Influence of Process Parameters on the Phase ... - CAVS€¦ · 2007 Annual Meeting; February 25-March 1, 2007 — Orlando, FL, USA L. Wang and S. Felicelli @ TMS Conference, 2007

2007 Annual Meeting; February 25-March 1, 2007 — Orlando, FL, USA

L. Wang and S. Felicelli @ TMS Conference, 2007

Influence of Process Parameters on the Phase Transformation and Consequent Hardness Induced by

the LENSTM Process

Liang Wang, Sergio FelicelliCenter For Advanced Vehicular Systems

Mississippi State University



LENS (Laser Engineering Net Shaping Process) is a new rapid fabrication process

(Hofmeister et al., 1999)

The LENS process performing a single-line build

What is LENS?



• A variety of materials can be used:– Stainless Steel (SS410, SS316)– Ti-based alloy (Ti-6Al-4V)– Inconel, copper, aluminum, etc.

• Application:– Aerospace repair & overhaul– Rapid prototyping and 3D structure fabrication– Product development for aerospace, defense,

and medical markets, etc. • Advantages:

– Low cost & time saving – Enhanced design flexibility and automation– Highly localized heat-affected zone (HAZ)– Superior material properties (strength and

ductility)

Processing Blade

Processing Bar

Introduction



• The mechanical properties are dependent on the microstructure of the material, which in turn is a function of the thermal history of solidification.

• An understanding of the thermal behavior and phase transformation of the fabricated part during the LENS process is of special interest.

• Numerical simulation methods have the potential to provide detail information of the thermal behavior and phase transformation.

Introduction



• 3-D model to simulate 10-pass single build plate LENS deposition of 410 stainless steel (SS410) powder with SYSWELD finite element code.

• Predict the temperature distribution, cooling rate, phase proportions, and hardness

• Investigate the effect of process parameter (laser power, travel speed) on the phase transformation and consequent hardness.

• Study the porosity formation due to lack of laser power

Objectives



• Weld direction: Same direction for each pass.• Material properties of the deposited part and the substrate are the

same.

Geometry & Process ParametersProcess

parametersCase

ICase

IICase

III7.62

mm/s20

mm/s0.5 s

0.5 s

1 s

10 s

1.3 s

0.7 s

2 s

20 s

Laser beam travel velocity

2.5 mm/s

Moving time of the laser beam for each pass

4 s

Idle time of consecutive

layers deposition

1 s

Time to finish one layer

5 s

Total time to finish the part

50 s



• Thermal properties depend on the temperature, and the phase proportions.

Thermal Properties (SS410)



• A dense mesh was used for the plate and the contact area with the substrate, where higher thermal gradients are expected.

Number of nodes: 104,535Number of elements: 132,400Element size in the part: 0.1X0.1X0.1 mm3

Mesh Structure



• Modified heat conduction equation:

• The latent heat effects due to phase changes are modeled with the specific heat variation.

Mathematical Model



• Evolution equation for austenitic, ferritic-perlitic, and bainitic transformation of steels

Mathematical Model

• These functions can be obtained by comparison of the prediction provided by the model with experimental results (i.e. CCT diagram).



• Martensitic transformation by Koistinen-Marburger Law

Mathematical Model



• Dummy material method is applied to the element activation:

• Fixed mesh is used for the plate and substrate.

Element Activation Technique

M1: Deposited layers + substrate– Material with actual thermal properties and

phase transformationM2: Layer being deposited

– Material with actual thermal properties and starting with dummy phase

– Dummy phase → Austenite phase (T>Taus)

M3: Layer to be deposited– Material with dummy low thermal properties

and without phase transformation



• 3D Conical Gaussian Function

• Part of energy generated by the laser beam is lost before being absorbed by the part.

• Absorbed laser power is used in the calculation. The nominal laser power is calibrated by matching the predicted temperature profile with measured data.

Heat Source

rQ



• Initial condition

• Boundary condition on the bottom of the substrate

• Boundary conditions for all other surface

• As new layers are activated, the surfaces are increased and the boundary conditions are updated.

0)0,,,( TtzyxT ==

0)0,,( TzyxT == 0>tfor

( ) ( ) Laserrea QTTTThnTk ΩΩΩΩ −−+−=⋅∇ 44)( εσr

Initial and Boundary Conditions



Laser Power Distribution

0

0.5

1

1.5

2

2.5

0 2 4 6 8 10

Lase

r Pow

er In

tens

ity

Width (mm)Layer Number

Nom

inal

Las

er P

ower

(W)

200

300

400

500

600

700

800

1 2 3 4 5 6 7 8 9 10

2.5 mm/s

7.62 mm/s

20 mm/s

• Laser power is varied with the number of the layers

• For each layer, the laser power is varies from one side to another.



4 mm

• The calibration calculation is performed only for the deposition of the top layer (the 10th layer).• T0 = 600°C, Pabs = 100W, Pl = 275W, E = 36.4% (30-50%) (Unocic and DuPont, 2004)

Temperature distribution (2-D View)

Model Calibration



2.5 mm/s7.62 mm/s20.0 mm/s

Tem

pera

ture

(°C

)

Height (mm)

Temperature Distribution



V=2.5 mm/s

V=7.62 mm/s

V=20.0 mm/s

Molten Pool Size Distribution• One and a half layers

are melted for each pass

• Larger molten pool size for higher travel speed



7.62 mm/s 20 mm/s2.5 mm/s

Plate

Substrate

• Austenite distribution after the 10th layer is deposited• Higher laser travel speed results in more austenite, which transforms

to fresh martensite, providing more uniform hardness in the plate. • The yellow lines show the unmelted powder due to lack of laser power

at the beginning of each layer and the layers close to the substrate

• This indicates that porosity could be formed due to lack of fusion

Austenite Distribution



Martensite Distribution

2.5 mm/s 7.62 mm/s 20 mm/s



Defects of 410SS LENS MaterialTop

Bottom

• The top part of the sample has much less defects than that in the bottom.

• The defects occur at the interface of the consecutive layers

• The defects indicate that the powder is not fully melted due to the lack of fusion

• More defects present in the bottom are due to the large heat dissipation in the substrate.



Conclusions

• A 3-D finite element model was developed to simulate the LENS deposition of a SS410 plate.

• This model predicts the temperature distribution, molten pool size, and volume fraction of the solid phases.

• Process parameter (laser power) was optimized to obtain steady molten pool size

• High laser travel speed may reduce the proportion of tempered martensite, and achieve more uniform microstructure and hardness distribution, however, it can lead to the presence of porosity due to lack of fusion.

Documents

Influence of Process Parameters on the Phase ... - CAVS€¦ · 2007 Annual Meeting; February 25-March 1, 2007 — Orlando, FL, USA L. Wang and S. Felicelli @ TMS Conference, 2007