Post on 19-Mar-2020
Journal of Electrical Engineering 3 (2015) 176-186 doi: 10.17265/2328-2223/2015.04.003
Laser Beam Welding of Alloyed Ultra-High Strength
Steels
Stefan Lindner1, Martin Dahmen2 and Benjamin Gerhards3
1. Outokumpu Nirosta GmbH, Krefeld Research Centre, Krefeld 47807, Germany
2. Fraunhofer Institute for Laser Technology, Aachen 52074, Germany
3. RWTH Aachen University ISF Welding and Joining Institute, Aachen 52062, Germany
Abstract: Stricter regulations for automotive car body engineering in point of CO2 emissions and safety requirements demand the application of ultra-high strength materials for lightweight constructions. But components manufactured with new materials need established, cost-efficient and their benefits supporting production processes. One high potential process is laser beam welding with its high welding speed, enormous flexibility, low thermal distortion and slim weld shape. The combination of new materials on the one side, and challenging welding methods on the other side, makes a good understanding for the joining process as well as for the behavior of the base materials during the heat input necessary to create constructions which exploit the full lightweight potential and fulfill the extensive requirements. The following paper contributes to the safety applying of new materials into the car body manufacturing process by giving laser joining recommendations for a verified weldability. Key words: Laser beam welding, chromium-manganese steels, ultra-high strength, press-hardening, lightweight, crash performance.
1. Motivation
The automotive engineering was characterized over
the last decades by a continuous increase of the car
body weight [1]. Different passenger advantages like
entertainment systems, passenger comfort, but also
electronic assistance systems to increase the active
passenger safety are responsible for this movement. At
the same time the worldwide regulations [2] demand a
continuous reduction of the CO2 emissions. These
contrary-seemed construction targets for automotive
engineers enforce new lightweight design possibilities.
One way to reach this approach is the application of
new high-strength materials which fulfill the
requirements in a cost-efficient way with a high
processability. Outokumpu Nirosta developed and
established a new material group called Forta H-series
with ultra-high strength properties for different
forming operations like cold-forming or
Corresponding author: Stefan Lindner, M.Sc. (IWE),
research fields: joining technologies, automotive constructions and stainless steel metallurgy.
press-hardening (“hot-formig”). For every grade the
weldability is one of the key processes [3] to reach the
targets in the automotive manufacturing process.
2. Cold-Forming Materials
For the production way of cold-forming parts
Outokumpu Nirosta designed a new austenitic
manganese-chromium alloyed steel group (MnCr
steels). The material family comprises the Forta H500
and the worked-hardened Forta H800 and Forta H1000
whereby the numbers declares the yield strength. The
mechanical-technological values represent a well
aligned relation between a high strength and a very
good elongation which are shown in Fig. 1 and Table 1.
The MnCr steels have a face centred cubic (fcc) lattice
structure depending on the concerted relation of the
chosen alloying elements. The fully austenitic
microstructure will be maintained after forming,
deformation, and welding without forming martensite.
A specific balanced stacking fault energy results in
the described stable austenitic microstructure. At the
same time the conditions to avoid any kind of delayed
D DAVID PUBLISHING
Fig. 1 Value
Table 1 Mec
cracking are
hardening e
steels (or c
converts in
induced plas
hardening e
hardening w
in the micro
microstructu
2,000 MPa i
operation or
The alloy
austenite for
increase of
manganese
from the gen
[5]. The sec
which influe
in a positiv
homogeneity
chromium-o
es of the austen
chanical-techn
e fulfilled [4
effect of me
called CrNi
nto martensit
sticity), the ne
effect (twinn
works by build
ostructure kee
ure. As a resu
in the harden
r a crash situa
ying elemen
rmer and cre
strength as w
has the effec
neration of ph
cond main a
ences the me
ve way [6],
y of the oth
oxid passivati
Laser Bea
nitic cold-form
nological value
4]. Instead of
etastable aus
steels), whe
te (TRIP =
ew MnCr ste
ning-induced
ding up more
eping at all ti
ult it is possib
ned material d
ation.
nt manganes
eates further
well as ductil
ct of prevent
hases sensitiv
lloying elem
echanical-tech
support the
her elements
ion layer on
am Welding o
ming H-grades
es of the austen
f the well-kn
stenitic stain
ere the auste
= transforma
eels use the TW
plasticity).
e and more tw
ime an austen
ble to reach o
during a form
se works as
benefits like
lity. Furtherm
ting the mat
ve to hot-crack
ment is chrom
hnological va
e solubility
and build u
the steel sur
of Alloyed Ult
in relation to o
nitic cold-formi
nown
nless
enite
ation
WIP
The
wins
nitic
over
ming
s an
e an
more
erial
king
mium
alues
and
up a
rface
whi
is d
coa
corr
irre
coa
pas
prot
chip
the
prop
T
abs
one
axia
For
pro
ene
and
D
mic
ra-High Stren
other construct
ing H-grades.
ich is well-kn
delivered brig
ating because
rosion protec
gularities. W
ating on th
sivation laye
tection syste
pping. The p
injured dip
perties.
The MnCr
orption comb
e example Fi
al crash of a
rta H800 in a
file shows a
ergy absorptio
d forming zon
During the i
crostructure a
ngth Steels
tion materials.
nown from sta
ght and does
e of this n
ction layer. T
With applyin
he manufac
er presents
m even after
assivation lay
coating bec
steels featur
bined with a h
g. 2 illustrat
a square prof
a thickness of
a great formi
on combined
nes.
impact situa
and the TWI
.
ainless steels.
not need a f
natural and r
This fact avo
g a typical
tured comp
a very goo
r a lattice cu
yer avoids an
cause of its r
re a very h
high impact r
tes a dynami
file manufac
f 1.5 mm. As
ing behavior
d with crack-
ation, the fu
P hardening
177
The material
further (zinc)
repassivating
oids welding
cathodic dip
ponents, the
od corrosion
ut or a stone
n undercut of
repassivating
high energy
resistance. As
c high-speed
ctured with a
s a result the
r and a high
-free welding
ull austenitic
effect of the
7
l
)
g
g
p
e
n
e
f
g
y
s
d
a
e
h
g
c
178
Fig. 2 High-
Fig. 3 Resul
Forta H-seri
will harden i
high resistan
the impact e
the axial cra
Therefore
For the exam
to a press-h
MPa) that a
under retent
-speed axial cr
lting hardness
ies develop th
in relation to t
nce to the imp
energy. Fig.
ash from Fig.
e a high direc
mple of a b-pi
ardened steel
a thickness r
tion of the in
Laser Bea
rash of Forta H
after the xial-
heir full bene
the impact fo
pact simultan
3 illustrates
2.
t lightweight
llar it can be
l (22MnB5 w
reduction of
ntrusion level
am Welding o
H800.
crash of Forta
efit: The mat
rce and subte
neously absorb
the hardenin
potential res
shown in rela
with Rm ≈ 1
35% is poss
but with a m
of Alloyed Ult
a H800 .
erial
end a
bing
ng of
sults.
ation
,500
sible
more
duc
T
tran
form
grou
up
long
cha
H-s
exp
ra-High Stren
ctile and crack
The MnCr s
nsport applic
m complex p
up. It is poss
to 50% for
gitudinal bea
annels, tanks
series enable
ploiting so f
ngth Steels
k-free crash b
steels enable
ations becau
parts with an
sible to reach
r complex c
ams, wheel
or battery pa
e further con
far not used
behavior.
e further ligh
use of their p
n ultra-high s
a lightweigh
cold forming
houses, sea
acks. Moreov
nstructive lig
geometrical
htweight for
possibility to
strength steel
ht potential of
g parts like
at structures,
ver the Forta
ghtweight by
l degrees of
r
o
l
f
e
,
a
y
f
freedom. O
processes l
combination
decrease th
component s
general.
3. LaserCold-Form
In genera
welding in
Especially
welding spee
Fig. 4 Ducti
Fig. 5 Hard
Other design
like roll for
n with a la
he weight
stiffness as w
r Beam mable MnC
al, austenitic
similar as w
laser beam
ed and locally
ile behavior of
dness curve and
Laser Bea
n ways and
rming or hy
aser beam w
and increas
well as the pa
WeldabilCr Steels
MnCr steels
well as in d
processes w
y concentrate
f a laser beam w
d micrograph o
am Welding o
d manufactu
ydro-forming
welding pro
e the resul
assenger safet
lity of
s are suitable
dissimilar we
with their h
ed heat input o
welded cross-t
of similar laser
of Alloyed Ult
uring
g in
ocess
lting
ty in
the
e for
elds.
high
offer
a gr
stee
F
stee
Mn
tran
and
test
L
beh
met
mea
dete
ensile-sample.
r-beam welded
ra-High Stren
reat potential
els and their p
First investig
els are describ
nCr steels a
nsmission of p
d a ductile
ting, as shown
Laser beam
havior of w
tallography
asurement. A
ected, as show
d H800 in lap j
ngth Steels
l in combina
physical prop
gations of la
bed in Ref. [7
as a base
power under
fracture beh
n in Fig. 4.
welded she
welded aust
inspection
A softening i
wn in Fig. 5. T
oint condition
tion with tho
perties.
aser beam w
7]. Laser beam
material of
every directio
havior during
eets exhibit
tenitic mate
with a
in the weld s
The often vis
.
179
ose austenitic
welded MnCr
m joints with
ffer a high
on of loading
g destructive
the typical
erials under
a hardness
seam can be
ible behavior
9
c
r
h
h
g
e
l
r
s
e
r
180
of ferritic ca
the heat-affe
with brittle e
materials.
Metallurg
effect is tha
process an
microstructu
H500 base
With the be
combination
zones are no
they are a
explanation
three-point b
The profile w
Fig. 6 Comp
arbon steels w
fected and w
effects, canno
gical backgro
at the weldi
d reverses
ure. As a res
material is r
enefit of a ful
n with a TWIP
ot a weak po
further cras
can be rep
bending test w
was worked o
parison of hard
Laser Bea
with an increa
elded zones,
ot be detected
ound of the
ing works li
locally the
sult the hardn
reached in th
ll austenitic m
P hardening e
oint in a con
sh and safety
presented wi
where a cup p
out as a tailor
dness for a wel
am Welding o
ase of hardnes
often comb
d for the austen
visible soften
ke an annea
e twins in
ness of the F
he welded zo
microstructur
effect, the we
nstruction, in
y potential.
th a high-sp
profile was u
red welded b
lded and then
of Alloyed Ult
ss in
ined
nitic
ning
aling
the
Forta
ones.
re in
lded
fact
The
peed
used.
lank
of a
to a
Fig
com
pro
A
twin
hard
low
The
con
the
cold
wel
safe
com
impact-loaded
ra-High Stren
a Forta H800
a zinc-coated
. 6 shows
mparison of
file.
A hardening
ns restart bu
dening speed
wer number o
erefore the jo
nstruction. Du
hardness of
d-formable
lded as well
ety requirem
mponents of a
d sample.
ngth Steels
with an H10
micro-alloye
a hardness
a non-tested
can be detec
uilding in the
d is much hig
of twins, here
oint areas are
uring a critica
the base mat
austenitic F
as in base m
ents for intru
a car body.
000, and then
ed steel as a l
measuremen
d and an imp
cted after the
e microstruc
gher for the
e the laser w
e not the wea
al impact the
terial again. T
orta H-serie
material condi
usion and cr
n spot-welded
locking plate.
nt with the
pact stressed
impact. The
ture and the
areas with a
welded seam.
ak point in a
e joints reach
Therefore the
es fulfill in
itions highest
rash relevant
d
.
e
d
e
e
a
.
a
h
e
n
t
t
The Forta
material eva
as well as
weldability
Aachen Uni
supports th
documentati
Different gu
dissimilar j
support the
about me
recommenda
4. Hot-For
The Forta
manufacturi
reached a
“press-harde
1,200 MPa
press-harden
ferritic-mart
changed af
martensitic-b
material is
carbon and 1
material adv
tensile stren
extra-ordina
hot-forming
Fig. 7 Mech
a H-series rea
aluation proce
s the confir
according to
versity, ISF W
e complete
ion guideline
uidelines and
joining com
processing i
etallurgical
ations are giv
rming Mat
a H1200PH
ng way and
fter hot-for
ening”, a yie
a and “PH”
ning. This
tensitic micro
fter the ho
based matrix
based on a
13 wt.% chro
vantage is the
ngth (Rm ≈ 1
ary elongati
, as shown in
hanical-technol
Laser Bea
ached success
ess according
rmation of
SEP1220-3
Welding and
process of
for the laser b
d test results
mbinations a
industry. Fur
aspects
ven in Refs. [
erials
represents a
material desi
rming, or
eld strength
” specified
stainless
ostructure in
ot-forming p
x with auste
steel contai
mium 1.4034
e combinatio
,850 MPa) c
ion (A80
n Fig. 7.
logical values o
am Welding o
sfully the gen
g to SEP1245
the laser-b
[9]. The RW
Joining Inst
the testing
beam weldab
s for similar
are available
rther informa
and wel
10, 11].
a complete o
ign. The mate
often ca
level of Rp0
the ability
steel has
initial state
process into
enite parts.
ining 0.45 w
4 (X46Cr13).
on of a very h
ombined with
≈ 13%) a
of different ho
of Alloyed Ult
neral
5 [8]
beam
WTH
itute
and
bility.
and
e to
ation
ding
other
erial
alled
0,2 ≈
for
a
and
o a
The
wt.%
The
high
h an
after
O
com
man
form
sub
400
of
adv
nee
alum
fast
add
stai
in
prop
tem
com
form
stai
she
wha
thic
pres
T
bas
adv
T
exc
ben
t-formed steels
ra-High Stren
Other hot-f
mparison to
nganese-boro
ming starts
sequent quen
0 °C for five
hot-forming
vantages: The
ed a furth
minum-siliciu
t heating proc
ditional diffus
inless steel fu
very homo
perties. Furth
mperature (M
mplex parts
ming operat
inless steels
ets (down to
at permit fu
ck plates up t
ssure tanks.
The building
ed microstru
vantage of the
This effect is r
cellent prope
nding tests wh
s [17].
ngth Steels
forming pr
o establishe
on steel 22M
with auste
nching in the
minutes. Nev
parameters
e chromium a
her scale-re
um (+AS) and
cesses with in
sion time is
unctions as an
ogeneous an
hermore the
Ms) supports
because mo
tion is ava
are industria
0.5 mm) over
urther lightwe
to 6.0 mm ar
of austenite
ucture after
e Forta H1200
responsible fo
erties during
here high ben
rocess para
ed materials
MnB5 are ne
nitization at
mould, and
vertheless thi
results in d
alloyed mater
esistance c
d can be there
duction or co
necessary. M
n air hardener
nd repeatab
low marten
the hot-form
re process t
ailable. The
al available
r a large widt
eight potenti
re possible fo
islands in th
hot-forming
0PH, as show
for the high el
g subsequent
nding angles
181
ameters in
s like the
ecessary: hot
t 1150 °C,
tempering at
is adjustment
different new
rial does not
oating like
efore used for
onduction. No
Moreover the
r what results
ble material
nsitic starting
ming of very
time for the
martensitic
also as thin
th (1,250 mm
ial. But also
or hot-formed
e martensitic
g is a key
wn in Fig. 8.
longation and
t three-point
over 90° can
n
e
t
,
t
t
w
t
e
r
o
e
s
l
g
y
e
c
n
m)
o
d
c
y
d
t
n
182
Fig. 8 Retai
be reached.
by the descr
Former li
martensitic s
as limited
some speci
although the
laser beam w
process poss
weld heat tre
mechanical p
5. LaserHot-Form
Two diffe
suitable to a
component
engineering:
butt joint in
tailored we
afterwards. F
is hot-forme
then welded
of the car bo
weldability
hot-formed s
ined and rever
Further a goo
ribed microstr
iterature desc
stainless steel
[12-14]. The
ial assistanc
e material w
welding whic
sible to be ap
eatment was
properties of
r Beam mable Mater
ferent ways o
apply the For
for transpor
: The first one
initial, cold-r
elded blank
For the secon
ed in a first ste
d as a lap or a
ody. Therefor
of the base m
state.
11
Laser Bea
sion austenite
od crash perf
ructure effect
cribed genera
ls with up to 0
erefore the m
ce in point
was proven to
ch is the only
pplied to this
proven to be
f the welds.
Weldabilrials
of manufactu
rta H1200PH
rt application
e uses laser b
rolled materia
which can
nd possibility
ep to create th
butt joint to o
re it is import
material in ini
50 °C/5 min/WQ
am Welding o
after hot-form
formance is g
ts.
al weldability
0.46 wt.% car
material requ
of weldab
o be suitable
y fusion wel
class of steel
e beneficial to
lity of
uring seem to
as a hot-for
ns and car b
eam welding
al state to crea
be hot-for
the base mat
he component
other compon
tant to ensure
itial as well a
1150 °C/5
of Alloyed Ult
ming and annea
given
y of
rbon
uires
bility
e for
ding
ls. A
o the
the
o be
rmed
body
as a
ate a
rmed
erial
t and
nents
e the
as in
S
mar
crac
hea
mar
star
inte
foll
diss
Fur
mat
tem
T
AiF
“Inv
chro
usin
(IG
of h
as w
wel
T
seam
trea
hard
lase
min/WQ + 400 °C/1
ra-High Stren
aling of FortaH
Short thermal
rtensitic stain
cks and high
at-control is
rtensitic micr
rting tempe
ercooling to
lowing temp
solution of th
rther referenc
terials and the
mperature can
The following
F-funded r
vestigations
omium steels
ng laser beam
GF project num
heat input and
well as disc l
lding paramet
The results of
m and the he
atments of the
dness of in
er welded is s
1150 °C1 min/WQ
ngth Steels
H1200PH.
cycles should
nless steels in
hardness gra
desirable. T
rostructure, a
erature (Ms
150 °C-200
ering below
e alloy carbid
ces to the w
e calculation
be given wit
g investigatio
research pr
on fusion w
s with a mart
m and gas-met
mber 17.433N
d seam geom
laser were us
ters are prese
f hardness mea
eat affected zo
e H1200PH ar
nitial cold-ro
shown with b
C/5 min/WQ + 400 °C
d be preferred
general. But
adients, a mat
To relax th
pre-heating t
) combined
°C after we
the temper
des is appropr
welding behav
of the marten
th Refs. [17-1
ns were work
roject FOS
welding of h
tensitic micro
tal-arc-weldin
N). To test di
metry, carbon
sed as beam
ented in Table
asurement ac
ones for diffe
re displayed i
olled sheets
lack graphs (
C/30 min/WQ
d for welding
to avoid cold
erial-suitable
he tetragonal
to martensitic
d with an
elding and a
rature of the
riate [15, 16].
vior of those
nsitic starting
19].
ked out in the
STA P905
high-strength
ostructure by
ng processes”
ifferent ways
dioxide laser
sources. The
e 2.
ross the weld
erent thermal
in Fig. 9. The
which are
(rhombs) and
g
d
e
l
c
n
a
e
.
e
g
e
5
h
y
”
s
r
e
d
l
e
e
Table 2 App
Fig. 9 Hard
clarifies an
gradient is
literature an
blue graphs
plied welding p
dness curves of
increase to
well-known
nd results in a
(squares) illu
Laser Bea
parameters for
f Forta H1200P
600 HV0.3
n in the m
a cold crack b
ustrate the har
am Welding o
r laser beam w
PH for differen
3. This hard
mentioned be
behavior. But
rdness of weld
of Alloyed Ult
welding of H120
nt thermal trea
dness
efore
t the
ds in
pres
the
The
pre-
ra-High Stren
00PH.
atment conditio
ss-hardened c
heat-affected
e dashed line
-heating, soli
ngth Steels
ons.
condition. Wi
d zone by 80
s represent to
id lines to p
ithout pre-hea
to 110 points
o welds produ
pre-heated we
183
ating a dip in
s is observed.
uced without
elds (TMs =
3
n
.
t
=
184
300 °C).
press-harden
positive infl
zones to th
hot-formed c
The fatigu
test the lifet
was analyz
amplitudes.
pulsator, wh
Fig. 10 Cho
Especially
ned conditio
uence and gr
e level of th
condition.
ue behavior is
time conditio
zed with a
The test were
hich enables a
sen sample geo
Laser Bea
for the
n, the pre-h
rades the hard
he base mate
s an importan
ons of welde
cyclic test
e carried out u
a test frequen
ometry (above
am Welding o
laser-welds
heating show
dness in the w
erial hardnes
nt investigatio
ed structures
under cons
using a resona
ncy of about
e) and resulting
of Alloyed Ult
in
ws a
weld
ss in
on to
and
stant
ance
100
Hz
rati
cho
test
sam
1·10
SEP
join
righ
hea
g fatigue behav
ra-High Stren
depending o
o RF = 0. A
osen as failure
t frequency dr
mples without
07 cycles. T
P1240 [20] h
nt vertical to
ht side. Fig. 1
at-control.
vior (below).
ngth Steels
n the specim
A crack leng
e criterion an
rop off by 1 H
t rupture, the
The specimen
as been chos
the cyclic l
10 also repre
men’s stiffnes
gth of about 0
nd could be d
Hz. In the ca
e test was sto
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Laser Beam Welding of Alloyed Ultra-High Strength Steels
185
As a result of this project extensive guidelines about
how to weld and how to treat the ultra-high strength
grade were created. The presentations at the
project-closing colloquium [21-23] enlarge significant
the knowledge of the former literature and add new
manufacturing possibilities for the processing industry.
6. Conclusions
Outokumpu’s new Forta H-series combine
ultra-high strength alloyed materials with different
microstructures and hardening effects for different
manufacturing processes like cold- or hot-forming. The
weldability depends on the microstructure and alloying
elements of the respective chosen material. But
independent, the process of laser beam welding with its
specific advantages occupies a key role by ensuring the
weldability of the materials or different material
conditions. Therefore laser welding enables the
production of lightweight and crash-safety components
with new ultra-high strength materials for future car
bodies and transport applications.
Acknowledgments
Special thanks are given to all research institutes,
especially to professor Dr. Uwe Reisgen (RWTH
Aachen University ISF Welding and Joining Institute),
Dr. Rainer Wagener (Fraunhofer LBF Darmstadt) and
Vitalij Janzen (LWF Paderborn).
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Laser Beam Welding of Alloyed Ultra-High Strength Steels
186
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