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Technical Report

Role of pulsed current on metallurgical and mechanical properties

of dissimilar metal gas tungsten arc welding of maraging steel

to low alloy steel

V. Rajkumar, N. Arivazhagan ⇑

School of Mechanical and Building Sciences, VIT University, Vellore 632 014, India

a r t i c l e i n f o

Article history:

Received 4 October 2013

Accepted 27 May 2014

Available online 9 June 2014

a b s t r a c t

This research work encompasses the investigations carried out on the mechanical and metallurgical

properties of maraging steel and AISI 4340 aeronautical steel weldments. The materials were joined by

continuous current gas tungsten arc welding (CCGTA) and pulse current (PCGTA) gas tungsten arc weld-

ing processes using ErNiCrMo-3 filler wire. Cross sectional macrostructures confirmed proper deposition

of the fillers and lack of discontinuities. Optical microscopy studies revealed that at the maraging steel–

weld interface, martensite in distorted and block forms prevailed in CCGTA and PCGTA weldments

whereas tempered martensite was predominant at the low alloy–weld interfaces of both the welds. Scan-

ning electron microscopy (SEM) with energy dispersive analysis of X-rays (EDAX) analysis apparently

showed less elemental migration in PCGTA weldments as compared to the other. Results of X-ray diffrac-

tion analysis recorded possible phase formations in various zones of the weldments. Microhardness pro-

files in either weld zones followed a constant trend whereas it showed a downtrend in the heat affected

zones (HAZ) of the maraging steel and very high hardness profiles were observed in the low alloy steel

side. Tensile studies on various factors and impact testing showed that PCGTA weldments outperformed

the continuous ones in terms of strength, ductility and toughness. Fractograph analysis depicted the nat-

ure of failures of tensile and impact tested specimens. Comparison analyses involving influence and nat-ure of pulsed current welds over continuous ones were done to determine the possibility of

implementing these joining processes in aerospace applications. Weldments fabricated using PCGTA

technique proved to be superior to the other, resulting in exceptional mechanical properties.

2014 Elsevier Ltd. All rights reserved.

1. Introduction

Maraging steel 250 and AISI 4340 aeronautical steels are widely

used in various aerospace applications such as aircraft landing

gears, helicopter shaft drives and especially the combination of

above are used in specific critical defense applications [1–3]. Marag-ing steel is a best alternative for aerospace application since this

steel possesses ultra-high strength combined with good toughness.

Readily weldability, good formability, high strength to weight ratio

and good dimensional stability are other attributes of this steel

which makes it an eligible material for critical aerospace applica-

tions [4–6]. Transformation from austenite to martensite followed

by ageing, gives this steel an exceptional strength [7] whereas high

strength low alloy steels possess excellent ductility [8,9].

Joining of bimetallic combinations are relatively challenging

due to differences in physical, thermal and metallurgical compat-

ibility of the filler wire etc. Welding processes involving low

energy inputs like electron beam welding and friction stir weld-

ing have been carried out for joining these materials individually

and as well using combination of above materials like mediumcarbon steel [10,11]. However, fusion welding like GTAW is

widely employed for various critical applications in view of the

consistency of weld quality and overall economy [12,13]. It i s

widely reported and researched by scientists that pulsed current

produced deep penetration (because of the high frequency pulse

current) with lesser heat input to the joint (quick solidification

due to alternating current) is obtained thereby giving refined

grain structure and enhanced mechanical properties [14].

Mohandas et al. [15] reported that the fusion (GTA) welds of dis-

similar combination of materials could cause considerable reduc-

tion in tensile properties. This can be rectified by selecting

http://dx.doi.org/10.1016/j.matdes.2014.05.055

0261-3069/ 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +91 9443034794; fax: +91 416 2243092.

E-mail address: [email protected] (N. Arivazhagan).

Materials and Design 63 (2014) 69–82

Contents lists available at ScienceDirect

Materials and Design

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / m a t d e s



suitable welding process and filler wire [16]. Balasubramanian

et al. [17] reported that the pulsed current GTA welding process

is particularly attractive, as it avoids the issue of macro/micro-

segregation in the interdendritic region by controlling the heat

input and weld thermal cycle.

Dilution of adverse elements is a major concern reported in

these applications as high heat input increases the same

thereby emanating the possibility of elemental migration. It isreported that the strength of these welds is dictated by welding

processes and filler metal selection to a greater extent [9]. The

segregation within the dendritic structure results in deteriora-

tion of mechanical properties and corrosion resistance of the

joints. However, such problems can be minimized by careful

and judicious selection of filler materials [18]. Correlation

between them is utmost necessary in order to predict the weld-

ment behaviour and analyse its performance characteristics

[19].

Carbon migration is a potential setback reported my many

researchers and is highly undesirable as it makes the material

more brittle by formation of certain carbides at undesired loca-

tions [20] Nickel-based filler materials are exclusively chosen

in joining the high alloy and low alloy steels, as the activity rate

and diffusion rate of carbon in highly alloyed materials are slow

[21]. Further, Madhusudhan Reddy et al. [9] employed friction

welding with and without using Nickel interlayer to join a sim-

ilar bimetallic combination and deduced that the former yielded

better results as Nickel acted as an effective barrier. Most impor-

tantly, sulphur (0.015 wt%), phosphor (0.02 wt%) and carbon

(0.1 wt%) contents are kept minimum in this filler which is a

requisite of a filler wire to weld low alloy steels as dilution by

above elements reduces ductility and toughness [22]. Out of

other fillers in its class, ErNiCrMo-3 has Niobium and Tantalum

content about (4% by wt) which obviates the formation of chro-

mium carbides (leaves the weldment brittle) by eliminating sen-

sitization. In this study, ErNiCrMo-3 filler wire was chosen as it

is widely used for joining nickel based and super alloys, high

nickel content of which enables good dissimilar weldability[23]. As the strength of this electrode and high-strength low-

alloy (HSLA) steels are almost equal welding of both can be per-

formed without sacrificing strength [24]. Choosing a filler wire

with a lesser tensile strength than the base metals or with very

high tensile strengths could result in known outcome as the

strength of the joint is governed by the weakest material in

the combination [25]. In some instances where stress relieving

the weld upon completion of welding is feasible, electrodes of

austenitic stainless steels or nickel alloys can be used [26]. Venk-

ata Ramana et al. [27] reported works on similar welding of

maraging steels using base materials compositions as fillers.

Rao et al. [28] have studied the influence of different post-weld

heat treatments on the properties of similar GTA weldments of

maraging steel.Attempts to join these material combinations neither using

GTA welding with super alloyed fillers such as one used in this

study nor using pulsed current GTA welding techniques has not

been reported so far. In this work, the bimetallic joints combi-

nations are fabricated using above two techniques keeping in

mind the merits, integrity and performance of these welds.

Most importantly that the choice of a proper filler wire, opti-

mized process parameters and weld configurations could result

in sound weld joints by constructively using the strength prop-

erty of the materials. Metallurgical analyses to characterize its

compositional changes and tests assessing mechanical

properties are done to investigate the possibility of employing

above welding techniques in aerospace applications thereby

improving the service performance and reliability of this alloycombination.

2. Experimental procedure

2.1. Candidate materials and welding process

Maraging steel (250 grade) and AISI 4340 aeronautical steel

received in the form of rods were EDM wire cut to

120 mm 50 mm 5 mm thick plates were used in this study.

The coupons were welded with standard butt joint configuration(single v-groove having a root gap of 2 mm, size land of 1 mm

and included groove angle of 60. Welds were deposited with com-

mercial TIG welding equipment (KEMPPI ARC) using ErNiCrMo-3

filler wire in both continuous and pulsed current welding modes.

The composition of parent and the filler materials used are pre-

sented in Table 1.

The welds were deposited parallel to the plate rolling direction

in the as received plates ground to 5 mm ± 0.005 lm using surface

grinding. The test plates were restrained in the fixed position

using clamps while welding to prevent lateral distortion. Stainless

steel wire brushing was employed between passes to remove heat

tint and excessive weld metal scales and impurities. Maraging

steel is readily weldable whereas preheat is mandatory for low

alloy steels [29]. According to standards, AISI 4340 samples were

preheated to a temperature of 120 C. Multilayered, multi pass

welds with three layers and a total of five passes were deposited.

The inter pass temperature never exceeded 150 C. Pre heat and

post weld cooling rate of samples were monitored using infra

red thermometer. Post-weld, the samples were cooled at a moder-

ate pace (air cooled at room temperature) as for maraging steel

very slow cooling rates are adverse and faster cooling of AISI

4340 weldments would initiate hot cracking [14]. The shielding

gas flow rate was periodically monitored. Arc current and voltage

were continuously recorded for each pass. Bead-on welding with

different current inputs were performed whose parameters are

tabulated in Table 2. Either due to lack of fusion or improper melt-

ing of fillers these parameters were discarded. The final optimized

set of parameters which resulted in mechanical and metallurgical

properties is tabulated in Table 3. The welded samples werenon-destructively tested by X-ray radiography and results showed

neither any lack of fusion nor any discontinuities. The welded

coupons were surface grinded to 4.5 mm thickness and samples

were removed of required dimensions using EDM wire cut for var-

ious analysis.

2.2. Weldment characterization

Macro-structural and micro-structural studies in the weldment

cross sections were done using Carl Zeiss optical microscope

equipped with image analysis software and SEM (JEOL JSM6380A)

equipped with energy dispersive X-ray (EDAX). Special care was

taken to reveal the microstructure of the dissimilar weldmentusing three respective etchants. Modified Fry’s reagent (50 ml

HCl, 25 ml HNO3, 1 g CuCl2 and 150 ml water was used to etch

maraging steel and 2% Nital was used to etch the low alloy steel

side. In order to etch the weld region etchant (20 ml of HCl, 1 ml

of HNO3, 5 ml of glycerol and 0. 25 g of CuSO 4) was used. The

etched samples were also analyzed by SEM/EDAX to find the pos-

sible formation of secondary phases and segregation. XRD analysis

was performed using BRUKER D8 ADVANCE powder diffractometer

to evaluate the formation of phases in the different zones of the

weldment. Micro hardness measurements were carried out to

characterize variation of hardness across different regions on the

weldment using Matsuzawa MMT-X Vickers hardness tester. The

indentations were made at every 0.025 mm distance and with a

load of 500gf for a dwell time of 15 s ranging through extremelengths of the sample.

70 V. Rajkumar, N. Arivazhagan/ Materials and Design 63 (2014) 69–82



Tensile tests were carried out on welded specimens of rectangu-

lar cross section of standard ASTM: E8/E8M-13a sizes usingINSTRON 8801 UTM. The tests were carried out at a strain rate of

3 mm/min which is the average value between the specified range

i.e. 105 to 101 s1, for testing done using hydraulic machines

[30]. Three samples from each weld coupon were tested to ensure

the repeatability, accuracy of testing and stability in welding.

Observations on fracture surface have been carried out to deter-

mine the nature/mode of tensile failure using SEM. All the samples

were ultrasonically cleaned prior the image was captured. The

impact test samples were prepared according to the ASTM:

E23-12c standards of a sub-size specimen and tested using Charpy

V-notch method.

3. Results and discussion

Sound dissimilar weldment of maraging steel 250 and AISI 4340

aeronautical steel has been fabricated by continuous and pulsed

current GTA welding technique using ErNiCrMo-3 filler and as –

welded samples represented in Fig. 1a and b.

3.1. Macro and microstructures

The macrostructure of the three phase region covering the weld

interface and heat affected zones of parent metals are shown in

Fig. 2a and b. The microstructure of parent metals is shown in

Fig. 3a and b. Lath martensite with unique habit planes and inter-

nal structure of parallel twins is seen as the major constituent in

the maraging steel surrounded by prior austenitic grain boundaries

(due to very less carbon content). This sets up good binding withdislocations having high resistance to deformation [31]. Each of

these parallel twins along with grain boundary act as a barrier to

the movements of slip planes enabling more dislocation pile-upswhich imparts maraging steel with high tensile strength and

toughness. Pearlite (in colonies) and cementite are found to be

the dominating constituents surrounded by prior austenitic grain

boundary in the parent metal of low alloy steel. Each colony repre-

sents different orientation of a lamellar pearlite.

The microstructures at the weld interfaces and weld zones of

the continuous and pulsed current weldments are shown in Figs. 4

and 5 respectively. Fig. 4a represents the microstructure of marag-

ing steel–weld interface of the CCGTA weld. It is observable that

lath martensitic structure is completely disturbed and there no

longer exists original grain boundaries as seen in parent metal

and instead resulted in uneven and irregular plate martensite

structures deposited randomly in clusters. Moreover equal

amounts of retained austenite are visible in between these platesas this zone is subjected to elevated temperatures, possesses very

good ductility as well.

On the other hand, micrograph of maraging steel–weld interface

of pulsed weld (Fig. 5a) shows phenomenal change in the orienta-

tion of martensitic structure, from plate to block form. Each mar-

tensitic block is being clearly surrounded by prior austenitic grain

boundary, is clearly visible. Subsequently, the sizes of these blocks

are reduced when moving away from the interface and can be seen

from Fig. 5c. It could be deduced that this re-orientation of martens-

itic structure must be due to narrow and localized intensity of

pulsedheat input subjecting to more but localized dilution. The fact

that block martensite has high dislocation density compared to its

other form adds to the reason for better strength exhibited at the

inter faces by the pulsed welded samples. Mixed region of intermetallic precipitates are present in this case near the interface

which enables better bonding and a steady transition in strength

from weld to parent metal. Whereas in the former case no such

traces are seen, resulting in less strength. Moreover, titanium is

quick and strongin forming carbides andhas formedTiC and depos-

ited along the grain boundary of the block martensite (Fig. 5a). High

amounts of retained austenite prevails in this case as well which

equalizes for the ductility and strength in this zone. These all

together resist the propagation of dislocations when the material

strain hardens.

Regarding the low alloy steel side, formation of tempered mar-

tensite is evident along with spherical (rounded) carbides, in the

heat affected zone of both welds Figs. 4b and 5b. As the low alloy

was preheated it has obviated hot cracking in the weld and in HAZby lowering the cooling rate and avoiding formation of

Table 1

Chemical compositions of parent metals and filler wire.

Parent/filler Composition (wt%)

C Ni Mo Co Ti Al Mn Si Cr Cu Nb Ta Fe

Maraging 250 0.01 18.06 5.19 8.34 0.47 0.14 – – 0.04 – – – 67.1

AISI 4340 0.38 1.65 0.2 – – – 0.6 0.18 0.95 – – – 95.9

ErNiCrMo-3 0.10 58 9 – 0.40 0.4 0.5 0.5 22 0.5 3–4 3–4 5

Table 2

Parametric studies on CCGTA and PCGTA.

Welding parameters CCGTA ERNiCrMo-3 CCGTA ERNiCrMo-3 CCGTA ERNiCrMo-3 PCGTA ERNiCrMo-3 PCGTA ERNiCrMo-3 PCGTA ERNiCrMo-3

Base current (Amps) 160 140 120 130 100 70

Peak current (Amps) – – – 200 170 140

Voltage (Volts) 13.4 12.1 11 14.2 12.3 11.1

Pulse frequency (Hz) – – – 6 6 6

Pulse on time (%) – – – 50 50 50

Wire diameter (mm) 2.4 2.4 2.4 2.4 2.4 2.4

Argon gas (lit/min) 14.4 14.4 14.4 14.4 14.4 14.4

Table 3

Optimized welding parameters.

Parameters Process

CCGTA PCGTA

Peak current I (Amps) 170 220

Base current I (Amps) – 150

Voltage V (Volts) 14 15

Pulse frequency (Hz) – 6

Pulse on time (%) – 50

Argon flow rate (L/min) 14.4 14.4

Filler wire Dia (mm) 2.4 2.4

Welding speed (mm/min) 2.1 2.1

V. Rajkumar, N. Arivazhagan/ Materials and Design 63 (2014) 69–82 71



untempered martensite. These fine rounded carbides possess high

resistance to void formation (during deformation) which accounts

for good ductility and strength [31]. Presence of more epitaxial

grain growths are clear in the interface of either sides of continu-

ous welds whereas completely absent in the other. This is due to

the fact that the rate of heat flow is more in the direction perpen-

dicular to the weld as reported by [10]. Comparison of Fig. 4b and d

reveals that size of needle shaped martensite has tended todecrease from the HAZ of AISI 4340 towards its parent metal and

change over to the unaffected parent metal microstructure. The

zone distinction is clearly evident from Fig. 5f and this was

observed in low alloy steel side of both welds.

In continuous welded sample spherical shaped and small

equi-axed dendrites were noticed (due to slow cooling) which

has comparatively less toughness and strength than other forms

of dendrites which is essentially a reason for the breakage of ten-

sile specimen in weld region. As the other process involves quick

cooling enabled by pulse-off time (50%) columnar dendrites are

formed which possess relatively high toughness and prevents for-

mation of epitaxial grains and its intrusion towards adjacent metal

[30]. This may be the reason due to which very less number of

grain growths was observed, leading to good weld strength.

3.2. X-ray diffraction studies

The characteristic X-ray peaks were recorded in the heat

affected regions (very near to inter face) and the weld zones of

the weldments in order to identify the phases formed during weld-

ing and are represented in Fig. 6. In the heat affected zones of the

maraging steel, Fe–Ni martensite (denoted as /) and Ni–Ti phases

are formed which are inter-metallic precipitates from which the

Fig. 1. Photograph of samples welded by process (a) GTAW and (b) PCGTAW using ErNiCrMo-3 filler wire.

Fig. 2. Cross-sectional macrostructure of (a) GTA and (b) PCGTA welded specimens.

Fig. 3. Micrographs of parent metals (a) Maraging steel 250 and (b) AISI 4340.




material derives its exceptional strength. In addition, martensite

peaks and individual peaks of Fe which is in accordance with the

results reported by Meshram et al. [11] and Velu et al. [32]. Simi-

larly, in HAZ of low alloy steel sides, different phases combining

Ni and Fe are noticed, as Fe (95%) and Ni (about 2%) are the major-

ity constituents present in them. Martensite and iron carbide peaks

are noticed here as well which is certainly the reason for relatively

high hardness values in the HAZ of the AISI 4340, and is in good

agreement with SEM/EDAX results. In the weld region, Ni–Cr–Fe

and Fe4Cr, the phases of iron with nickel and chromium are

obvious.

3.3. SEM/EDAX analysis

The SEM images captured at the heat affected zone very near

the interface and the weld regions of CCGTA and PCGTA welded

samples are shown in Figs. 7 and 8 respectively. It is vivid on com-

paring Fig. 7a and b that Iron (Fe) has migrated from low alloy steelto the weld region (about 20% by weight) along with carbon to a

considerable extent. Traces of cobalt diffusing from the maraging

steel side to weld and low alloy steel are also noticed.

It is evident from the EDAX analysis of the PCGTA welded sam-

ples that there is no much elemental migration as was seen in the

CCGTA welds. Comparing EDAX results of all three zones of pulsed

welds, the weight% of all elements remains nearly same as that of

parent metals eventually proving less dilution. Only 5% of iron

seemed to have moved from low alloy to weld matrix, especially

migration of carbon being effectively controlled. The EDAX analysis

taken on the dark spot in the HAZ of the AISI 4340 (Fig. 8c) con-

firms the presence of iron carbides which are evident in the micro-

structures as well. Martensites in the same micrograph are seen as

white needle like structures.

3.4. Tensile testing

The testing was done at a strain rate of 3 mm/min and at room

temperature. The average UTS of maraging steel and AISI 4340 arefound to be 1020 MPa and 730 MPa respectively. The tensile

Fig. 4. Micrographs taken at different zones of GTA welded sample (a) Maraging side-weld interface. (b) Low alloy steel and weld interface. (c) Macrograph of GTA weld. (d)

Heat affected zone of low alloy steel. (e) Heat affected zone of maraging steel side. (f) Weld region.




results show that in the CCGTA welded specimens the elongation

and fracture has occurred in the weld region Fig. 9a. In the PCGTA

welded specimens, necking has occurred in the weld region but has

fractured in the parent metal of AISI 4340 (Fig. 9b). It is evidentthat the weld region has demonstrated better yield strength than

AISI 4340 in the latter case. This affirms that pulsed welds perform

better than continuous in the case of dissimilar combination. Due

to less bondage between parent and weld metals rendered by

CCGTA welds at the either side of interface, the weld region and

the interface has not contributed much to the strain hardening.

This is why, fracture must have happened in the weld region. In

addition, adverse influencing factors such as presence of more epi-

taxial grain growth and less amount of dilution of parent metals at

the interface leading to disturbance of grain boundaries (which

was not observed in pulsed welds) must have also been the cause.

The tensile stress versus tensile strain graphs are given in

Fig. 10 and an overlapped graph of averaged stress–strain values

is provided in Fig. 11. The nature of the stress–strain graphs of both

the samples affirms that the material has shown excellent

toughness with gradual increase and decline in the slope and has

underwent typical breakage conforming to a ductile material. In

certain trials after inception of necking, the graph of CCGTA speci-

mens have plummeted towards the end of fracture and the loadhas correspondingly decreased leading to a ductile rupture. This

is attributed to the extreme strain hardening property of the sam-

ple where the strain compensates for the gradual decrease in the

load until it meets breakage. The graphs of the PCGTA specimens

showed steady increase ending without any downtrend. This is

because the fracture has occurred in the parent metal of AISI

4340 suddenly at a point after considerable necking in weld region.

Even though the fracture was instantaneous, it cannot be consid-

ered brittle because enough elongation had taken place in the

weld. During the time of tension test, various important parame-

ters that were populated are listed in Tables 4 and 5. The results

show that the PCGTA welded specimens has absorbed more

amounts of load (32.1%), ultimate stress (6.74%), stress at break

(26.61%), strain at break (16.68%) and more amount of elongation

at break (10.89%) as that of the CCGTA ones. Also, the average

Fig. 5. Micrographs showing (a) Maraging steel–weld interface. (b) AISI 4340-weld interface. (c) Heat affected zone of maraging steel. (d) Macrograph of PCGTA weld.

(e) Weld region. (f) Phase transition from pearlite to martensite.




ultimate strength values of CCGTA and PCGTA are 706.3 MPa and

757.3 MPa which are only 30.79% and 25.71% lesser than the ten-

sile strength of maraging steel (highest of the three materials used

in the study). The UTS obtained from PCGTA welds in this research

is 40% more than the ones obtained using friction welds employing

Nickel as inter layer [9], provided the metallurgical aspects being

satisfactory.

3.4.1. Fractography

SEM fractograph image of the CCGTA and PCGTA specimen is

shown in Fig. 12a and b. It depicts the occurrence of spherical dim-

ples that initiate the crack propagation with voids indicating a typ-

ical ductile failure. The fractograph of the PCGTA sample in Fig. 12b

shows cleavage planes symbolizing a brittle fracture as discussedearlier.

3.5. Hardness measurements

The hardness plot of the CCGTA and PCGTA welded samples

across middle of cross section of the weld are shown in

Fig. 13. In both joining processes the hardness values on the

HAZ of AISI 4340 side has tremendously increased and in the

maraging steel side it has declined when compared to values

of its own parent metals. This is due to the reason that the mar-

aging steel was welded in solution annealed condition as is con-

sistent with results reported by Shetty et al. [33]. This is in

conformance with the findings by Floreen et al. [34] who

observed in the case of maraging steels that high annealing tem-

peratures results in reduction of strength. However post-weld

ageing treatments can improve hardness and put them back to

normalcy [35].

Fig. 6. XRD patterns of GTA welded samples (a) HAZ of MDN 250. (b) Weld. (c) HAZ of AISI 4340 and PCGTA welded samples. (d) HAZ of MDN 250. (e) Weld. (f) HAZ of AISI

4340.




Fig. 7. SEM/EDAX analysis of GTA welded samples showing (a) HAZ of MDN 250. (b) Weld region. (c) HAZ of AISI 4340.




Fig. 8. SEM/EDAX analysis of PCGTA welded samples showing (a) HAZ of MDN 250. (b) Weld region. (c) HAZ of AISI 4340.






the pulsed welds. Step mode of decrement of the hardness plot in

the low alloy steel side of PCGTA process owes to the narrow heat

input of pulsed weld. This has not significantly affected maraging

steel side. The difference in the widths of the HAZ on either sides

of the dissimilar weldment is due to the difference in thermal

conductivity of material [10]. The hardness values have corre-

spondingly decreased from HAZ towards parent metal of AISI

4340 which can be attributed to the decrease in the size of tem-

pered martensite and then to unaltered parent microstructure.

Presence of iron carbides have disappeared traversing towards

Fig. 11. Averaged stress versus strain graphs of CCGTA and PCGTA welded samples.

Table 4

Tensile test parameters of CCGTA welded samples.

Parameters GTA

Units Trial 1 Trial 2 Trial 3 Average

Load at break kN 14.66 7.00 16.12 12.5933

UTS MPa 692 716 711 706.3333

Tensile stress at break MPa 610.84 291.74 671.60 524.7267

Tensile strain at break % 10.731 13.679 14.532 12.98067

Tensile strain at yield mm/mm 0.09437 0.11043 0.12968 0.111493

Tensile extension at yield mm 3.01994 3.53386 4.14982 3.567873

% Elongation at break % 9.74030 13.13767 12.64037 11.83945

Table 5

Tensile test parameters of PCGTA welded samples.

Parameters PCGTA

Units Trial 1 Trial 2 Trial 3 Average

Load at Break kN 18.42 18.99 20.38 18.9633

UTS Mpa 730 734 808 757.3333Tensile stress at Break Mpa 682.05 703.23 759.67 714.9833

Tensile strain at Break % 14.709 14.659 17.635 15.57767

Tensile strain at Yield mm/mm 0.12487 0.12389 0.14996 0.132907

Tensile extension at Yield mm 3.99579 3.96437 4.79872 4.25296

% Elongation at Break % 13.55721 13.32599 15.21773 13.29051

Fig. 12. Fractograph of the (a) GTA and (b) PCGTA tensile specimens welded using ErNiCrMo-3 filler.




the parent material. Most importantly it is to be noted that the ten-

sile failure was also in the transition zone of martensite to pearlite

structure. At this zone, the strain hardening by martensite struc-

ture is not supported by pearlite structure. Remarkable variations

in hardness values between these phases add to the above. The

hardness profiles of the parent metals are identified to be uniform.

In the weld region, constant trend was observed which is inferable

that the hardness of the filler material is unchanged irrespective of type of the weld. Various informations populated with reference to

hardness profiles across various zones are tabulated in Table 6.

3.6. Impact testing

Impacts testing of weldments are important especially in aero-

space applications as they are subjected to high impact forces in

their service conditions (especially at the end of a mission). For

above reasons the knowledge of impact energy absorbed by fabri-

cated structures is to be analyzed. As observed from Fig. 14a and b,

the parent metal of maraging steel and the AISI 4340 fractured in a

ductile and brittle manner respectively whereas the CCGTA and

PCGTA weldments yielded in a brittle and a ductile manner. The

Charpy V-notch impact values and their nature of failure are given

Fig. 13. Hardness plot of the GTA and PCGTA welded samples using ErNiCrMo-3 filler wire.

Table 6

Micro hardness properties.

Parameters Process

GTA PCGTA

Average hardness in weld zone (HV) 214.65 214.65

Peak hardness in weld zone (HV) 224.6 236.5

Average hardness in HAZ of MDN 250 (HV) 326.2 325.2

Peak hardness in HAZ of MDN 250 (HV) 354.1 340.7

Average hardness in HAZ of AISI 4340 (HV) 555.8 644.5

Peak hardness in HAZ of AISI 4340 (HV) 632 663.7

Fig. 14. Impact tested specimens (a) Parent metal of Maraging steel. (b) Parent metal of AISI4340. (c) GTA welded sample and (d) PCGTA welded sample using ErNiCrMo-3filler.




in Table 7. The parent metal of maraging steel has highest impact

energy value and that of low alloy steel which simply implies the

reason for difference in the mode of failure. The impact energy val-

ues of both the weldments are identified to be almost equal.

3.6.1. Fractography

The fractographs of the impact tested CCGTA and PCGTA

welded specimens given in Fig. 15a and b shows dark cleavage

planes (due to presence of 95% Fe in low alloy steel) suggest com-

pletely brittle mode of fracture. Bright rectangular dimples (due

to high Ni content in weldment) with direction of crack propaga-

tions in parallel planes are visible meaning a sudden impact yet

indicating a ductile failure. Voids are rarely visible in the same

and comparatively less when compared to tensile fractographs

which is an indication of a difference in loading condition. This

clearly proves that the PCGTA weld joints are tougher than CCGTA

welds when subjected to impact loading as well. Even though AISI

4340 is high tensile steel, the impact load bearing capacity is less

when compared to the maraging steel and the weld regions.

Therefore fabrication of this steel with alloy rich fillers in the

bimetallic combinations would result in highly impact resistant

structures.

4. Conclusions

This article furnishes results on joining of dissimilar metals

using continuous and pulsed current GTA techniques with ErNi-

CrMo-3 filler. Listed below are some of the important conclusions

deduced out of this research.

(1) The distinctions in the metallurgical nature and mechanical

performance of both welding techniques are addressed. Fil-

ler ErNiCrMo-3 has exhibited its compatibility to this bime-

tallic combination by showing excellent weldability and

performance thereby enabling a successful filler wire

selection.

(2) Microstructures of continuous and pulsed welds showed

remarkable changes due to the difference in the nature

and heat input of weld process. Relevance between micro-

structures and their influence on mechanical properties are

well colligated.

(3) SEM/EDAX analysis provided informations on the elemental

composition and migration post-weld. On observing and

comparing the values of EDAX analysis of both processes,

it reads that pulsed weld has arrested its elemental migra-

tion (especially carbon) than the continuous welds which

is highly desirable.

(4) Presence of inter metallic precipitates such as Ni–Ti, Fe–Ni

(/) in maraging steel, Fe–Cr and Ni–Cr–Fe phases in the weld

and Fe–C in the low alloy steel portions were affirmed by

XRD analysis.

(5) The tensile studies on various factors proved that the pulsed

welded specimens have outperformed the continuous ones

in every single measure. Moreover, fracture occurring in

the parent metal of low alloy steel indicates that the filler

wire has exhibited pronounced yield and ultimate strength

than the former. This implies filler wire suitability and effi-

ciency of pulsed weld process.(6) The hardness traverse showed lesser values in the heat

affected zones of maraging steel in both the welds whereas

it is seemed to be exceptionally high in the low alloy steel

side. Even though, the hardness profiles of both weld regions

are constant, the nature of failure under impact loading is

totally contrasting. The hardness profiles obtained are in

good agreement with the phases obtained in microstructural

examinations and the reasons correlating the mechanical

and metallurgical aspects of both the welds are well

substantiated.

(7) In the impact testing, parent metals of maraging and low

alloy steel seems to be extreme (ductile and brittle respec-

tively) in terms of both impact values and failure mode.

The weldments have exhibited almost similar impact values

Table 7

Impact energy values of parent and weld metals used in the study.

Parameters Units Trial 1 Trial 2 Average Failure mode

Maraging 250 (Parent metal) J 51 49 50 Ductile

AISI 4340 (Parent metal) J 13 12 12.5 Brittle

GTA (ErNiCrMo-3) J 47 45 46 Brittle

PCGTA (ErNiCrMo-3) J 47 47 47 Ductile

Fig. 15. Fractographs of impact tested (a) GTA welded and (b) PCGTA welded specimens using ErNiCrMo-3 filler.




but the nature of failure being brittle in continuous welds

and ductile in the case of the pulsed welds. Further, the

pulsed welding technique can be used to improve the tough-

ness of low alloy steels at the joints and overall weldment as

well.

(8) Pulsed current tungsten inert gas welds has demonstrated

superior weldability characteristics and warrants for better

performance in every aspect tested and has once againproved that it is highly reliable for joining dissimilar mate-

rial combinations.

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