U.S. patent application number 10/492262 was filed with the patent office on 2005-02-10 for process for avoiding cracking in welding.
Invention is credited to Fernandes De Lima, Milton, Kurz, Wilfried, Rappaz, Michel, Wagniere, Jean-Daniel.
Application Number | 20050028897 10/492262 |
Document ID | / |
Family ID | 8184183 |
Filed Date | 2005-02-10 |
United States Patent
Application |
20050028897 |
Kind Code |
A1 |
Kurz, Wilfried ; et
al. |
February 10, 2005 |
Process for avoiding cracking in welding
Abstract
The new welding process avoids cracking in welding, in repair
welding or in cladding of parts of metallic alloys which are
sensitive to hot cracking. The process is using a first heat source
(15), directed to the parts (11, 12) of the metallic alloy forming
a melt pool (14) on the parts (11, 12) of metal or metallic alloy.
The heat source (15) and the parts (11, 12) are moved relative to
each other. The process is characterized in that there is one (13)
or more additional heat sources directed to the parts (11, 12) of
metal or metallic alloy and following the first heat source (15) in
a distance and with substantially the same speed and in the same
direction as the first heat source (15). The additional heat source
(13) or heat sources are directed to the solidification region
(mushy zone) (144) of the melt pool (14) generated by the first
heat source (15). The power of the additional heat source (13) is
set such as to reduce the local cooling rate of the solidification
region (144) of the melt pool (14), or to even shortly reheat this
region without substantial remelting or with no remelting it at all
and thereby reducing the tensile stresses or even inducing
compressive stresses. During this process a central equiaxed zone
might also be enhanced. By this new process the formation of hot
cracks is avoided.
Inventors: |
Kurz, Wilfried; (La
Conversion, CH) ; Wagniere, Jean-Daniel; (Boussens,
CH) ; Rappaz, Michel; (Lausanne, CH) ;
Fernandes De Lima, Milton; (Sao Paulo, BR) |
Correspondence
Address: |
TOWNSEND AND TOWNSEND AND CREW, LLP
TWO EMBARCADERO CENTER
EIGHTH FLOOR
SAN FRANCISCO
CA
94111-3834
US
|
Family ID: |
8184183 |
Appl. No.: |
10/492262 |
Filed: |
April 7, 2004 |
PCT Filed: |
October 8, 2002 |
PCT NO: |
PCT/EP02/11270 |
Current U.S.
Class: |
148/525 |
Current CPC
Class: |
B23K 26/0608 20130101;
B23K 26/0604 20130101 |
Class at
Publication: |
148/525 |
International
Class: |
C22F 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 9, 2001 |
EP |
01810986.8 |
Claims
1. Process for avoiding cracking in welding, in repair welding or
in cladding of parts of metallic alloys (11, 12) which are
sensitive to hot cracking, the process using a first heat source,
directed to the parts of the metallic alloy and forming a melt pool
(14) on the parts of metallic alloy, the heat source (15) and the
parts being moved relative to each other, the process being
characterized in that there is one or more second additional heat
sources (13) directed to the parts (11, 12) of metallic alloy and
following the first heat source (15) in a distance and with
substantially the same speed and in the same direction as the first
heat source, the second heat source(s) (13) being directed to the
solidification region of the melt pool (14) generated by the first
heat source (15), the power of the additional heat source being set
such as to reduce the local cooling rate of the solidification
region (144) of the melt pool (14) or to even shortly reheat this
region (144) without substantial remelting, thereby reducing the
tensile stresses or even inducing compressive stresses, thus
avoiding formation of hot cracks.
2. Process as claimed in claim 1, one or more additional heat
source (13) directed to the solidification region/the mushy zone
(144) of the melt pool (14), the power of the additional heat
source (13) being set such as to reduce the local cooling rate or
to even shortly reheat this region of the melt pool (14), without
remelting at all or without substantial remelting, thereby reducing
the tensile stresses or even inducing compressive stresses, thus
avoiding formation of hot cracks.
3. Process as claimed in claim 1, the first (15) and/or the
additional heat source(s) (13) being beams of high energy sources
such as, laser sources, electron beam sources, electric arc
sources, plasma sources, or a combination of those.
4. Process as claimed in claim 1, the first (15) and/or the second
(13) heat source being beams of one single or of multiple heat
sources.
5. Process as claimed in claim 1, using wire, powder or ribbons
powder of an alloy of the same or of different composition as the
parts to be welded, for filling and cladding purposes.
6. Process as claimed in claim 1, using wire, powder, ribbons or
preapplied powder material of an alloy of the same or of different
composition as the parts to be cladded.
7. Use of a process as claimed in claim 1, for avoiding cracking in
welding, repair welding or cladding of parts of super alloys and/or
steels.
8. Use of a process as claimed in claim 1, for avoiding cracking in
welding, repair welding or cladding of parts of aluminum alloys.
Description
[0001] The invention is related to a welding, repair welding or
cladding process of metallic alloys according to the preamble of
the independent claim 1. It is further related to the use of the
welding, repair welding or cladding process and to work pieces
welded or clad with the process. Its main aim is the prevention of
hot-crack formation during the process.
[0002] The general behavior of solidification and hot cracking
(solidification cracking) is very similar in the three processes
mentioned. It is further general to many metallic alloys like
steels, super alloys, aluminium alloys. Therefore only the welding
process will be explained in more detail, with the aid of hot
cracking of prone aluminium alloys. Aluminium alloys are
traditional materials in transport technology such as aerospace,
automobiles and trains, because of a good combination of mechanical
properties and low weight.
[0003] Different joining techniques are used for producing
aluminium parts. Riveting, TIG and MIG welding are traditional
processes in the manufacturing industry although they present some
important weaknesses. Rivets form weak joints and are especially
vulnerable to stress corrosion cracking. TIG and MIG produce large
heat-affected zones (HAZ), where alloys experience additional
solution treatments and averaging, thus leading to a degradation of
material properties and a reduction in lifetime. Laser welding is a
particularly interesting approach for the construction of metallic
structures. New developments in laser technology, such as
fiber-optic delivery of YAG beam and high-power diode lasers, have
increased and will increase in the future their use in high volume
production.
[0004] Many aluminium alloys are weldable provided the
solidification interval is relatively small. Some classes of
aluminium alloys such as 2xxx (Al--Cu), 5xxx (Al--Mg), 6xxx
(Al--Mg--Si) and 7xxx (Al--Zn-- . . . ) often crack during
autogenous welding. Industrial experience has shown that hot
cracking can be avoided by the addition of a eutectic-forming
alloy, such as a Al--Si wire, to the weld. This methodology is
widely applied to the construction of aluminium parts, even if the
mechanical properties of the weld are not as good as those of the
base material.
[0005] The term "hot cracking" is used to denote brittleness at
temperatures above the solidification end (often the eutectic
temperature) which is due to the presence of residual liquid films
in-between the dendritic grains of the solidifying alloy. Materials
in which such cracking occurs invariably possess a large
solidification interval, since pure metals and eutectic alloys are
not susceptible to hot cracking. During cooling of these alloys
from the liquid, the formation of primary dendrites begins close to
liquidus temperature, and during subsequent cooling these dendrites
grow at the expense of the liquid. When the proportion of liquid is
still large, the alloys have essentially the properties of a
liquid. Later in the process of solidification, however, the
dendrites interlock and form a coherent network with the remaining
liquid occupying the interstices. During the formation of this
network, there is a progressive increase in strength. However, if
the nearly-completely solidified alloy has a high-strength in
compression, it is still weak with respect to the transmission of
shear/tensile stresses as long as inter-dendritic and
inter-granular liquid films are present. In parallel to this, the
interdendritic liquid experiences an increasing difficulty to flow
through the high tortuosity paths in order to compensate shrinkage
and deformation of the solid skeleton. The combination of shrinkage
and shear/tensile stresses will therefore lead to underpressure in
the remaining liquid, and thus finally to solidification cracking.
The alloy is therefore susceptible to cracking while it is in the
brittle temperature range (BTR), i.e. at a temperature
corresponding approximately to the last 10% of liquid. The
invention is related to an improved welding process that overcomes
the problem of cracking and in particular of hot cracking.
[0006] The process according to the invention is characterized by
the features of the characterizing part of the independent claim 1.
The depending claims are related to favorable improvements of the
invention. The process provides for crack free welding of work
pieces and in particular metal sheets.
[0007] The invention and the prior art are illustrated and
explained in details with reference to the pictures and
drawings.
[0008] The figures show the following:
[0009] FIG. 1a show a schematic, perspective view of two work
pieces that are welded according to the invention,
[0010] FIG. 1b is a schematic side view of the welding area and a
solidification profile of the different solid fractions during the
solidification of a dendritic network of the welded area of a work
piece.
[0011] FIG. 1c is a schematic representation of dendritic
solidification with associated solid fraction as a function of
distance and phase diagram.
[0012] FIG. 2a shows a schematic side view of the welding
setup;
[0013] FIG. 2b is a schematic side view of the laser setup and one
possible configuration of a gas supply and gas suction nozzles of
the welding setup.
[0014] FIGS. 3a and 3b show the pictures of two sheets welded with
a CO.sub.2 laser according to a prior art method (FIG. 4a) and two
sheets welded with the process according to the invention (FIG.
4b);
[0015] FIG. 4 is an example of a temperature-time curve measured by
a thermocouple placed close to the weld trace for welding process
according to the prior art FIG. 4a and the temperature-time curve
of the welding process according to the present invention FIG.
4b;
[0016] FIG. 5 is the picture of a specimen welded in the lower part
with a conventional welding process and in the upper part with the
process according to the invention, showing crack healing over the
transient zone for an overlapped joint.
[0017] On the mesoscale, hot cracks can be distinguished from other
cracks formed at distinctly lower temperature by detached grains
and crack surfaces decorated by dendrites. The residual liquid
remains on both fractured surfaces, which sometimes shows a
eutectic layer. A few spikes resulting from the opening of
inter-granular grain boundaries are also characteristic of
hot-cracked surfaces. In most cases, these effects can only be
perceived with a scanning electron microscope.
[0018] A possible arrangement for practicing welding process is
shown schematically in FIG. 1a and FIG. 1b, as well as in FIG. 2a
and FIG. 2b. The two metal sheets 11 and 12 are arranged next to
each other, thereby forming a gap 10 The metal sheets 11 and 12 are
moved in the direction of arrow A. The laser beam 15 of a CO.sub.2
laser is directed to the surface are of the two sheets 11 and 12,
and bridging the gap 10. The laser beam 15 is meting the two sheets
11 and 12 and forms a melt pool 14. A second laser beam 13 of a YAG
laser is directed to the mushy zone 144 region of the melt pool 14.
When the sheets 11 and 12 are moved in the direction of arrow A,
the laser beam 13 is following the laser beam 15. It would of
course also be possible that the laser beams 15 and 13 are moved in
stead of the two sheets 11 and 12. In this case the laser beams 15
and 13 would be moved in the direction opposite to the direction
indicated by arrow A. In another arrangement both, the sheets 11
and 12 as well as the laser beams 11 and 12 can be moved relative
to one another. As can be seen there is no overlap of the spots of
the energy sources 15 and 13 on the sheets.
[0019] FIG. 2 illustrates in more detail and schematically
dendritic solidification with associated solid fraction as a
function of distance and the phase diagram. In this example the
mushy zone 144 is also named the dendritic solidification region or
zone, where the f.sub.s the solid fraction is 0<f.sub.s<1.
This means that in the mushy zone/dendritic solidification
region/zone there is dendritic solid material, but that there is
still liquid material in this zone. 0<x %<100% of the
material is still in the liquid phase.
[0020] At the temperature T.sub.t solidification of the material
starts and dendrites start building up. At the temperature T.sub.1
lower than T.sub.t, down to the temperature T.sub.2 there is the
zone of the interdendritic film. In the shown example in this area
the solidification factor is 0.6<f.sub.x<0.9.
[0021] When in this application it is stated that the wording "or
even shortly reheat this region without substantially remelting" is
used it is meant, that the temperature of the part of the mushy
zone exposed to the second heat source will not go up to T.sub.t.
As a consequence of this, there will always be dendrites. Only the
percentage of liquid will be increase by such "non substantial
remelting" and there will always be solid dendrite material in the
mushy zone 144.
[0022] As can bee seen in FIG. 2, the process is performed under
gas protection. The gas supply nozzle G supplies the inert gas and
the gas suction nozzle S sucks the gas, so that the melt pool 14 is
well protected by the gas flowing from nozzle G to nozzle S. In the
enlarged part of the sheets 11 and 12 of FIG. 2b can be seen, that
the sheets 11 and 12 are arranged overlapping each other in the
area that is welded.
[0023] The process may also be performed with lasers beams of the
same type. In such an arrangement the laser beams may come from two
separate laser sources or the laser beams laser beams.
[0024] Based on present experience of hot cracking phenomena in
alloys, the conditions for avoiding cracks can be analyzed. Under
normal welding conditions, the transverse stress distribution near
the melt pool along the weld centerline consists of three typical
regimes. Firstly, compression forces are observed ahead of the melt
pool due to heating and thermal expansion of the solid. Secondly,
liquid formation with a free surface accommodates the stresses.
Thirdly, tensile forces build up as soon as the mushy zone begins
to behave as a continuous solid. These tensile forces, which can
result in final deformation of the welded part and/or in residual
stresses, are often responsible for hot cracks.
[0025] The control of process conditions, such as the geometry of
the weld, the clamping of the parts, and the laser power and speed
could reduce stresses behind the melt pool. For example, the
clamping distance directly influences stresses. For an edge-mounted
sample, a small clamping distance decreases tensile stresses
because of the expansion of the sheet. If the thermal conductivity
and interaction time are sufficiently large this effect leads to
compression of the mushy zone and prevents cracking. However, if
the sheets are overlapped this effect leads to sliding, thereby
producing cracks in the interface between the sheets. Usually this
occurs after complete solidification thus producing cold cracks.
Reducing welding heat input and speed also decreases the transverse
stresses, increasing the resistance against cracking.
[0026] Rappaz et al. (M. Rappaz, J.-M. Drezet and M. Gremaud, Met.
Trans. 30A (1999) 449) assessed the influence of stain rate on the
HCS (Hot Cracking Susceptibility); their model is based on the
maximum tensile/shear strain rate which can be supported by the
mushy zone before cracks appear. The stain rate can be decreased if
the cooling rate during solidification is reduced, i.e. if the
solidification speed and/or the thermal gradient are reduced. Some
thermal gradient control is possible with preheating, but this
cannot always be applied in industrial manufacturing.
[0027] The addition of a eutectic-forming alloy to the weld is
recommended as it increases the permeability of the mushy zone in
the regions where shrinkage and stresses occur. For example, the
addition of a 4043 Al--Si alloy wire to the 6061 alloy weld reduces
the hot cracking susceptibility. However both the yield and
ultimate strengths are reduced by 50%. Furthermore, there might be
variable heat input resulting from the occasional feeding of filler
wire directly into the beam, causing inconsistent penetration and
weld-pool instability.
[0028] The weld microstructure also plays a role in hot cracking.
It is essentially controlled by the growth speed V and the thermal
gradient G at the solidification front, but also by the inoculation
conditions. For columnar structures growing from the edge of the
weld microsegregation usually produces a centreline channel which
is the last part to solidify, and which is especially sensitive to
cracking. Two mechanisms can decrease the HCS of this centreline
boundary: formation of equiaxed grains and a variation in grain
orientation.
[0029] It has been observed that fine equiaxed grains are less
susceptible to hot cracking than columnar grains because the
strains are more evenly distributed among numerous grain
boundaries. Another possible technique for avoiding crack formation
consists of changing the directionality of grain growth towards the
weld centreline by producing a tortuous path.
[0030] The laser welding process according to the invention in
principle includes a precisely controlled cooling cycle and
associated stress build-up evolution. This is achieved by the
combination of two or more heat sources such as laser beams (FIG.
1). Positioning the laser source over the sensitive region of the
melt pool which is the mushy zone, this results in
[0031] a reduced contraction or even in a compression of this
zone;
[0032] a decreased strain rate. The strain rate can be decreased by
a decrease of the cooling rate during solidification, especially at
high volume fraction of solid;
[0033] an increased intergranular and interdendritic feeding
time;
[0034] an enhancement of equiaxed grains. An equiaxed structure can
be produced by decreasing the thermal gradient and by producing
mechanical stirring due to a pulsed laser, which may lead to an
additional fragmentation of dendrites.
[0035] In the following the invention is explained in detail with
the aid of an example. A 6016 aluminium alloy (Table 1) which is
used in automobiles was chosen for the experiments since it is
susceptible to hot cracking. FIG. 2 shows a schematic
representation of the welding setup, showing in FIG. 2a the fixture
system and the sheets and in FIG. 2b the setup with the CO.sub.2
laser beam and the YAG laser beam, together with the gas supply
nozzle G and the gas suction nozzle S. The alloy was delivered in a
T6 condition, after solution heat-treatment at 540.degree. C. for a
short time, air cooling, and a precipitation (aging) treatment at
205.degree. C. for several hours. The sample dimensions were
100.times.50.times.1 mm sheets which were welded with an overlap of
8 mm FIG. 2a This geometry is usually used in transportation
industry because mounting of butt plates easily leads to
uncontrolled gaps.
1TABLE 1 6016 alloy composition in wt. % Si Fe Cu Mn Mg Cr Ni Zn Ti
Al 1.11 0.24 0.07 0.06 0.41 0.013 0.0052 0.015 0.012 bal.
[0036] The laser workstation consisted of two lasers, one CO.sub.2
and one YAG laser, a CNC controlled table (with linear scanning
velocities up to 0.5 m/s) and a gas protection system, FIG. 2b. The
1.7 kW CW--CO.sub.2 laser produced a minimum focal spot of about
0.26 mm diameter with an off-axis parabolic mirror with 152 mm
focal length. The 1.2 kW pulsed mode YAG laser produced a 0.6 mm
focal spot given by the diameter of the optical fibre. For the
present application, the mean spot size of the second laser was
defocused giving an elliptical 1.2.times.1.5 mm spot with the
longer axis aligned in the direction of the laser movement
[0037] Inert gas was applied through a nozzle and evacuated through
a suction system to direct the gas stream and protect the optics.
This suction system also moved the plasma plume away from the
second laser spot allowing a free interaction of the second laser
beam with the sample surface without plasma formation. Pure helium
was found to be better than argon, or a mixture of Ar/He, since the
plasma was smoother and less metal particles were ejected. The gas
flux was set at an intermediate value of 5 l/min (too high fluxes
disturbed the liquid bath and too low fluxes did not protect
against oxidation). The best gas injection angle was found to be
about 30.degree. from the sample surface plane (FIG. 2b).
[0038] The surface cleanness is important for a high quality weld.
Dirty and oxidized surfaces produce bubbles in the welds. Washing
with water and ethanol followed by ultrasonic cleaning gave
sufficient surface cleanness to produce good welds. Laser-cleaning
prior to welding was also tried. A Q-Switched YAG laser was used to
clean some samples as an alternative to traditional cleaning with
excellent results.
[0039] Various process parameters have been considered in order to
obtain sound weldings by the dual beam method. The welding speed
was fixed at 60 mm/s with 1700 W CO.sub.2 power. In other
arrangements welding speeds of 100 mm/s and more are possible. The
average YAG laser power was fixed at 1200 W, and the following
range of process parameters were investigated:
[0040] YAG energy and frequency: 4/300, 6/200, 8/150, 10/120, and
12/100 (J/Hz)
[0041] YAG pulse length: 0.2, 1, and 2 ms.
[0042] Distance between sources: 0, 1, 2, 3, 4, 5, and 6 mm.
[0043] Among the above process variables, the best conditions for
crack-free welds at 60 mm/s weld speed were obtained when the YAG
beam was placed 3 mm behind the CO.sub.2 beam. Eight joules and 2
ms were the best compromise between an insufficient thermal
transfer (4-6 J and/or 0.2-1 ms) and burning (12 J), but
satisfactory results were also obtained using 10 J. Using 8 J pulse
energy, and a frequency of 150 Hz, the intensity per pulse was
about 30 W/cm.sup.2. In comparison with single beam CO.sub.2
welding see FIG. 3a, where there is a crack C.
[0044] The dual beam method produces an enlarged liquid bath. The
result of the dual beam method is shown in FIG. 3b, where there is
no crack at all. Bubbles in the weld disappeared when the dual
laser method was applied as length of the liquid bath (L) was
increased, thus increasing the time for bubbles to rise to the
surface.
[0045] FIG. 4 shows the temperature-time curve for a welding
process with a single laser beam (curve a) and the temperature-time
curve for a welding process with a dual laser beam (curve b) with a
welding process according to the invention. In other words FIG. 4
shows the thermal history (a) and the cooling rate (b) during
welding for single laser beam (curve a) and dual laser beam (curve
b) experiments. The peak temperature was unaffected by the use of
the second source, but the cooling rate in the critical location
was substantially reduced, from 2600.degree. C./s to 1500.degree.
C./s (curve b).
[0046] The results also showed an increase in the pool length at 8
J, which corresponds to the best welding condition. At low power
levels of the second beam, the liquid pool length (L) and width (W)
was only slightly changed since the heat input was low. At high
energy levels (above 10 J) the YAG laser interacted directly with
the plasma over the weld, formed by the first laser, both laser
beams acting as a single heat source.
[0047] As proposed by Clyne and Davies (T. W. Clyne and G. J.
Davies, J. British Foundry 74 (1981) 65), two different
solidification periods can be considered: a free feeding time (FT),
where the interdendritic spacing permits an unimpeded flux of
liquid, and a constrained feeding time (CT) where the dendritic
bridging leads to an increasing underpressure in the residual
liquid. Here these two periods of time were considered as the
interval between 60 and 10% and 10 and 1% liquid, for FT and CT
respectively. These characteristic times can be calculated from the
extension of the solidification interval giving at centerline
divided by the scanning velocity, giving:
2 Single source: FT = 0.01 s and CT = 0.005 s Process according to
the invention: FT = 0.043 s and CT = 0.005 s
[0048] The feeding time is four times greater in the welding
process according to the invention than in the single source case,
and the HCS according to Clyne is now 0.12 in comparison with 0.5
for the single source process. Therefore, the welding process
according to the invention reduces cracking by extending the time
where the liquid can feed the growing solid.
[0049] The thermal gradient can be estimated with a semi-empirical
thermal model. The results at the centerline show that the thermal
gradient at the beginning of mushy zone was reduced from 400 K/mm
to 175 K/mm This decrease has consequences on the microstructure,
leading to equiaxed dendrites near the centerline.
[0050] To prove the interest of the proposed laser welding process
according to the invention, a specimen was welded first with the
single source producing a longitudinal crack, and then the
secondary laser was switched on during the experiment. Two steady
regimes were observed: a cracked weld when a single source was used
and a crack-free weld when the second laser source was turned on.
Few millimeters of YAG interaction were required in the transient
regime to close the crack (FIG. 5). The crack disappears after
about 60 milliseconds of YAG interaction in a sample where the YAG
beam was turned on at the middle of the experiment (speed of the
laser beams 60 mm/s, YAG energy 8J). A series of ten consecutive
experiments confirmed the trend without a single exception.
[0051] The difference between the laser welding process according
to the present invention, and conventional welding processes is the
use of two or more locally and intensity wise well-controlled heat
sources. Although this can be performed by any combination of
available heat sources, as far as they are localized enough, it
appears that laser technology has a major advantage over others
methods because of the very precise control of spot size, position
and heat input, essential to the effectiveness of the present
technique.
[0052] The effect of the dual laser system on strain rate can be
discussed in the following way: under the assumption of a fully
constrained weld, the mechanical stain rate is equal to the
opposite of the thermally induced strain rate. This later value is
proportional to the cooling rate, which can be controlled by the
present welding method. Taking the values shown in FIG. 4, the
strain rate at the critical location is decreased to nearly half of
the value for conventional CO.sub.2 welding by the use of the
second laser source. The important point is that this second laser
acts directly on the final part of the mushy zone, thus reducing
the cooling rate most effectively, where it is needed.
[0053] It is possible that a dynamic correlation of the
inter-source distance and the other process parameters can also
produce sound weldments. Moreover, any combination of heat sources,
laser or otherwise, which reproduce this process window could be
used to avoid, cracks.
[0054] The new welding process avoids cracking in welding, in
repair welding or in cladding of parts of metallic alloys which are
sensitive to hot cracking. The process is using a first heat source
15, directed to the parts 11, 12 of the metallic alloy forming a
melt pool 14 on the parts 11, 12 of metal or metallic alloy. The
heat source 15 and the parts 11, 12 are moved relative to each
other. The process is characterized in that there is one 13 or more
additional heat sources directed to the parts 11, 12 of metal or
metallic alloy and following the first heat source 15 in a distance
and with substantially the same speed and in the same direction as
the first heat source 15. The additional heat source 13 or heat
sources are directed to the solidification region (mushy zone) 144
of the melt pool 14 generated by the first heat source 15. The
power of the additional heat source 13 is set such as to reduce the
local cooling rate of the solidification region 144 of the melt
pool 14, or to even shortly reheat this region without substantial
remelting or with no remelting it at all and thereby reducing the
tensile stresses or even inducing compressive stresses. During this
process a central equiaxed zone might also be enhanced. By this new
process the formation of hot cracks is avoided.
* * * * *