INVESTIGATION OF ELECTRON BEAM WELDING OF AHSS BY

PHYSICAL AND NUMERICAL SIMULATION

Raghawendra P. S. Sisodia1, Marcell Gáspár2

PhD student1, Assistant Professor2 1,2Institute of Materials Science and Technology, University of Miskolc, Miskolc

3515, Hungary

ABSTRACT

The electron beam welding (EBW) sets new standards as it facilitates very high

quality and effective welding of high strength structural steels. The technology

ensures high-quality critical welded joints in thicker structural metals. It has a high

energy density in comparison to the conventional arc welding processes (e.g.

GMAW). As a result of less overall energy input and higher velocity, the effect of

welding on the base material in the heat-affected zone (HAZ) and the distortion is

much smaller compared to conventional arc welding processes. The low heat input

result in a small HAZ and a reduced extension of critical HAZ areas which can be

favourable in high strength steels when the mechanical properties can drastically

decrease in the HAZ. In comparison with experimental studies, a numerical modelling

study can provide detailed information concerning the welding process and

parameters, and the number of costly experiments can be reduced. Finite element

modelling (FEM) of EBW enables the estimation of temperature field, time

temperature curve, weld pool geometry and welding distortion etc. The determination

of the temperature field can be very useful in terms of the further investigations since

the t8/5 cooling time can be less than 2 s during EBW. In this paper, by the application

of Sysweld software, the time-temperature curve was determined and the physical

simulation of the critical HAZ subzones were performed using a GLEEBLE 3500

physical simulator in order to analyse the properties of HAZ during extremely short

cooling time.

Keywords: Electron Beam Welding (EBW), Finite element modelling (FEM), HAZ

characteristics, Gleeble 3500 thermophysical simulator, t8/5 cooling time, 3D conical

heat source

INTRODUCTION

In keeping the view of modern world where advancement in technology is

prime requirement to drive economic growth and sustainable development objective

within the world. Several steel manufacturers have increased their interest in ultra-

high strength quenched and tempered steels [1][2]. The development of high strength

weldable steels has diversified the field of application, range of design alternatives

subjected to more severe operation conditions than previous time. Nowadays high

strength steels are gaining more popularity in welded structures, even in ship building

and machinery components. Advanced High Strength Steel (AHSS) are more

complex, particularly through their microstructures, which are usually multiphase for

MultiScience - XXXIII. microCAD International Multidisciplinary Scientific ConferenceUniversity of Miskolc, 23-24 May, 2019, ISBN 978-963-358-177-3

DOI: 10.26649/musci.2019.051

an improved combination of strength and ductility. This balance is carefully used to

meet performance requirements while maintaining excellent formability [3]. This

high strength can be achieved with minimum yield strength up to 1300 MPa, AHSS

often has other advantageous mechanical properties, such as high strain-hardening

capacity. They provide high strength to weight ratios, very good acceptable

weldability especially where overmatched welds are used, improved toughness and

sufficient deformation capacity. Financial benefits can also be realized through

reduced transportation and lifting costs (reduced weight), material savings

(smaller/lighter sections) and reduced weld volumes (thinner plates). Easier and

simple structural components and construction techniques are also possible,

especially for large structures, and establishment costs may also be reduced due to

lower dead weight [4]. High strength steels are available for structural applications,

as in bridges, buildings, offshore applications etc., all around the world. A very high

carbon equivalent value indicates poor weldability and these steels are not suitable

for structural applications, where welding is very important to assure structural safety

[5]. The future of AHSS for automotive applications is bright. The weldability of high

strength steels consists of more challenges compared to mild steels: hardening of the

heat-affected zone (HAZ) and higher cold cracking sensitivity; reduction of strength

and/or toughness of HAZ; selection of filler material (mismatch) [6]. Therefore, new

welding conceptions and special welding technology should be developed for these

steels. For the advanced high strength steels innovative welding technologies can be

needed (e.g. laser beam, electron beam, laser-GMAW hybrid welding technology)

which may have benefits for the joint properties and the production costs. Generally,

these welding processes result narrow HAZ, which is often the most problematic part

of the welded joint in AHSS. By these technologies the extension of the critical areas

(softened or hardened zones) can be minimized [7]. Regarding the economic

advantages the high productivity and the less filler material costs should be

emphasised. Due to the narrow and deep penetrated weld structure (keyhole

technique) lower amount of filler material is needed. It can be also beneficial to the

mechanical properties of the whole welded joints since in AHSS the weld can be

similarly critical as the HAZ.

Electron Beam Welding (EBW) is a highly efficient, flexible [8] and precise welding

method increasingly used within the manufacturing chain and of growing importance

in different industrial environments such as the automobile, aeronautical, aerospace

(3A), construction, and power generation sectors [9]. It is also observed from two

referred research paper that numerically simulated cooling time can be extremely

short: t8/5 = 0.49 s [10] & t8/5 = 2 s [11] for EBW.

With the development of various new high strength steel and their difficulty in

weldabilities arises the other promising welding technology that can be applied for

high and ultra- high strength steels is the electron beam welding process [12]. So far,

the studies of electron beam welding on high strength quenched and tempered steel

and thermo mechanically controlled steel with different yield strength have not been

carried out extensively, therefore their suitability should be investigated before. Since

they are very expensive technologies, therefore the application of numerical and

physical simulation is suggested before the real welding experiments and the

production.

EXPERIMENTAL METHODOLOGY

The first phase of experimental steps involves the numerical simulation that

containing steps like creation of geometry of the required model, selecting type,

order, method of element for further mesh creation, providing boundary conditions,

weld path, reference line, clamping conditions etc. Then analysis is performed which

can be either thermo-metallurgical or mechanical analysis together or separately

depending upon requirement and finally simulated data were generated with detailed

analysis. The important parameters considered for numerical simulation for S960QL

material were presented in Table 1, which seems quite satisfactory with real

experiment data with EBW for 10 mm thickness plate and velocity 10 mm/s [13].

The needed material properties of the steel S355J2 are integrated in the FE software

SYSWELD and used for the calculations because it is similarly low alloyed as

S960QL, therefore there is no a big difference in the thermo-physical properties. The

efficiency is a very important parameter and it is considered η=1 for EBW analysis.

The efficiency influenced the amount of the heat source power. [14]

Table 1

Simulation parameters

Welding

process

Heat input

(J/mm)

Velocity

(mm/s)

Simulated

time (t8/5)

(s)

Cooling

medium

EBW 600 14 2 Free air

cooling

Second phase of experimental steps is the physical simulation and the simulation

parameters were determined iteratively by running calculations with more parameters

in order to find the optimal EBW parameters [15]. Unfortunately, proper toughness

evaluation of HAZ is far more difficult than for a homogeneous material. Because of

the very local heat input, a weld offers various zones with different microstructures

and therefore different properties (metallurgical notch) [16] and the third phase is the

materials testing. A Gleeble 3500 thermomechanical simulator was used to simulate

the welding thermal cycles of the heat-affected zone of S960QL steel. Square base

specimen (10 mm x10 mm x 70 mm) were prepared and subjected to thermal cycle

[15].

The high cost of the material and the process does not allow for trial and error

approach in optimizing the process and, therefore, modelling and simulations

approaches are being used increasingly with continuous improvement brought on

better heat source modelling and more reliable predictions on the influence of process

parameters on the distribution of HAZ, residual stresses and distortions. Finite

element (FE) simulations have been the most common numerical method used to

simulate the welding process, thermal and mechanical analyses [17]. Modern finite

element codes not only allow for calculation of deformations and stresses due to the

welding process but also take into account the change of microstructure due to

different heating and cooling rates [18]. For an advanced high strength steel structure

or sample such effects shall be investigate and quantify using advanced material

modelling and FE simulation [19]. The electron beam welding (EBW) numerical

model was created using SYSWELD software. Weld plate is modelled and simulated

on 3D finite element method (FEM) using the SYSWELD software and this is

specifically used for thermal analysis. It basically involves three important steps in

welding simulation, these are modelling, analysis and post processing. The sample

geometry, materials and heat sources fitting belong to the first steps i.e. modelling in

weld simulation sequence.

Meshing

The main aim was to simulate the EBW of a 15 mm thick butt welded joint. The

meshed model of plate with butt joint used in simulation is presented in Figure 1(a).

The model contains total 49,470 nodes, forming quad triangular elements. The

dimensions of the plate used for simulation is 150 x 100 x 15 mm and the number of

plates is two for butt welded joint with a gap of 2 mm. A finer mesh has been created

in the joint area can be seen in Figure 1(a), in order to get the steep temperature

gradients with higher accuracy. The total number of elements involved in the meshing

is 52,764. After creation of the base model in the SYSWELD software, the necessary

groups needed to define the welding trajectory, starting and end points, reference line

together welding path defines the plane, respective to which heat source coordinate

frame is oriented [20]. The red line in below Figure 1(b) is referring to the reference

line. Using clamping configurations can mitigate the welding distortion of floor

structure. Here in the simulation, “Stop” under the Clamping Condition step was used

in order to block a structure in only one given direction. Thermal losses to the

surrounding environment were modelled using element groups termed “skins” which

consisted of 4-node quadrilateral elements superimposed on the free surfaces [21]. In

order to model the ambient temperatures around the model, a skin consisting of 2D

elements must be generated.

Fig. 1 Finite element model

(a) Geometry of a butt joint and (b) Reference line, skin for cooling, clamping

Heat source model

The welding operation is modelled through the Welding Wizard module, which

makes it possible to flexibly and accurately assign the necessary parameters for

processing the analytical model for volumetric heat release. Volumetric heat release

takes place as a result of the introduction of heat in accordance with several thermal

models incorporated into the program. For modelling electron beam, a conical shaped

source with 3D Gaussian distributed thermal energy density source model [22][23] is

applied to get thermal field analysis, shown in Figure 2a-b.

Fig. 2 (a) Heat source model [23] (b) Temperature field

This model is described by Eq. (1-2). At any plane perpendicular to z-axis, the heat

intensity is distributed in a Gaussian form and may be written as [23].

2 2

0 2

0

( , , ) exp( )

x yQ x y z Q

r z

+= −

(1)

the height of the conical heat source is H = Ze - Zi, the z-coordinates of the top and

bottom surfaces of the conic region, and it can be expressed as

( )0 ( ) i ee e

i e

r rr z r z z

z z

+= + −

− (2)

where Q0 is power intensity, re and ri are top and bottom radiuses, remainder (ze-zi) is

penetration, r0 is the distribution parameter and r is the radial coordinate from the

source centre.

RESULT AND DISCUSSION

Numerical Simulation

Numerical modelling of welding processes has proven to be highly efficient for

design and production engineering [24]. The numerically simulated temperature field

is presented in the cross section at the heat source in Figure 3a-b.

Fig. 3 (a) Iso section heat source [22] (b) volumetric section

Thermal analysis

After numerical simulation, one node in the HAZ is selected (Figure 4a) for

finding t8/5 cooling time and exported the curve as shown in Figure 4b, it is observed

that cooling time is 2 s.

Fig. 4

(a) Selected node in HAZ (b) Exported HAZ thermal cycle

Microscopic examination

Microscopic examinations were performed by the Zeiss Evo MA10 Scanning

Electron Microscope. The SEM images as shown in Figure 5, demonstrate the

successful physical simulation of the desired heat zone i.e. peak temperature 1350 °C

and t8/5=2.5 s, and the effect of the cooling time on the microstructure of the HAZ

subzone. It can be seen from the images that when the coarse grain zone structure

was examined, the grain size in CGHAZ is finer than in the t8/5 range of conventional

arc welding technologies in the 5…30 s cooling time range, since during EBW is less

time for grain coarsening (when the t8/5 is just around 2…2.5 s) and below also the

needle martensite’s and the block sub-substructure are well observable.

The scanning electron microscopic pictures of coarse-grained heat affected zone,

S960QL (Tmax = 1350 ºC, 2.5 s for different magnifications (M=500x, M=1000x,

M=1500x & M=2000x) are shown in Figs. 5a, b, c-d.

0

200

400

600

800

1000

0 5 10 15 20

Tem

p, °

C

t, s

t8/5=2 s

Fig. 5 Coarse grained HAZ (S960QL), Tmax = 1350 ºC, t8/5=2.5 s, (a) M=500x, (b)

M=1000x, (c) M=1500x & (d) M=2000x [15]

The hardness values thus obtained after physical simulation for CGHAZ (Figure 6a)

and the cooling time, t8/5=2 s was calculated from the equation in Figure 6b which is

440 HV10. It can be stated that CGHAZ has fulfilled the requirement of EN ISO

15614-1 standard [25] even in 2 s cooling time, which allows 450 HV10 maximum

hardness for non-heat-treated welded material group 3. Compared to the HV10

hardness of the raw material 340, the measured higher average hardness for t8/5=2 s.

With the maximum allowable hardness, the 2...2.5 s cooling time technology proved

to be the most critical, but it did not reach the 450 HV10 limit.

Fig. 6 Hardness test

(a) Thermal cycle with Rykalin-3D model (b) second degree polynomial curve [15]

0

200

400

600

800

1000

1200

1400

0 50 100 150 200 250

T, ºC

t, s

2.5 s

850 ºC

500 ºC

y = 0,0071x2 - 1,9961x + 437,48R² = 0,9947

200

250

300

350

400

450

500

0 20 40 60 80 100

HV

10

t8,5/5, s

In the case of the coarse grain zone, the effect of the cooling time t8/5 was also plotted

on the diagram Figure 6b, with the second-degree polynomial on the hardness values

[15]. So, it can be observed from above result that hardness may increase further with

shorter cooling time than 2 s. However, we did not reach the permitted maximum

value, there is a high risk in EBW that we reach this value, therefore we should

estimate higher crack sensitivity.

CONCLUSIONS

Thermo-metallurgical analysis of electron beam welding was performed on

SYSWELD software considering conical heat source model. A numerical model was

used for the accurate thermal analysis and prediction of t8/5 cooling time which is

usually shorter in case of electron beam welding compared to conventional arc

welding technologies (e.g. gas metal arc welding). and here in our results it was found

to be 2 s which is conforming to various results performed by another researchers [1]

[13]. On the basis of numerical simulation results, the time-temperature curve was

determined and various mechanical properties like hardness and microstructure of

numerically simulated thermal cycle for t8/5= 2 s cooling time in HAZ was compared

with the results of previously determined t8/5=2.5 s thermal cycle from physical

simulation using GLEEBLE 3500 HAZ tests, the following conclusions were drawn:

(a) Thermo-metallurgical simulation of advance high strength steels (S960QL) is

possible for electron beam welding using SYSWELD.

(b) To model the weld penetration for required thickness (15 mm) of material was

performed successfully and ensured the full depth penetration with the

optimized parameters by the FEM.

(c) The physical simulation results was used for comparison with numerically

simulated thermal cycle results successfully which allow studying materials

behaviour at conditions very close to real industrial processing or applications,

and by means of physical simulation a large variety of microstructures and

associated mechanical properties can be obtained and studied in a short time

and for a tiny fraction of full-scale industrial experiments.

(d) The thermal cycle exported from simulated results and found that the t8/5

cooling time is 2 s for electron beam welding is quite small and conforming to

the result.

(e) The hardness values thus obtained after physical simulation for CGHAZ, t8/5=2

s was 440 HV10. It can be said that CGHAZ has fulfilled the requirement of

EN ISO 15614-1 for required test cooling period, which allows 450 HV10

maximum hardness for non-heat-treated welded material group 3.

(f) Through microscopic examinations of the coarse grain HAZ, it was observed

that the size of the grains is finer for shorter cooling time (i.e. t8/5=2.5 s)

compared to the generally applied 5…30 s cooling time of arc welding

process.

(g) Also, the needle martensite’s and the block sub-substructure are well

observable.

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ACKNOWLEDGEMENT

The described article was carried out as part of the EFOP-3.6.1-16-2016-00011

“Younger and Renewing University – Innovative Knowledge City – institutional

development of the University of Miskolc aiming at intelligent specialisation” project

implemented in the framework of the Szechenyi 2020 program. The realization of

this project is supported by the European Union, co-financed by the European Social

Fund.