p. s. wei xi-wan chair professor department of mechanical and electro-mechanical engineering

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P. S. WeiXi-Wan Chair Professor

Department of Mechanical and Electro-Mechanical EngineeringNational Sun Yat-Sen UniversityKaohsiung, Taiwan 80424, ROCE-mail: [email protected]

Heat Transfer Lab for Manufacturing and Materials Processing

NSYSUMechanical & Micro-Mechanical

Engineering

Understanding of workpiece defects induced by laser beam

Journal of Lasers, Optics and Photonics


Engineering


This presentation deals with (1) defects of surface rippling and humping and root spiking and (2) pore formation due to super-saturation and liquid entrapment after solidification. Surface rippling and humping often accompany solute segregation, porosity, crack, deformation, etc. Spiking accompanies cold shut and porosity is another severe defect. Incapable drilling also results from collapse of the induced keyhole. Finding mechanisms of these defects is essentially required to control qualities of workpieces.

Abstract


Engineering


Introduction

Laser welding or melting (http://www.rofin.com/en/applications/laser_welding/welding_methods/)

http://www.rofin.com/en/applications/laser_welding/welding_methods/





Engineering


TORVAC EBW, max. 60 kV, 50 mA, 60 mm/s, 3000 W

Experimental setup

Rippling and spiking are decreased by increasing welding speed. Porosity

can also be seen near the spiking tip (Wei et al. 2012, IEEE Trans.

CPMT)


Engineering


Observation and measurements

Spiking and humping are decreased by increasing welding speed and raising focal location. Porosity can also be seen near the spiking tip (Wei et al. 2012. IEEE Trans. CPMT)


Engineering


(continued)


Engineering


(continued)

Spiking tendency by considering energy conservation in welding and vertical directions is given by (Wei et al. 2012, IEEE Trans. CPMT)

sh w~

h

22 1

1

w 1 Ste1 c 1 ( ) ( c )

2h Pe Ste 1

where melting efficiency is

s

1 2

In above equations, h , h, w, and are spking amplitude, average fusion zone depth and width, and beam radius. Pe and Ste are the Peclet and Stefan numbers, c and c empirical constants, respectively.


Engineering


(continued)

21/2 2 2/3

r m

1 h dγ qw~ ( ) [ ( ) ]

n w γ dT μk

23/2 2 2/3

r m

1 h dγ qw~ ( ) [ ( ) ]

n w γ dT μk

Average pitch of humping or spiking for alloys in the absence and presence of volatile elements are, respectively,

mwhere ,γ , ,μ,dγ/dT,q, and k are density, surface tension, kinematic and dynamic viscosities, surface tension coefficient,incidentflux and liquid thermal conductivity,respectively.


Engineering


Bubble nucleated due to super-saturationPore formation due to liquid entrapment in keyhole welding (Pastor et al. 2001, Weld. Int.)

Pore formation


Engineering


Experimental setup

Experimental Setup (Wei et al. 2003, Metall. Mater. Trans B; Wei et al. 2004, JCG)


Engineering


(continued)

Bubbles trapped in solid at different times or locations near the location of 1 cm (a) 0, (b) 5, (c) 20, (d) 60, (e) 120, (f) 150, (g) 180, and (h) 206 s during the freezing of water containing oxygen gas content of 0.0041 g/100 g and temperature of the constant temperature sink of -250C (Wei et al. 2004, JCG).


Engineering


(continued)

Bubbles trapped in solid at different times or locations near a location of 1 cm (a) 0 s,(b) 450 s, (c) 540 s, (d) 810 s, (e) 900 s, (f) 1170 s, (9) 1350 s, (h) 1440 s during the freezing of water containing oxygen gas content of 0.0037 g/100 g and temperature of -250C of the constant temperature sink (Wei et al. 2004).

NSYSU

Mechanical & Micro-Mechanical Engineering


Pore formation due to super-saturation

g gg

dp dndVV p RT

dt dt dt

2B

dV dsr

dt dt

2B

gD ,w

dnh r (C C )

dt

Differentiating equation of state with time

g ,wp KC

Mass transfer to the bubble is given by

Henry’s law is

Volume change rate is

g

D B w

gp R are, respectively, pressure, volume, mole of gas and temperature in the pore, and specific gas constant, h ,r ,C ,C ,s,and K the mass transfer coefficient,pore radius, concent

where , V, n ,T,and

ration at the bubble cap and infinity, solification front displacement and Henry constant.

(continued)NSYSU



Development of pore shape for dR/ds= 0.04sin(0.4s) (Wei and Hsiao, 2012)

(continued)NSYSU



Equations of mass, momentum are, respectively

s

Du P u v u 2 u v2

Dt x x x y x y 3 x x y

d d( T ( T) ) ( C ( C) ) n n

dT dC

n n n n n i

u vu v 0

t x y x y

s

Dv P v v u 2 u v2 g

Dt y y y x x y 3 y x y

d d( T ( T) ) ( C ( C) ) n n

dT dC

n n n n n j

(continued)NSYSU



Conservation equations of energy, concentration and phase field equations are, respectively

( h) ( uh) ( vh) T Tk k [ L uL vL ]t x y x x y y t x y

CC (uC) (vh) C CD D [ uC vC ]t x y x x y y t x y

2 12 2

Lt pf

v

,where surface curvature 1 2

n

2s n nDelta 3 funct (1 ) 4i /on


Engineering


(continued)

Predict pore formation in aluminum

Pore formation due to liquid entrapment NSYSUMechanical & Micro-Mechanical

Engineering


Equations of mass, momentum and energy are, respectively

c c c c

c c cc

dρ du dA dW

ρ u WA

2 cimc c c c c c c

h c

dW4 dsu du gdz dp u (1 ) 0

D W

2c i

c c Ei

u dWdq dh d (H H )

2 W

2E

cc 1 2

j 1 1p =p +Γ( + )

ρ R R

• The higher the gas pressure, the easier and smaller the pore can be formed

h cim

c E c

D , , and W

h , H , H

where , are shear stress, hydraulic diameter, axial velocity component ratiobetween entrainment and mixture through the core region, and mass rate through the keyhole,q,

E 1 2, j , R , R and the absorbed energy,total energy of entrainment and mixture gas, entrainment flux, radii of principle curvatures, and surface tension paramter.

(continued)


Engineering


Pore formation or keyhole collapse for energy absorption for a supersonic flow (Wei et al. 2014, IEEE Trans. CPMT)

Conclusions


Engineering


Mechanisms of different types of surface patterns such as rippling, gouging, undercut, and humping, and root spiking are still unclear.

Pore formation is characterized by different mechanisms: (1) super-saturation of dissolved gases in liquid ahead of the solidification front, and (2) liquid entrapment such as keyhole collapse during keyhole welding.

All these defects involve strong deformation of the free surface and different types of instabilities coupled with complicated transport processes. Controlling factors need to be clarified and determined.

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p. s. wei xi-wan chair professor department of mechanical and electro-mechanical engineering

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