Humping is a frequently observed welding defect in laser welding which is caused when the welding speed exceeds a certain limit while the other welding conditions remain unchanged. Humping is characterized by the appearance of unsmooth and discontinuity of humps at the surface of the weld. The formation of humping is generally understood to be caused by the complex heat transfer and melt flow in a high speed welding process. However, so far the fundamental mechanisms causing humping are not fully understood, and research on determining the onset of humping has been based on the “trial-and-error” procedure. In this paper, mathematical models previously developed by the authors for the transport phenomena in laser welding have been extended to investigate the formation of the humping defect. In this study, the transient heat transfer and melt flow in the weld pool during the keyhole formation and collapse, and melt solidification are calculated for a 3-D moving laser welding. Different humping patterns have been predicted by the present study in different laser power levels and welding speeds. From the present study, it was found that the formation of humping in laser welding is caused by the interplay between two important factors: a) the strong liquid metal flow in the real part of the keyhole induced mainly by the laser recoil pressure and b) the rapid solidification rate of the liquid metal. The humping pattern can be well explained by the calculated melt flow and the solidification process.

1.
Bradstreet
B. J.
,
Effect of surface tension and metal flow on weld bead formation
,
Welding Journal
47
(
7
),
1968
, pp.
314s–322s
314s–322s
.
2.
L. Rayleigh, Theory of sound, Vol. 2, 2nd Ed, 1896, London: Macmillan reprinted 1945 New York: Dover, pp. 351–362.
3.
Savage
W. F.
,
Nippes
E. F.
and
Agusa
K.
,
Effect of arc force on defect formation in GTA welding
,
Welding Journal
58
(
7
),
1979
, pp.
212s–224s
212s–224s
.
4.
A. Scotti, U. Larsen and J. Norrish, Bead instability of mechanized P-MIG welding in vertical-up position, Proceeding of International Conference on Joining of Materials JOM-5, 1991, Denmark, pp. 127–134.
5.
M.L. Lin and T.W. Eagar, Influence of surface depression and convection on arc weld pool geometry, Transport Phenomena in Materials Processing, ed M.M. Chen et al, American Society of Mechanical Engineers, New York, 1984.
6.
Mills
K. C.
and
Keene
B. J.
,
Factors affecting variable weld penetration
,
International Material Review
,
35
,
1990
, pp.
185
216
.
7.
P.F. Mendez and T.W. Eagar, Penetration and defect formation in high-current arc welding, Welding Journal, October 2003, pp. 296s–306s.
8.
C.E. Albright and S. Chiang, High-speed laser welding discontinuities, Journal of Laser Applications, 1988, pp. 18–24.
9.
S. Hiramoto, M. Ohmine, T. Okuda and A. Shinmi, Deep penetration welding with high power CO2 laser, LAMP’87, Osaka, Japan, 1987, pp. 157–162.
10.
M. Beck, P. Berger, P. Nagendra and H. Hugel, Aspects of keyhole/melt interaction in high speed laser welding, Proc. 8th Int. Symp. On Gas Flow and Chemical Lasers, 1991.
11.
Gratzke
U.
,
Kapadia
P. D.
,
Dowden
J.
,
Kroos
J.
and
Simon
G.
,
Theoretical approach to the humping phenomenon in welding processes
,
Journal of Physics D: Applied Physics
25
,
1992
, pp.
1640
1647
.
12.
J. Zhou, Modeling of three-dimensional laser welding process, Ph.D. Dissertation, University of Missouri-Rolla, 2003.
13.
V.V. Semak, J.A. Hopkins, M.H. McCay and T.D. McCay, A concept for a hydrodynamic model of keyhole formation and support during laser welding, ICALEO, 1994, pp. 641–650.
14.
V.V. Semak, J.A. Hopkins, M.H. McCay and T.D. McCay, Dynamics of penetration depth during laser welding, ICALEO, 1994, pp. 17–20.
15.
Klemens
P. G.
,
Heat balance and flow conditions for electron beam and laser welding
,
J. Appl. Phys.
47
,
1976
, pp.
2165
2174
.
16.
Semak
V. V.
and
Matsunawa
A.
,
The role of recoil pressure in energy balance during laser materials processing
,
J. Phys. D: Appl. Phys.
30
,
1997
,
2541
2552
.
17.
P. Solana, P. Kapadia and J. Dowden, Surface depression and ablation for a weld pool in material processing: a mathematical model, ICALEO, Sec. F, 1998, pp. 142–147.
18.
A. Clucas, R. Ducharme, P. Kapadia, J. Dowden and W. Steen, A mathematical model of laser keyhole welding using a pressure and energy balance at the keyhole walls, ICALEO, Sec. F, 1998, pp. 123–131.
19.
M. M. Chen and J.A. Bos, Melt flow in deep penetration welding, ICALEO, Sec. F, 1998, pp. 187–196.
20.
Ducharme
R.
,
Williams
K.
,
Kapadia
P.
,
Dowden
J.
,
Steen
B.
and
Glowacki
M.
,
The laser welding of thin metal sheets: an integrated keyhole and weld pool model with supporting experiments
,
J. Phys. D: Appl. Phys.
27
,
1994
, pp.
1619
1627
.
21.
Sudnik
R.
,
Rada
D.
,
Breitschwerdt
S.
and
Erofeew
W.
,
Numerical simulation of weld pool geometry in laser beam welding
,
J. Phys. D: Appl. Phys.
33
,
2000
, pp.
662
671
.
22.
P. Kapadia, J. Dowden and R. Ducharme, A mathematical model of ablation in the keyhole and droplet formation in the plume in deep penetration laser welding, ICALEO, Sec. B, 1996, pp. 106–114.
23.
Solana
P.
and
Negro
G.
,
A study of the effect of multiple reflections on the shape of the keyhole in the laser processing of material
,
J. Phys. D: Appl. Phys.
30
,
1997
, pp.
3216
3222
.
24.
E.A. Metzbower, Absorption in the keyhole, ICALEO, Sec. G, 1997, 16–25.
25.
D.B. Kothe, R.C. Mjolsness and M.D. Torrey, Ripple: A computer program for incompressible flows with free surfaces, LA-12007-MS, Los Alamos National Laboratory, 1991.
26.
Chiang
K. C.
and
Tsai
H. L.
,
Shrinkage-induced fluid flow and domain change in two-dimensional alloy solidification
,
Int. J. Heat Mass Transfer
35
,
1992
, pp.
1763
1769
.
27.
Dowden
J.
,
Postacioglu
N.
,
Davis
M.
and
Kapadia
P.
,
A keyhole model in penetration welding with a laser
,
J. Phys. D: Appl. Phys.
20
(
1987
), pp.
36
44
.
28.
I. Miyamoto, E. Ohmura and T. Maede, Dynamic behavior of plume and keyhole in CO2 laser welding, ICALEO, 1997, section G, pp. 210–218.
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