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Effect of fusion zone shape on the composition uniformity of laser surface alloyed iron
Scripta
METALLURGICA
Vol. 16, pp. 65-68, 1982 P r i n t e d in the U . S . A .
P e r g a m o n P r e s s Ltd. All r i g h t s r e s e r v e d
EFFECT OF FUSION ZONE SHAPE ON THE cOMPOSITION UNIFORMITY OF LASER SURFACE ALLOYED IRON
P .A.Mollan Research Asslstant, Department of Materials Science Oregon Graduate Center, Beaverton, Oregon-97006 (Received October (Revised November
23, 12,
1981) 1981)
INTRODUCTION Laser surface alloying is a unique process designed to produce different alloy compositions and microstructural features on the surface of a substrate material, thereby improving its resistance to wear, corrosion, fatigue and impact. The basic process consists of applying a thin coating, on the order of i0 to 200 microns, to the surface of a substrate and then scanning a high power laser beam to melt ~nd ~use the c~ating into the substrate to form an allo~ (F~§ure i). High energy density (i0 -i0 watts/cm ) and short interaction times (I0 v-lO sec) involved in this process naturally rises questions on whether uniform composition can be achieved in laser alloyed fusion zones. Welnman et al (1) had shown, by electro microprobe analysis of laser surface alloyed chromium on AISI 1018 steel,that uniform mixing of chromium had occurred in the melt pool within 50 microseconds. Cline and Anthony (2,3) have proposed that concentration homogenization of coating into the substrate during laser surface alloying depends on the diffusivity of coating elements and the degree of convection currents produced in the melt pool. In the present study, laser surface alloying of chromium on pure iron was performed, using a 1200 watts continuous wave CO~ gas laser, to produce z different fusion zone shapes. The composition uniformity of chromium in the fusion zones was then investigated as a function of fusion zone shape. EXPERIMENTAL High purity iron (Ferro vac E- C=0.008%, Si=0.007%, Mn=0.O07%, P=0.008%, S=0.003%), received in the form of cold rolled ingot, was vacuum degassed (10-7 torr) at b~O~C and cut to specimens of size 7.5 x 2.5 x 0.64 cm. Chromium was then electrodeposited on these specimens to a thickness of 50 microns using conventional plating methods. After chromium deposition, the samples were again vacuum degassed (i0 tort) at 205°C (400°F) to remove the entrapped hydrogen in the coating. The chromium deposited iron specimens were then mounted on a numerically controlled x-y table and irradiated with a continuous wave COp laser beam of 10.6 microns wavelength at a power level of 1200 watts. The beam was defocuse~ to obtain the spot sizes of 0.0066, 0.025 and 0.040 cm. Single scan laser melting was employed with a scan rate of 2.10 cm/sec (50 In/mln). The laser beam power was measured by directing the beam into a watercooled calorimeter and measuring the temperature drop associated with heat flow across a copper element lamination. The actual focal point of the lens was determined directly from experiments on thin plexiglass sheet. The effective spot size at the surface of specimen was then calculated using appropriate optical relations. After laser treatment the specimens were sectioned, polished, etched and then examined by scanning electron microscope. Energy and wavelength x-ray microprobe analysis were used to study the compositions of fusion zones. RESULTS AND DISCUSSION Figure 2 shows the transverse sections of laser alloyed fusion zones produced under different laser beam parameters and the corresponding chromium concentration profile plots from the top surface of fusion zone to the fuslon/base interface. It is seen that the variation Jn chromium content is more pronounced in Fig.2a, intermediate in Fig.2b and relatively none in Fig.2c. Figure 2a is a representative of deep penetration melts, where the laser beam delivers energy more rapidly than it can be removed by thermal conduction. A 'key-hole' is
65 0036-9748/82/010065-04503.00/0 C o p y r i g h t (c) 1982 P e r g a m o n P r e s s
Ltd.
66
LASER
SURFACE
ALLOYED
IRON
Vol.
16, No.
i
then drilled into the material and laser energy is deposited through the depth of hole, resulting a deep penetration fusion zone. On the contrary, Figure 2c is a shallow fusion zone primarily produced by conduction heat transfer mechanism (hereafter called as 'conduction melt'). Figure 2b appears to be a hybrid of Figures 2a and 2c. Figure 2a, corresponding to 'deep penetration' melting mode, exhibits a hour-glass shape with a high aspect ratio ( aspect ratio = Penetration depth % Melt width). In contrast, Figure 2c corresponding to 'conduction' melting mode, possesses a semicircular shape with a small aspect ratio. The fusion zone of Figure 2b exhibits a triangular shape. The results of chromium concentrations plotted in Figure 2, corresponding to the fusion zones indicate that uniform concentration of chromium is obtained in semicircular fusion zone compared to hour-glass and triangular fusion
zones.
[lib,
L~SER SEA.
OF TRAVEL
Fig.l. A schematic of laser surface alloying process The follwlng table serves to illustrate the laser beam parameters employed in this study to obtain the above described fusion zones. In addition, the resulting penetration depths of fusion zones and depth o f unlforr~ concentration of chromium that can be obtained in the fusion zones based on the models proposed by Cllne and Anthony (2,3) are given. Laser beam parameters Hour-glass
Fusion zone shape Triangular
Semicircular
Power density watts/cm
3.5 x l0 7
2.4 x 10 6
1.0 x i0 6
Interaction time seconds
3.12 x 10-3
1.2 x 10-2
1.9 x 10 -2
Penetration depth microns *Depth of uniform concentration of chromium (Diffusion model) microns *Time that the alloy was in molten state seconds *Fluid flow velocity required to achieve homogenization (Surface tension model)
1430
190
5.8 x 10 -2
4500
880
380
4.6 x 10 -2
2550
cm/sec
*Estimated based on diffusion and surface tension models (2,3)
250
470
1.F x 10 -2
500
ffl
0
3
4
6
Fig.2.
Ilg
~
U~
If,
m
0
Composition
I 500
0
,
12
0
I 300
I 600
MELT DEPTH:
* 900
880/~m
of
fusion
zone
as
0
.
of
fusion
I I00
-
.
zone
I 200
MELT DEPTH:
(x 10 " 4 c M )
0
30
40
a function
DISTANCE FROM MELT SURFACE
' 1500
uniformity
, I000
MELT DEPTH: 1430/~m
shape
I 300
250/~m
-~
° Z
~< ~
G
>
;>
U3
©
< ©
68
LASER
SURFACE
ALLOYED
IRON
Vol.
16, No.
1
For u n i f o r m c o m p o s i t i o n t o o c c u r i n t h e f u s i o n z o n e , two mechanisms have b e e n p o s t u l a t e d (2,3). One i s b a s e d on t h e d i f f u s i o n o f s o l u t e i n t h e m e l t p o o l and t h e o t h e r b a s e d on t h e s u r f a c e t e n s i o n e f f e c t s c a u s e d by t h e t e m p e r a t u r e g r a d i e n t s i n t h e m e l t p o o l and s u r r o u n d i n g area. The d i f f u s i o n model i n v o l v e s t h e l a s e r beam i n t e r a c t i o n t i m e ( t ) and t h e d l f f u s i ~ i ~ y o f s o l u t e e l e m e n t i n t h e l i q u i d s t a t e (D~) a s g i v e n i n t h e f o l l o w i n g e q u a t l o ~ , X= (D~t) "~ where X i s t h e d i f f u s i o n d i s t a n c e o v e r which t h e c o n c e n t r a t i o n h o m o g e n i z a t i o n can o c c u r . C l l n e and Anthony ( 2 , 3 ) p r o p o s e d t h a t w i t h i n t e r a c t i o n t i m e s o f 50 m i c r o s e c o n d s t o 2 m i l l i s e c o n d s , t h e d i f f u s i o n d i s t a n c e s (X) i n t h e m e l t p o o l a r e too s m a l l t o o b t a i n u n i f o r m m i x i n g i n f e r r o u s s y s t e m s . S u r f a c e t e n s i o n g r a d i e n t mechanism was n o t a l s o p e r m i t t e d t o be o p e r a t i v e w i t h i n t h i s t i m e f r a m e , a c c o r d i n g t o them, s i n c e t h e c o n v e c t i o n c u r r e n t s p r o d u c e d i n t h e m e l t p o o l by s u r f a c e t e n s i o n g r a d i e n t f o r c e s had b e e n shown t o l a g t h e advance of s o l i d i f i c a t i o n f r o n t and h e n c e c o u l d n o t c a u s e enough m i x i n g . Let us assume t h a t t h e f l u i d f l o w i n m o l t e n i r o n , i n d u c e d by s u r f a c e t e n s i o n g r a d i e n t s , would p r o d u c e a more u n i f o r m a l l o y e d r e g i o n t h a n d i f f u s i o n o f s o l u t e i n t h e m e l t p o o l . Then t h e time t h a t t h e a l l o y was i n t h e l i q u i d s t a t e ( t ' ) and t h e f l u i d f l o w v e l o c i t y [ U ( y , t ' ) ] r e q u i r e d t o homogenize t h e l i q u i d had been e s t i m a t e d ( s e e t h e above t a b l e ) e m p l o y i n g t h e e q u a t i o n s , d e v e l o p e d by C l i n e and Anthony ( 2 , 3 ) , which a r e g i v e n a s f o l l o w s :
u(y,t') = 1 (d~) aT [(Vut')0"5--y] where
y = t'= = dUO = dt
p e n e t r a t i o n d e p t h (cm) time that the alloy was in molten state (a~c) surface energy per unit ares (dyne-seclcm-) surface tension temperature gradient
(ergs/cm 2 °K)
AT = temperature difference between the liquid below the laser beam and the liquid at the sylid-liquid interface of the melt pool (°K) D
u = P/P,
"diffusivity"
of velocity profile
(dyne-sec/gm)
Fluid flow velocity, estimated from the above equation as a function of fusion zone shape (see the above table), indicates the degree of convection currents required to obtain uniform concentration of chromium in the laser alloyed fusion zone. Contrary to these mechanisms, Welnman et al (i) had shown, using electron microprobe analysis of laser surface alloyed chromium steel, that complete mixing of chromium had occurred in 50 microsecond interaction time. They postulated a different mechanism namely that mixing effect~ caused by superheated gases above the melt pool were sufficient to stir the melt pool. In the present investigation, the diffusion mechanism has been followed to explain the uniform concentration of chromium in semicircular fusion zones (see the table). However the uniform compositions obtained over a large distance in hour-glass and trlangular fusion zones than predicted by the diffusion models suggest that surface tension gradient and superheated gas (1-3) mechanisms are also operative during laser surface alloying process. REFERENCES l . L.S.Weinman, processing, 2. T.R.Anthony 3. T.R.Anthony
J.N.Devault and P.G.Noore, Symposium on laser-solid interactions and laser Nov.28-Dec.l, 1978, Boston, HA and H.E.Cllne, Journal of Applled Physics, 49, 3, 1248, (1978) and H.E.Cline, Journal of Applled Physics, 48, 9, 3888, (1977)
METALLURGICA
Vol. 16, pp. 65-68, 1982 P r i n t e d in the U . S . A .
P e r g a m o n P r e s s Ltd. All r i g h t s r e s e r v e d
EFFECT OF FUSION ZONE SHAPE ON THE cOMPOSITION UNIFORMITY OF LASER SURFACE ALLOYED IRON
P .A.Mollan Research Asslstant, Department of Materials Science Oregon Graduate Center, Beaverton, Oregon-97006 (Received October (Revised November
23, 12,
1981) 1981)
INTRODUCTION Laser surface alloying is a unique process designed to produce different alloy compositions and microstructural features on the surface of a substrate material, thereby improving its resistance to wear, corrosion, fatigue and impact. The basic process consists of applying a thin coating, on the order of i0 to 200 microns, to the surface of a substrate and then scanning a high power laser beam to melt ~nd ~use the c~ating into the substrate to form an allo~ (F~§ure i). High energy density (i0 -i0 watts/cm ) and short interaction times (I0 v-lO sec) involved in this process naturally rises questions on whether uniform composition can be achieved in laser alloyed fusion zones. Welnman et al (1) had shown, by electro microprobe analysis of laser surface alloyed chromium on AISI 1018 steel,that uniform mixing of chromium had occurred in the melt pool within 50 microseconds. Cline and Anthony (2,3) have proposed that concentration homogenization of coating into the substrate during laser surface alloying depends on the diffusivity of coating elements and the degree of convection currents produced in the melt pool. In the present study, laser surface alloying of chromium on pure iron was performed, using a 1200 watts continuous wave CO~ gas laser, to produce z different fusion zone shapes. The composition uniformity of chromium in the fusion zones was then investigated as a function of fusion zone shape. EXPERIMENTAL High purity iron (Ferro vac E- C=0.008%, Si=0.007%, Mn=0.O07%, P=0.008%, S=0.003%), received in the form of cold rolled ingot, was vacuum degassed (10-7 torr) at b~O~C and cut to specimens of size 7.5 x 2.5 x 0.64 cm. Chromium was then electrodeposited on these specimens to a thickness of 50 microns using conventional plating methods. After chromium deposition, the samples were again vacuum degassed (i0 tort) at 205°C (400°F) to remove the entrapped hydrogen in the coating. The chromium deposited iron specimens were then mounted on a numerically controlled x-y table and irradiated with a continuous wave COp laser beam of 10.6 microns wavelength at a power level of 1200 watts. The beam was defocuse~ to obtain the spot sizes of 0.0066, 0.025 and 0.040 cm. Single scan laser melting was employed with a scan rate of 2.10 cm/sec (50 In/mln). The laser beam power was measured by directing the beam into a watercooled calorimeter and measuring the temperature drop associated with heat flow across a copper element lamination. The actual focal point of the lens was determined directly from experiments on thin plexiglass sheet. The effective spot size at the surface of specimen was then calculated using appropriate optical relations. After laser treatment the specimens were sectioned, polished, etched and then examined by scanning electron microscope. Energy and wavelength x-ray microprobe analysis were used to study the compositions of fusion zones. RESULTS AND DISCUSSION Figure 2 shows the transverse sections of laser alloyed fusion zones produced under different laser beam parameters and the corresponding chromium concentration profile plots from the top surface of fusion zone to the fuslon/base interface. It is seen that the variation Jn chromium content is more pronounced in Fig.2a, intermediate in Fig.2b and relatively none in Fig.2c. Figure 2a is a representative of deep penetration melts, where the laser beam delivers energy more rapidly than it can be removed by thermal conduction. A 'key-hole' is
65 0036-9748/82/010065-04503.00/0 C o p y r i g h t (c) 1982 P e r g a m o n P r e s s
Ltd.
66
LASER
SURFACE
ALLOYED
IRON
Vol.
16, No.
i
then drilled into the material and laser energy is deposited through the depth of hole, resulting a deep penetration fusion zone. On the contrary, Figure 2c is a shallow fusion zone primarily produced by conduction heat transfer mechanism (hereafter called as 'conduction melt'). Figure 2b appears to be a hybrid of Figures 2a and 2c. Figure 2a, corresponding to 'deep penetration' melting mode, exhibits a hour-glass shape with a high aspect ratio ( aspect ratio = Penetration depth % Melt width). In contrast, Figure 2c corresponding to 'conduction' melting mode, possesses a semicircular shape with a small aspect ratio. The fusion zone of Figure 2b exhibits a triangular shape. The results of chromium concentrations plotted in Figure 2, corresponding to the fusion zones indicate that uniform concentration of chromium is obtained in semicircular fusion zone compared to hour-glass and triangular fusion
zones.
[lib,
L~SER SEA.
OF TRAVEL
Fig.l. A schematic of laser surface alloying process The follwlng table serves to illustrate the laser beam parameters employed in this study to obtain the above described fusion zones. In addition, the resulting penetration depths of fusion zones and depth o f unlforr~ concentration of chromium that can be obtained in the fusion zones based on the models proposed by Cllne and Anthony (2,3) are given. Laser beam parameters Hour-glass
Fusion zone shape Triangular
Semicircular
Power density watts/cm
3.5 x l0 7
2.4 x 10 6
1.0 x i0 6
Interaction time seconds
3.12 x 10-3
1.2 x 10-2
1.9 x 10 -2
Penetration depth microns *Depth of uniform concentration of chromium (Diffusion model) microns *Time that the alloy was in molten state seconds *Fluid flow velocity required to achieve homogenization (Surface tension model)
1430
190
5.8 x 10 -2
4500
880
380
4.6 x 10 -2
2550
cm/sec
*Estimated based on diffusion and surface tension models (2,3)
250
470
1.F x 10 -2
500
ffl
0
3
4
6
Fig.2.
Ilg
~
U~
If,
m
0
Composition
I 500
0
,
12
0
I 300
I 600
MELT DEPTH:
* 900
880/~m
of
fusion
zone
as
0
.
of
fusion
I I00
-
.
zone
I 200
MELT DEPTH:
(x 10 " 4 c M )
0
30
40
a function
DISTANCE FROM MELT SURFACE
' 1500
uniformity
, I000
MELT DEPTH: 1430/~m
shape
I 300
250/~m
-~
° Z
~< ~
G
>
;>
U3
©
< ©
68
LASER
SURFACE
ALLOYED
IRON
Vol.
16, No.
1
For u n i f o r m c o m p o s i t i o n t o o c c u r i n t h e f u s i o n z o n e , two mechanisms have b e e n p o s t u l a t e d (2,3). One i s b a s e d on t h e d i f f u s i o n o f s o l u t e i n t h e m e l t p o o l and t h e o t h e r b a s e d on t h e s u r f a c e t e n s i o n e f f e c t s c a u s e d by t h e t e m p e r a t u r e g r a d i e n t s i n t h e m e l t p o o l and s u r r o u n d i n g area. The d i f f u s i o n model i n v o l v e s t h e l a s e r beam i n t e r a c t i o n t i m e ( t ) and t h e d l f f u s i ~ i ~ y o f s o l u t e e l e m e n t i n t h e l i q u i d s t a t e (D~) a s g i v e n i n t h e f o l l o w i n g e q u a t l o ~ , X= (D~t) "~ where X i s t h e d i f f u s i o n d i s t a n c e o v e r which t h e c o n c e n t r a t i o n h o m o g e n i z a t i o n can o c c u r . C l l n e and Anthony ( 2 , 3 ) p r o p o s e d t h a t w i t h i n t e r a c t i o n t i m e s o f 50 m i c r o s e c o n d s t o 2 m i l l i s e c o n d s , t h e d i f f u s i o n d i s t a n c e s (X) i n t h e m e l t p o o l a r e too s m a l l t o o b t a i n u n i f o r m m i x i n g i n f e r r o u s s y s t e m s . S u r f a c e t e n s i o n g r a d i e n t mechanism was n o t a l s o p e r m i t t e d t o be o p e r a t i v e w i t h i n t h i s t i m e f r a m e , a c c o r d i n g t o them, s i n c e t h e c o n v e c t i o n c u r r e n t s p r o d u c e d i n t h e m e l t p o o l by s u r f a c e t e n s i o n g r a d i e n t f o r c e s had b e e n shown t o l a g t h e advance of s o l i d i f i c a t i o n f r o n t and h e n c e c o u l d n o t c a u s e enough m i x i n g . Let us assume t h a t t h e f l u i d f l o w i n m o l t e n i r o n , i n d u c e d by s u r f a c e t e n s i o n g r a d i e n t s , would p r o d u c e a more u n i f o r m a l l o y e d r e g i o n t h a n d i f f u s i o n o f s o l u t e i n t h e m e l t p o o l . Then t h e time t h a t t h e a l l o y was i n t h e l i q u i d s t a t e ( t ' ) and t h e f l u i d f l o w v e l o c i t y [ U ( y , t ' ) ] r e q u i r e d t o homogenize t h e l i q u i d had been e s t i m a t e d ( s e e t h e above t a b l e ) e m p l o y i n g t h e e q u a t i o n s , d e v e l o p e d by C l i n e and Anthony ( 2 , 3 ) , which a r e g i v e n a s f o l l o w s :
u(y,t') = 1 (d~) aT [(Vut')0"5--y] where
y = t'= = dUO = dt
p e n e t r a t i o n d e p t h (cm) time that the alloy was in molten state (a~c) surface energy per unit ares (dyne-seclcm-) surface tension temperature gradient
(ergs/cm 2 °K)
AT = temperature difference between the liquid below the laser beam and the liquid at the sylid-liquid interface of the melt pool (°K) D
u = P/P,
"diffusivity"
of velocity profile
(dyne-sec/gm)
Fluid flow velocity, estimated from the above equation as a function of fusion zone shape (see the above table), indicates the degree of convection currents required to obtain uniform concentration of chromium in the laser alloyed fusion zone. Contrary to these mechanisms, Welnman et al (i) had shown, using electron microprobe analysis of laser surface alloyed chromium steel, that complete mixing of chromium had occurred in 50 microsecond interaction time. They postulated a different mechanism namely that mixing effect~ caused by superheated gases above the melt pool were sufficient to stir the melt pool. In the present investigation, the diffusion mechanism has been followed to explain the uniform concentration of chromium in semicircular fusion zones (see the table). However the uniform compositions obtained over a large distance in hour-glass and trlangular fusion zones than predicted by the diffusion models suggest that surface tension gradient and superheated gas (1-3) mechanisms are also operative during laser surface alloying process. REFERENCES l . L.S.Weinman, processing, 2. T.R.Anthony 3. T.R.Anthony
J.N.Devault and P.G.Noore, Symposium on laser-solid interactions and laser Nov.28-Dec.l, 1978, Boston, HA and H.E.Cllne, Journal of Applled Physics, 49, 3, 1248, (1978) and H.E.Cline, Journal of Applled Physics, 48, 9, 3888, (1977)