Materials and Design 57 (2014) 128–134

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Materials and Design

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Experimental investigation on dissimilar pulsed Nd:YAG laser weldingof AISI 420 stainless steel to kovar alloy

0261-3069/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.matdes.2013.12.050

⇑ Corresponding author. Tel.: +98 21 82084096; fax: +98 2188006076.E-mail addresses: hamze.baghjari@modares.ac.ir, hamze_baghjari@yahoo.com

(S.H. Baghjari), akbarimusavi@ut.ac.ir (S.A.A. AkbariMousavi).

S.H. Baghjari, S.A.A. AkbariMousavi ⇑School of Metallurgy and Materials, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran

a r t i c l e i n f o

Article history:Received 6 April 2013Accepted 20 December 2013Available online 30 December 2013

Keywords:Electron microscopyHardness measurementX-ray diffractionAusteniteWeldingPrecipitation

a b s t r a c t

This paper presents the results of an investigation on autogeneous laser welding of AISI 420 stainlesssteel to kovar alloy using a 100 W pulsed Nd:YAG laser. The joints had a circular geometry and buttwelded. The joints were examined by optical microscope for cracks, pores and for determining the weldgeometry. The microstructure of the weld and the heat affected zones were investigatedby scanning elec-tron microscope. The austenitic microstructure was achieved in the weld. The morphology of weld zonesolidification was basically cellural, being influenced by the temperature gradient. It was found that thestart of solidification in the kovar side of weld zone occurred by means of epitaxial growth. When thetemperature gradient was high, the columnar grains were created in the fusion boundary of 420 stainlesssteel side toward weld zone. Measurements taken by X-ray spectrometry for dispersion of the energy inthe weld zone indicated a significantly heterogeneous distribution of chromium element. The variationsin chemical compositions and grains morphologies significantly alter the Vickers microhardness values inthe weld zone.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Joining of dissimilar materials is one of the challenging tasksfacing modern manufacturers. Dissimilar joining technologies findapplications in many sectors including microelectronics, medical,optoelectronics and microsystems [1]. Dissimilar-metal joints areused widely in various industrial applications due to both technicaland economic reasons. The adoption of dissimilar-metal combina-tions provides possibilities for flexible design of the product byusing each material efficiently, i.e., benefiting from the specificproperties of each material in a functional way [2].

Martensitic stainless steels (MMSs) are commonly used formanufacturing components with excellent mechanical propertiesand moderate corrosion resistance, so that they can work underhigh and low temperatures. Unlike other stainless steels, theirproperties could be changed by heat treatment; hence, these steelsusually are used for a wide range of applications such as steamgenerators, pressure vessels, mixer blades, cutting tools and off-shore platforms for oil extraction [3]. Kovar is an iron–nickel–co-balt alloy with a coefficient of thermal expansion similar to thatof hard (borosilicate) glass. This makes it especially suitable forusers that require a matched-expansion seal between metal andglass parts. Thus, kovar finds wide usage in the electronics industry

for metal parts bonded to hard glass envelopes. In addition, kovaras soft magnetic alloys are used extensively in applications such aselectromagnetic and radio frequency shields, amplifiers, torquemotor and so on because of their high permeability, high satura-tion magnetostriction and low hysteresis-energy loss [4,5].

The weldability of dissimilar metals is determined by theiratomic diameter, crystal structure and compositional solubility inthe liquid and solid states. Diffusion in the weld pool often resultsin the formation of intermetallic phases, the majority of which arehard and brittle and are thus detrimental to the mechanicalstrength and ductility of the joint. In laser welding, the risk of seg-regation and excessive formation of brittle phases is significantlyreduced due to the high cooling and solidification rates [6]. How-ever, in laser beam welding of dissimilar metals, the differencesin physical properties of the materials lead to an asymmetry inthe pertinent heat and fluid flow characteristics, as well as percep-tible complexities in the solute segregation patterns. These, in turn,leave their imprints on the final welded microstructure [7].

There are no results reported in the literature on the welding ofAISI 420 stainless steel to kovar alloy. Therefore, the purpose of thisstudy was to investigate the feasibility of laser welding for joiningtwo materials with different properties. However, some investiga-tions were carried out for dissimilar laser welding. Akbari Mousavi[8] studied metallurgical investigations of pulsed Nd:YAG laserwelding of AISI 321 and AISI 630 stainless steels. They found veryfine cellular and dendritic austenite structures achieved in theweld zone. The microhardness tests showed that the maximum

S.H. Baghjari, S.A.A. AkbariMousavi / Materials and Design 57 (2014) 128–134 129

hardness was produced for the 630 stainless steel side and theminimum hardness was occurred for the 321 stainless steel side.Mai [6] investigated pulsed Nd:YAG laser welding of tools steeland kovar. Their results showed substantial intermixing of bothmaterials within the fusion zone. No hot cracking of the jointwas observed, possibly due to the positive effect of Mn and Si dis-solution into the weld. In addition, the hardness of the tool steelside was slightly higher than that of the kovar side in the weldpool.

The objective of this work is to study the microstructure andmechanical properties of the weld zone and the HAZ in pulsedNd:YAG laser welding of dissimilar AISI 420 martensitic stainlesssteel and kovar.

2. Experimental procedures

The cylindrical samples with the diameter of 20 mm and thick-ness of 2 mm were made from the AISI 420 stainless steel and ko-var rods were used with 25 mm diameter in annealed condition.Typical shapes of the kovar and AISI 420 stainless steel samplesmanufactured by turning are shown in Fig. 1. Chemical composi-tions of AISI 420 stainless steel and kovar alloy are tabulated in Ta-ble 1. The Nd:YAG-pulsed laser welding tests were carried out witha100 w laser machine. There was no surface preparation for sam-ples before the tests and the samples were butt welded (Fig. 1).The list of Nd:YAG pulsed laser welding parameters considered istabulated in Table 2. For all tests, the argon gas with the 99.99%purity and gas flow rate of 10 L/min was used. After each weldingthe samples were cut in cross section for metallurgical investiga-tions. The etching process was performed by immersion of thesamples in kalling 2 (5 g CuCl2, 100 mL ethanol, 100 mL HCl).Metallographic studies were conducted by utilization of opticalmicroscopy, scanning electron microscopy (SEM) and X-ray diffrac-tion (XRD) technique. SEM was equipped with an energy dispersiveX-ray spectroscopy (EDS) apparatus. Micro-Vickers hardness mea-surements were also conducted according to ASTM:E384 11e1 onthe etched cross section of the welds with a load of 0.1 kgf holdingfor 15 s.

3. Results and discussions

3.1. Base metals microstructure

The SEM micrograph of AISI 420 stainless steel is shown inFig. 2a. The annealed microstructure mainly consists of inter- andintragranular precipitation of M23C6 carbide in ferrite matrix. X-rayDiffraction (XRD) pattern of AISI 420 verify presence of thesephases (Fig. 3). The intergranular precipitation at the grain

Fig. 1. Typical shapes of the kovar and AISI 420 samples before welding.

boundary is coarser than intragranular precipitation of the matrix.The kovar alloy microstructure after annealing has austenitic equi-axed grains microstructure and some twins (Fig. 2b). The averagegrain size of the kovar alloy is 21.1 lm. XRD pattern of kovar alloyis shown in Fig. 4. In addition to the austenite phase, some ferritephase is existed in kovar alloy.

3.2. Weld metal microstructure

Energy dispersive X-ray spectroscopy (EDS) results of sampleno. 1 weld metal is shown in Fig. 5. By using the chemical compo-sitions of weld metal the amounts of nickel and chromium equiv-alent could be obtained. The weld metal phase could be predictedby Schneider graph (Fig. 6) [9–11] by calculating the nickel andchromium equivalents of the weld metal. According to the Schnei-der graph of Fig. 6, the presence of austenitic microstructure in theweld metal is predicted which is also verified by the EDS results ofFig. 5.

The weld cross section of the sample no. 4 is shown in Fig. 7.Fig. 8 also shows the microstructure of weld metal for samplesnos. 1 and 3. The weld metal is full austenite due to high promotingaustenite elements such as nickel and cobalt in the weld metal.

The morphology of austenite phase solidification is significantlycellular. The main factor for determining the solidification mode isG/R. The growth rate R of the metal, that is, the travel speed of theSolid/Liquid interface, can be adjusted by adjusting the speed ofthe weld. The temperature gradient G in the liquid metal at theS/L interface can be adjusted by adjusting heating and cooling.The effect of the temperature gradient G and the growth rate Ron the solidification microstructure of alloys is summarized inFig. 9. Together, G and R dominate the solidification microstruc-ture. The ratio G/R determines the mode of solidification whilethe product GR governs the size of the solidification structure[13]. It has been observed that the higher the cooling rate, thatis, the shorter the solidification time, the finer the cellular or den-dritic structure produces.

By decreasing the magnitude of the G/R ratio, solidificationmode changes from planar to cellular and cellular to columnardendrite and columnar dendrite to equiaxed dendrite, respectively.Due to high cooling rate produced in the laser welding process, thegrowth rate is very high and the solidification mode is equiaxeddendrite. However, due to large temperature gradient in laserwelding, the G/R ratio is high enough to solidification occurs in cel-lular mode (Fig. 8a). In some grain, the magnitude of G/R ratio islower than others and solidification mode is columnar dendrite.A grain with columnar dendrite form is shown in Fig. 8b. The sizeof solidified microstructure can be determined by G/R factor(Fig. 5). In laser welding, the magnitudes of both G and R are high.Therefore, in laser welding, the microstructure should be very fineupon solidification. The average size of cells in the weld micro-structures for sample no. 1 is as fine as about 1–2 lm. The averagecell size achieved in GTAW welding of 304 stainless steel reportedby [10] is 20 lm.

In the fusion boundary of kovar alloy side, due to similarity ofweld metal and kovar crystal structure, crystal orientations of thebase metal affect on the microstructure of the weld and cause toform the epitaxial growth region. In the fusion welding process,the initial base metal grains act as a substrate for nucleation atthe fusion line. During the welding process, the liquid metal inthe weld zone is in intimate contact with the grains of substrate.Since the liquefied metal wets the grains of the substrate com-pletely, crystals nucleate from the liquid metal toward the sub-strate grains without difficulties. Under such circumstances, uponsolidification, nucleation occurs by arranging atoms from the liquidmetal toward the substrate grains without altering their existingcrystallographic orientations. Such a growth initiation process is

Table 1Chemical composition of AISI 420 stainless steel and kovar alloy (wt%).

S wt% P wt% Co wt% Ni wt% Cr wt% C wt% Fe wt% Alloy

0.022 0.028 0.038 0.257 13.44 0.186 Remaining 420 Stainless steel0.043 0.013 15 27 0.23 0.016 57 Kovar

Table 2Welding parameters used.

Sample no. Pulse voltage (V) Focused beam diameter (mm) Pulse frequency (Hz) Pulse duration (ms) Welding speed (mm/s)

1 375 0.7 6 6 1.42 400 0.7 6 6 1.43 425 0.7 6 6 1.44 450 0.7 6 6 1.45 375 0.7 6 6 0.76 375 0.7 6 6 2.1

Fig. 2. SEM micrograph of as annealed a-AISI 420 stainless steel b-kovar alloy.

Fig. 3. XRD patterns of AISI 420. Fig. 4. XRD patterns of kovar.

Fig. 5. EDS analysis of weld metal of sample no. 1.

130 S.H. Baghjari, S.A.A. AkbariMousavi / Materials and Design 57 (2014) 128–134

called epitaxial growth. This zone is shown in Fig. 10 for samplesnos. 1 and 3. For materials with face-centered-cubic (fcc) orbody-centered-cubic (bcc) crystal structures, the trunks of cellsgrow in the h100i direction. As shown, each grain grows withoutchanging its h100i direction. If the solidification mode is more af-fected by the thermal gradients than by the crystallographic orien-tation, the epitaxial growth is stopped and the growth would bepersuaded toward the opposite direction of the heat transfer [12].

In the fusion boundary of AISI 420 stainless steel side, the AISI420 stainless steel and the weld metal exhibit two different crystalstructures after welding. The crystal structure of 420 and weld me-tal are bcc and fcc, respectively. Therefore, epitaxial growth is no

Fig. 6. Schneider graph [9].

Fig. 7. Weld pool geometry of sample no. 1.

Fig. 9. Effect of temperature gradient G and growth rate R on the morphology andsize of microstructure upon solidification [12].

S.H. Baghjari, S.A.A. AkbariMousavi / Materials and Design 57 (2014) 128–134 131

longer possible in the AISI 420 stainless steel and new grains nucle-ate at the fusion boundary. Two kinds of grains could be formed inthe welds: columnar grains and equiaxed grains. The most impor-tant parameter that affects the shape of grains is temperature gra-dient. If the temperature gradient is high, the columnar grains arecreated. In sample no. 6, the temperature gradient is high due to

Fig. 8. SEM micrograph of weld metal,

high welding speed (2.1 mm/s) [13]. If the welding speed is high,the percent of pulse overlapping decreases which in turn causesto reduce the effect of preheating of pervious pulse on the nextpulse and increases the temperature gradient. [11] shows themicrostructure of the weld zone of sample no. 6. Columnar grainwith the length of 130 lm produced at the fusion boundary of AISI420 stainless steel side toward the centerline of weld pool.However, for sample no. 5, due to lower welding speed (0.7 mm/s) and temperature gradient, equiaxed grains produced in the weldpool (Fig. 11b). Fine equiaxed grains have more strength and areoften less susceptible to solidification cracking than coarse colum-nar grains. Because, fine equiaxed grains can accommodatecontraction strains occurred upon solidification more easily, there-fore, fine equiaxed grains are more ductile than the columnargrains. In this study, the welding parameters were determined insuch a way to result in lower temperature gradient [12].

3.3. Hot crack formation the weld zone

Both metallurgical and mechanical factors would participate inthe formation of hot cracks in the weld zone. Austeniticmicrostructures are hot crack sensitive due to their low solubilityof harmful elements such as phosphor and sulfur during

(a) sample no. 1 (b) sample no. 3.

Fusion

boundaryFusion

boundary

epitaxial

growth

epitaxial

growth

kovar kovara b

Fig. 10. SEM micrograph of epitaxial growth zone, a-sample no. 1 b-sample no. 3.

a b

µm150

AISI 420

Fig. 11. Weld cross section of (a) sample no. 6 (b) sample no. 5.

132 S.H. Baghjari, S.A.A. AkbariMousavi / Materials and Design 57 (2014) 128–134

solidification. Because, sulfur and phosphor show strong tendencyto segregate at grain boundaries and form low-melting-point com-pounds [14,15]. Due to existence of high percent of sulfur andphosphor in the weld metal (0.045 wt%), the weld metal has strongtendency to hot cracking. Therefore, in order to reduce the forma-tion of crack in the weld pool, the thermal stresses must be re-duced [16]. In sample nos. 2 and 3, hot crack forms in the weldmetal (see Fig. 12a and b), however in sample no. 1 which was per-formed in lower voltage than samples 2 and 3, no cracks was foundin the weld metal. The main reason for formation of hot cracks insamples 2 and 3compared with the sample no. 1 is the use of high-er voltage for samples nos. 2 and 3 than that for the sample no. 1.Higher heat input and stress induced-shrinkage of the molten me-tal results in the rupture of solidified grains along the melted grainboundaries in samples 2 and 3.

3.4. Heat affected zone (HAZ)

The optical microscopic image of the HAZ in the AISI 420 stain-less steel side for the sample no. 2 is shown in Fig. 13a. The totalwidth of HAZ is very small relative to other fusion welding pro-cesses due to low heat input of Nd:YAG pulsed laser welding (thewidth of the HAZ of AISI 420 stainless steel side for sample no. 2is about 30 lm). Due to high diffusion coefficient of carbon andalso super saturation of carbon in the ferrite phase at temperatureabove 925 �C, over precipitation and coarsening of M23C6 carbideprecipitation in the ferrite grain boundary occurs in the HAZ side

of the AISI 420 stainless steel. This precipitation has adverse effecton the mechanical and corrosion property of the weld. Because ofCr carbide precipitation at the grain boundary, the areas adjacentto the grain boundary are depleted from Cr. These areas behaveas anode compared to the rest of the weld zones and hence theseregions are preferentially attacked in the corrosive media, resultingin intergranular corrosion [12]. Post weld annealing in the range of650–815 �C encourages diffusion of Cr atoms to the Cr-depleted re-gion adjacent to Cr carbide precipitates and thus helps to re-estab-lish a uniform Cr composition to resist intergranular corrosion [12].Fig. 13b shows optical microscopic image of HAZ in the kovar sidefor the sample no. 2. Due to low heat input of Nd:YAG pulsed laserwelding, no distinct change in the grain size of kovar alloy isobserved.

3.5. Microhardness tests

Micro-Vickers hardness profile for samples nos. 1 and 4 areshown in Fig. 14. Weld zone microhardness of these samples arehigher than that of kovar base metal in spite of having similar crys-tal structures in the two zones. The higher hardness of the weldzones compared with the kovar is due to having fine cellular struc-ture in weld metal. In addition, the weld metal consists of morehardening element such as chromium and carbon in comparisonwith that of kovar. Measurements taken by X-ray spectrometryfor dispersion of the energy in the weld zone indicated significantlyheterogeneous distributions of alloying elements. The variations in

Fig. 12. SEM micrograph of hot crack in weld metal for (a) samples no. 2 (b) samples no. 3.

a b

kovar

AISI 420

Weld

HAZ Fusion

boundary

Weld

Fig. 13. HAZ of sample no. 1 in (a) AISI 420 base (b) kovar base.

0

50

100

150

200

250

300

350

400

450

-600 -400 -200 0 200 400 600

Har

dnes

s (H

V)

Distance from weld center (µm)

sample 1-3 sample 4-3

HAZ

Weld pool

Kovar

HAZ

AISI 420Fusion boundary

Fusion boundary

Fig. 14. Micro-Vickers hardness profile of sample no. 1 and no. 4.

0

1

2

3

4

5

6

-400 -300 -200 -100 0 100 200 300 400

Wei

ght

of C

r (%

)

Distance from weld center (µm)

sample 1-3 sample 4-3

Kovar AISI 420

Fig. 15. X-ray spectrometry for dispersion of the energy in the WZ of sample no. 1and no. 4.

S.H. Baghjari, S.A.A. AkbariMousavi / Materials and Design 57 (2014) 128–134 133

chemical compositions and grains morphology significantly alterthe Vickers microhardness values in the weld zone [17]. As it isshown in Fig. 15, the percentages of hardening elements such aschromium and carbon are higher in the AISI 420 stainless steel sidecompared with those of kovar side. It should be noted the chro-mium and carbon only exists in the AISI 420 stainless steel mate-rial and therefore the microhardness of the AISI 420 stainless

steel side is higher. The chromium and carbon elements increasethe hardness of austenite phase by two mechanisms: solid solutionstrengthening and chromium carbide precipitation strengtheningmechanisms [18–20]. Fig. 16 shows the presence of chromiumcarbide precipitations in the fracture surface in the bottom of thedimples in the weld zone of sample no. 1. However, the reasonfor reduction of hardness in the fusion boundaries of the two base

Chromium carbide

precipitations

Fig. 16. SEM photograph of fracture surface of sample No. 1.

134 S.H. Baghjari, S.A.A. AkbariMousavi / Materials and Design 57 (2014) 128–134

metal sidesin the weld zone and for sample no. 4 is the formationof coarse microstructures such as epitaxial growths in the kovarside and creation of columnar grain in the AISI 420 stainless steelside in the weld zone. The hardness in the HAZ of AISI 420 stainlesssteel side is high because of coarsening of M23C6 carbide precipita-tion in the ferrite grain boundary. The hardness of this zone insample no. 4 is higher due to higher heat input and more coarsen-ing of M23C6 carbide precipitation in the ferrite grain boundary.The hardness in the kovar HAZ side in samples nos. 1 and 4 doesnot change, since no distinct change in grain size of kovar alloyoccurs.

4. Conclusions

The main conclusion of this investigation can be summarized asfollows:

(1) The morphology of austenite phase solidification is basicallycellular and the size of cells is fine.

(2) In the fusion boundary of kovar side, due to similarity ofweld metal and kovar crystal structure, epitaxial growthregion was produced in this region.

(3) The most important parameter that affects on the shape ofgrains is temperature gradient. If the temperature gradientis high, the columnar grains would be created in the fusionboundary of AISI 420 stainless steel side toward weld zone.

(4) By laser beam welding, sound welds can be achieved in join-ing of alloys when the heat input is low. In sample no. 1 withlowest heat input hot cracks did not produce in the weldzone.

(5) HAZ in the AISI 420 stainless steel shows high microhard-ness. The reason might be attributed to over precipitationand coarsening of M23C6 carbide precipitation in the ferritegrain boundary in the HAZ. No distinct change in grain sizeof kovar alloy was observed.

(6) Measurements taken by X-ray spectrometry for dispersionof the energy in the weld zone indicated significantly heter-ogeneous distributions of chromium element. The variationsin hardening elements and grain morphology significantlyalter the Vickers microhardness values in the weld zone.

(7) The chromium and carbon elements increase hardness ofaustenite phase by two mechanisms: solid solutionstrengthening and chromium carbide precipitation strength-ening mechanisms.

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