melt pool depth compare to layer thickness?

A

usually 3-4 times the layer thickness

48
Q

What regions are remelted during L-PBF?

A

Layers underneath and overlapping layers

49
Q

What experiment considers only melting and solidification?

A

single track experiments

50
Q

What is the thermal gradient and cooling rate for L-PBF?

A

10^6-10^7 K/m and K/s

51
Q

Why does L-DED have a smaller thermal gradient and cooling rate than L-PBF?

A

larger laser beam, layer thickness much thicker

52
Q

What are equiaxed microstructure?

A

no temperature gradient and growing in random and very direction

53
Q

How can planar microstructures be controlled?

A

temperature gradient

54
Q

What does a fine microstructure mean?

A

Stronger materials, high mechanical stress

55
Q

What are the assumptions about phase?

A

single-phase only

56
Q

When AM with 316L, what elements are difficult to control?

A

C, O, H, N

57
Q

What do residual stresses cause?

A

Warping/change in shape

58
Q

What is a fusion boundary?

A

change in composition due to fusion

59
Q

What are high-angle grain boundaries aligned with?

A

scan direction

60
Q

What is the amount of low-angle grain boundaries?

A

~50-60%

61
Q

What is the angle for low-angle grain boundaries?

A

2-10 degrees

62
Q

What is the local misorientation angle?

A

below 2 degrees, difficult to measure

63
Q

How high is Si’s diffusivity?

A

Low

64
Q

What happens to elements with low diffusivity?

A

They remain in the element

65
Q

Does annealing remove impurities?

A

Not all

66
Q

Are low angle grain boundaries stable?

A

yes

67
Q

What are the size of impurities (N,H)?

A

< 1 nm

68
Q

What are the size of elemental segregations?

A

< 1 nm

69
Q

What are the size of nanopaticipates?

A

10-100 nm

70
Q

What are the size of cellular sizes?

A

.1 - .9 micrometers

71
Q

What are the size of local misorientations?

A

1-2 degrees/micrometers

72
Q

What are size of grain widths?

A

2-50 micrometers

73
Q

What are the size of dislocations?

A

~micrometers

74
Q

What are the size of grain lengths?

A

up to 100 micrometers?

75
Q

What are the size of fusion boundaries?

A

30-400 micrometers

76
Q

What angle is the Hall-Petch relationship measured?

A

10-15 degree

77
Q

Which types of boundaries have the highest slope?

A

solidification cell boundaries

78
Q

How does Orowan looping prevent dislocation?

A

The particles block the dislocation

79
Q

What elements decrease strength of steel?

A

Si and Mn

80
Q

Where do Cr and Si tend to segregate?

A

Along boundary walls

81
Q

What does a lattice strain deviation mean?

A

Presence of residual stresses

82
Q

What does annealing do?

A

reduces differences in hardening rate and lattice strain deviation

83
Q

Why should we anneal after 3D printing L-PBF even though yield strength goes down?

A

eliminate local residual stresses, improve creep, ductility, fatigue

84
Q

What is good about AM 316L?

A

strength, ductility (fatigue is very bad)

85
Q

Better work hardening gives you better ________?

A

ductility

86
Q

What does twinning do?

A

gives additional boundaries to block dislocations, stronger work hardening

87
Q

What is Cr used for?

A

salty water environment

88
Q

What is Mo used for?

A

higher temp oxidation resistance

89
Q

What surface surface has the worst roughness?

A

downskin (overhang)

90
Q

What is the breakdown potential?

A

Surface breaks down (higher breakdown potential is better)

MAT SCI 298 (11 decks)

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