A. GRAJCAR et al.: EFFECT OF GAS ATMOSPHERE ON THE NON-METALLIC INCLUSIONS ...
945–950
EFFECT OF GAS ATMOSPHERE ON THE NON-METALLIC
INCLUSIONS IN LASER-WELDED TRIP STEEL WITH Al AND Si
ADDITIONS
VPLIV PLINSKE ATMOSFERE NA NEKOVINSKE VKLJU^KE V
LASERSKO VARJENEM TRIP JEKLU Z DODATKOM Al IN Si
Adam Grajcar1, Maciej Ró¿añski2, Ma³gorzata Kamiñska3,
Barbara Grzegorczyk1
1Silesian University of Technology, Institute of Engineering Materials and Biomaterials, Konarskiego Street 18a, 44100 Gliwice, Poland
2Institute of Welding, Bl. Czes³awa Street 16-18, 44100 Gliwice, Poland
3Institute of Non Ferrous Metals, Sowinskiego Street 5, 44100 Gliwice, Poland
adam.grajcar@polsl.pl
Prejem rokopisa – received: 2015-08-12; sprejem za objavo – accepted for publication: 2015-11-04
doi:10.17222/mit.2015.253
The present study aims to characterize the weldability of a multiphase, automotive steel containing Al and Si additions from the
point of view of its tendency to form non-metallic inclusions. Laser welding tests of 2-mm-thick sheets were performed using
the keyhole-welding mode and a solid-state laser. The tests were carried out in air and with the use of an argon atmosphere. The
distribution, type and chemical composition of the non-metallic inclusions formed in the base metal and fusion zones were
analysed. The effect of applying the protective gas on the type and amount of non-metallic inclusions was determined using
light and scanning electron microscopy. The chemical composition of the identified particles was assessed using the EDS
method. It was found that a protective gas has a beneficial effect on reducing the non-metallic inclusions, but only to a limited
extent. The boundary between the complex oxides and the pure aluminium oxides was determined to be 2–3 μm.
Keywords: TRIP steel, laser welding, non-metallic inclusions, protective gas, Ar atmosphere, oxidation
Namen te {tudije je opredeliti varivost s stali{~a tvorbe nekovinskih vklju~kov v ve~faznem jeklu, ki vsebuje Al in Si, za avto-
mobilsko industrijo. Izvedeni so bili varilni preizkusi z laserjem na 2 mm debelih plo~evinah z V zarezo. Preizkusi so bili
izvedeni na zraku in v za{~itni atmosferi argona. Analizirana je bila razporeditev, vrsta in kemijska sestava nekovinskih
vklju~kov v osnovnem materialu in v coni zlivanja. Vpliv uporabe za{~itnega plina na vrsto in koli~ino nekovinskih vklju~kov je
bil dolo~en s pomo~jo svetlobne in vrsti~ne elektronske mikroskopije. Kemijska sestava najdenih delcev je bila ugotovljena
z metodo EDS. Ugotovljeno je bilo, da ima za{~itni plin ugoden vpliv na zmanj{anje nekovinskih vklju~kov le v omejenem
obsegu. Meja med kompleksnimi oksidi in ~istimi oksidi aluminija je bila dolo~ena kot 2-3 μm.
Klju~ne besede: TRIP jeklo, lasersko varjenje, nekovinski vklju~ki, za{~itni plin, atmosfera Ar, oksidacija
1 INTRODUCTION
Multiphase steels with a transformation-induced
plasticity (TRIP) effect belong to the advanced high-
strength steels (AHSS) used in the modern automotive
industry. The beneficial balance between strength and
ductility requires increased contents of Mn, Al and Si.
Their total content in TRIP steels can reach 4 %.1–3
Medium-Mn and high-Mn alloys require an even higher
concentration of alloying elements.4–8 Whereas a lot of
efforts focus on the relationships between hot-working,
heat-treatment, microstructure and mechanical pro-
perties, the weldability of AHSS has not received much
attention so far.
M. S. Weglowski et al.9 reported that the maximum
hardness in the fusion zone of 0.07C-1Mn-0.4Cr
dual-phase steel (which belongs to the 1st generation of
AHSS) reaches 340 HV. The hardness increases to
approximately 430 HV with increasing carbon and
manganese contents in the 0.13C-1.3Mn-0.2Cr-0.2Cu
steel.10 Another problem in high-strength, multiphase
steels is the heat-affected zone (HAZ) softening because
of the tempering of the pre-existing martensite. Since the
AHSS contain a high alloying content their weldability
should also be affected by the presence of non-metallic
inclusions. Different sulphide and oxide particles have
been identified by M. Amirthalingam et al.11 and A. Graj-
car et al.12 in different types of TRIP steels containing
Mn, Si and Al additions. It is well known that brittle
oxides and ductile manganese sulphides affect the
fatigue endurance limit, the fatigue-crack propagation
rate, the fracture toughness, the anisotropy of the tensile
properties with respect to the rolling direction and the
weld quality, too.13,14
Recently, A. Grajcar et al.12 analysed various types of
non-metallic inclusions in laser-welded Fe-1.5Mn-
0.9Si-0.4Al TRIP steel. Numerous oxide-type particles
have been revealed in the fusion zone formed under air
conditions. Therefore, the aim of the present work is to
investigate the effect of a protective gas on the quantity,
type and chemical composition of non-metallic inclu-
sions in Si- and Al-alloyed TRIP steel.
Materiali in tehnologije / Materials and technology 50 (2016) 6, 945–950 945
UDK 621.791.754'293:544.022.344.3:621.791.725 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(6)945(2016)
2 EXPERIMENTAL PART
The work addresses the laser welding of thermo-me-
chanically processed, 2-mm-thick, TRIP steel sheets
with the following chemical composition: 0.24 % C,
1.55 % Mn, 0.87 % Si, 0.4 % Al, 0.03 % Nb, 0.023 % Ti,
0.004 % S, 0.01 % P and 0.0028 % N. Rare-earth ele-
ments (REE) such as mischmetal (~50 % Ce, ~20 % La,
~20 % Nd) were added to modify the chemical compo-
sition and the shape of the non-metallic inclusions. The
25-kg, vacuum-melted ingot was cast using a protective
atmosphere of argon. The Si+Al addition is required in
TRIP steels to prevent carbide precipitation because C is
needed to enrich the retained austenite.1,2,11 The Nb+Ti
micro-additions are added to increase the strength level
as a result of the precipitation strengthening and grain
refinement. These phenomena are especially useful for
thermo-mechanically processed steels.15,16
The initial hot-working included the hot forging and
rough rolling of the ingot to a thickness of 5 mm. The
fundamental thermo-mechanical rolling consisted of
3 passes with a finishing rolling temperature of 850 °C.
The major step of the controlled cooling following the
hot rolling was isothermal holding of the sheets at a tem-
perature of 350 °C within 600 s. The final thickness of
the sheets was 2 mm.
Laser welding generally includes two major modes:
keyhole welding and conductive welding.17 The conduc-
tive welding utilizes the natural thermal conduction into
the material. The welds obtained in this welding mode
have good quality and do not contain pores and spalls.
However, the keyhole welding is more efficient. Because
of this the present tests were carried out using keyhole
welding. In this treatment mode the power density is
much higher compared to the conductive welding and the
fusion depth is much higher compared to the diameter of
the liquid pool. The welding tests were performed using
a solid-state laser integrated with a robotized laser-treat-
ment system. The welding station is equipped with the
TruDisk 12002 solid-state laser type Yb:YAG characte-
rized by a maximum power of 12 kW. The heat input
value of 0.048 kJ/mm was applied. The welding tests
were performed using an Ar atmosphere. To assess the
effect of the protective atmosphere, tests under air con-
ditions were performed too.
The distribution, type and chemical composition of
the non-metallic inclusions formed in the base metal
(BM) and fusion zone (FZ) of both types of samples
(using Ar gas and without any protective gas) were com-
pared. Samples for light microscopy (LM) and scanning
electron microscopy (SEM) were prepared. The chemical
composition of the non-metallic inclusions was assessed
using EDS point analyses. The distribution of the parti-
cular alloying elements was revealed using mapping. The
quantitative measurements of the chemical composition
for the identified inclusions were carried out using a
JCXA 8230 X-ray micro-analyser with an accelerating
voltage of 15 kV. The microstructures of the BM, the
heat-affected zone (HAZ) and the FZ were revealed
using the SUPRA 25 SEM at an accelerating voltage of
20 kV after nital etching.
3 RESULTS AND DISCUSSION
3.1 Distribution of non-metallic inclusions
The use of different gas atmospheres influences the
quantity of non-metallic inclusions. The distributions of
non-metallic inclusions formed in the fusion zones under
the conditions of an air atmosphere and Ar protective gas
are compared in Figure 1. The intense oxidation takes
place when the laser welding was conducted without any
protective gas. As a result numerous particles of different
sizes can be observed in the fusion zone. The clear boun-
dary near the fusion line is easily visible in Figure 2 for
both the air- and Ar-treated samples. The amount of
particles in the heat-affected zone is a few times lower
when compared to the fusion zones.
The size of the non-metallic inclusions is similar
when the protective gas was applied. The quantity of
particles is lower compared to the laser treatment with-
out any protective atmosphere. However, the amount of
non-metallic inclusions for Ar-protected samples is
higher than would be expected. This means that the Ar
atmosphere has a limited efficiency in the reduction of
harmful, non-metallic inclusions formed in the fusion
zone. The explanation is the essence of the keyhole-
welding technique. The high-density power in the region
of the laser beam’s exposure creates a gas-dynamic
A. GRAJCAR et al.: EFFECT OF GAS ATMOSPHERE ON THE NON-METALLIC INCLUSIONS ...
946 Materiali in tehnologije / Materials and technology 50 (2016) 6, 945–950
Figure 1: Distribution of non-metallic inclusions in the fusion zones
formed under conditions of air atmosphere and Ar protective gas
Slika 1: Razporeditev nekovinskih vklju~kov v podro~ju zlivanja,
nastalih na zraku in v za{~itni atmosferi Ar
channel, i.e., a deep and narrow capillary filled with
gases and metal steams.17,18 These turbulent gases
partially break the protective atmosphere of the argon.
That is why the amount of non-metallic inclusions for
laser welding tests with the use of Ar gas is higher than
expected.
3.2 Microstructure of the steel
The microstructure of the base metal consists of
polygonal ferrite (F) grains elongated along the hot-roll-
ing direction, bainite and retained austenite (Figure 3).
Retained austenite (RA) is the most favourable structural
constituent of TRIP steels due to its beneficial effect on
increasing the steel’s plasticity. This phase is usually
found as small, blocky, granules along ferrite grain
boundaries or between bainitic ferrite laths. Hence, a
large fraction of the microstructure constitutes bainite-
austenite (BA) constituents.
The rapid cooling rate typical for laser welding
influences the microstructure of the fusion zone. Typical
SEM microstructures of the fusion zone are shown in
Figure 4. Both microstructures are characterized by the
presence of martensite laths. Due to the chemical con-
trast typical for observations using back-scattered elec-
trons (BSE) white interlath retained austenite (RA) can
also be observed. The amount of non-metallic inclusions
is slightly smaller for the samples welded using the
argon gas. Moreover, a lot of sub-micron-sized particles
are formed in this steel.
3.3 Non-metallic inclusions
The amount of non-metallic inclusions in the base
metal is very small (Figures 1 and 2). It is related to the
high metallurgical cleanliness of the laboratory-melted
steel and the use of rare-earth elements. A result of the
mischmetal addition is a partial or total substitution of
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Materiali in tehnologije / Materials and technology 50 (2016) 6, 945–950 947
Figure 3: SEM microstructure of the base metal consisting of ferrite
(F), bainite-austenite constituents (BA) and retained austenite (RA)
Slika 3: SEM-posnetek mikrostrukture osnovnega materiala, ki jo
sestavljajo ferit (F), bainit-avstenit (BA) in zaostali avstenit (RA)
Figure 2: Distribution of non-metallic inclusions near the fusion line
in the fusion zones and heat-affected zones formed under conditions
of air atmosphere and Ar protective gas
Slika 2: Razporeditev nekovinskih vklju~kov blizu linije podro~ja
zlivanja in toplotno vplivane cone, nastalih na zraku in v za{~itni
atmosferi Ar
Figure 4: Martensite laths and globular non-metallic inclusions
formed in the fusion zones under conditions of: a) air atmosphere and
b) Ar protective gas
Slika 4: Martenzitne late in globularni nekovinski vklju~ki, nastali v
coni zlivanja: a) na zraku in b) v za{~itni atmosferi Ar
Mn in sulphide inclusions and Al in oxide inclusions by
the REE. These elements have a higher chemical affinity
for sulphur and oxygen compared to Mn and Al. The
example of such a particle is shown in Figure 5. In this
case Ce and La totally replaced Mn and Al, forming a
globular oxysulphide. Such particles are hardly deformed
during hot rolling and reduce the anisotropy of the me-
chanical properties of flat products.4,15 The mapping of
the globular inclusions indicates that Ce and La as well
as S and O are distributed uniformly within the particle
(Figure 6).
Figure 7 presents typical non-metallic inclusions
formed in the fusion zone of the sample laser-welded in
A. GRAJCAR et al.: EFFECT OF GAS ATMOSPHERE ON THE NON-METALLIC INCLUSIONS ...
948 Materiali in tehnologije / Materials and technology 50 (2016) 6, 945–950
Figure 7: Complex oxides of various size formed in the fusion zone of
the sample welded in the air atmosphere: a), b) spectrum of the
particle from point 1, c) spectrum of the particle from point 2 and d)
the chemical composition of the large particle (point 1) determined
using EDS
Slika 7: Sestavljeni oksidi razli~nih velikosti: a) nastali v coni zlivanja
vzorca zvarjenega na zraku, b) spekter delca v to~ki 1, c) spekter delca
v to~ki 2 in d) kemijska sestava velikega delca (to~ka 1), dolo~ena s
pomo~jo EDS
Figure 5: Complex oxysulphide containing La and Ce: a) formed in the base metal, b) spectrum of the inclusion and c) its chemical composition
determined using EDS
Slika 5: Sestavljeni oksisulfid, ki vsebuje La in Ce: a) nastal v osnovnem material, b) spekter vklju~ka in c) njegova kemijska sestava, dolo~ena s
pomo~jo EDS
Figure 6: Elemental mapping of the particle from Figure 5
Slika 6: Razporeditev elementov v delcu prikazanem na Sliki 5
the air atmosphere. There are a few large inclusions with
a size ranging from approximately 3 μm to 7 μm and
numerous small particles smaller than 1 μm. All the
inclusions have a globular shape. EDS analyses of the
large particles indicate that these are complex oxides
containing Al, Mn and Si (Figures 7b to 7d). It indicates
that there is a strong oxidation of the alloying elements
in the air atmosphere. Elemental mapping indicates that
there is no sulphur in the inclusions. Moreover, it should
be noted that Mn and Si are located only at the largest
inclusions, whereas the smaller ones are pure aluminium
oxides (Figure 8).
The use of a protective atmosphere of argon causes a
decrease in the amount of non-metallic inclusions. It is
especially true for large particles, the quantity of which
is much smaller (Figure 9a). As previously, the spectral
lines in Figures 9b and 9d do not contain sulphur peaks.
The intense evaporation during the keyhole welding re-
sults in the partial oxidation of the same alloying ele-
ments, like for the samples welded in the air atmosphere.
Hence, many oxides can be identified in Figure 9. The
analysis of the chemical composition indicates that the
concentrations of Mn and Si decrease with a decrease in
the particle diameter (Figures 9c and 9e). The presence
of Fe in the spectrum of the smaller particle (point 2 in
Figure 9) is caused by a similar diameter of the inclu-
sion and a beam diameter. The occurrence of Mn and Si
only in the largest inclusions is confirmed by Figure 10,
where these elements are located in the largest particle of
a diameter of approximately 4 μm. Some content of
titanium was also revealed.
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Materiali in tehnologije / Materials and technology 50 (2016) 6, 945–950 949
Figure 8: Elemental mapping of the particles from Figure 7
Slika 8: Razporeditev elementov v delcih, prikazanih na Sliki 7
Figure 10: Elemental mapping of the particles from Figure 9
Slika 10: Razporeditev elementov v delcih, prikazanih na Sliki 9
Figure 9: Complex oxides of various sizes formed in the fusion zone
of the sample: a) welded in the Ar atmosphere, b), d) spectra of the
particles from points 1 and 2, c) the chemical composition of the
particles from point 1 and e) point 2 determined using EDS
Slika 9: Sestavljeni oksidi razli~nih velikosti nastalih v coni zlivanja:
a) vzorca zvarjenega v atmosferi Ar, b), d) spekter delcev iz to~ke 1 in
2, c) kemijska sestava delcev iz to~ke 1 in e) to~ke 2, dolo~enih s po-
mo~jo EDS
Ti, Al, Si and Mn are also located in the largest inclu-
sion in Figure 11. However, the amount of such large
inclusions is reduced compared to the samples welded in
the air atmosphere. The region around such large inclu-
sions is usually free of any other particles. The limit bet-
ween Mn- and Si-containing large inclusions and small
pure aluminium oxides can be assessed as 2–3 μm.
4 CONCLUSIONS
The effect of the use of an Ar atmosphere during
laser welding in the keyhole-welding mode was assessed
for the Al-Si-bearing TRIP steel. It was found that the
intense evaporation of gases and metal steams partially
destroys the protective atmosphere of the argon. As a
result, some oxidation of the weld pool takes place. The
amount of non-metallic inclusions in the fusion zone of
the samples welded using Ar is smaller compared to the
samples treated without a protective gas. It concerns
especially the large inclusions with a diameter between
3 μm and 7 μm. The Ar atmosphere has no effect on the
chemical composition of the formed particles. In both
cases globular oxides of various sizes are formed in the
fusion zone. The particles with a diameter ranging from
3 7 μm to 7 μm constitute the complex inclusions con-
taining Al, Si and Mn (sometimes also Ti). The nume-
rous particles smaller than 2–3 μm are pure aluminium
oxides.
Acknowledgment
This work was financially supported with statutory
funds of Faculty of Mechanical Engineering of Silesian
University of Technology in 2015.
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950 Materiali in tehnologije / Materials and technology 50 (2016) 6, 945–950
Figure 11: Elemental mapping of the complex oxides formed in the
fusion zone of the sample welded in the Ar atmosphere
Slika11: Razporeditev elementov v sestavljenih oksidih, nastalih v
coni zlivanja, vzorca zvarjenega v atmosferi Ar