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Effect of GMAW on the Mechanical Properties of In-Line Galvanised

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					     EFFECT OF GMAW ON THE MECHANICAL PROPERTIES OF IN-LINE GALVANISED
                            COLD-FORMED STEEL


                                  Tim Wilkinson and Gregory J Hancock


Abstract

This paper describes tensile tests to examine the effect of various welding procedures on the
mechanical properties of in-line galvanised cold-formed steel. Cold-formed flats in either Grade C350
or Grade C400 were butt welded. The dip transfer method or the spray transfer method were used
to provide a variety of heat inputs, and two different welding electrodes, Autocraft LW1 and Autocraft
Mn-Mo, were used. The thicker (8 mm) Grade C350 steel displayed a small reduction in the yield and
ultimate stresses when welded compared to the unwelded steel. The 3.8 mm Grade C400 samples
displayed a more significant drop in yield and ultimate stresses when welded, and the drop in strength
was greater when the higher heat input spray method was used. On some occasions the ultimate
strength of the welded 3.8 mm specimens dropped below the yield stress of the parent material.

Keywords

Cold-formed steel, GMAW, butt welds, fillet welds, heat affected zone, mechanical properties, yield
stress, heat input, structural steel, galvanised steel.

Author Details

Dr Tim Wilkinson is a Lecturer, and Professor Greg Hancock is the BHP Steel Professor of Steel
Structures, Centre for Advanced Structural Engineering, Department of Civil Engineering, The
University of Sydney, Sydney, NSW, 2006, Australia.


 IIW (International Institute of Welding) Asian Pacific Congress, Paper No. 35, (Published by Welding Technology
 Institute of Australia - WTIA), Melbourne, Australia, November 2000.




                                                        1
1      INTRODUCTION

A recent innovation in steel products in Australia is the DuraGal range of cold-formed in-line
galvanised hollow and open sections produced by BHP Structural and Pipeline Products (formerly
known as Tubemakers) [1]. The typical steel strip used in the manufacturing process has a nominal
yield stress (fy) of 300 MPa. After cold-forming, the final product has a nominal yield stress in the
range 350 - 400 MPa, depending on the exact process and the shape of the product.

During the welding process, the grains of the cold-worked steel recrystallise, and the heat affected
zone will soften compared to the cold-formed hardness [2]. Consequently, the ultimate tensile
strength (fu) in the heated affected zone (HAZ) may be less than the yield stress of the parent
material.

There are several instances in which a steel structure has to demonstrate ductile behaviour. In plastic
design, the plastic hinges must rotate sufficiently for moment redistribution to take place in the
structure, in order to obtain the strength increase afforded by plastic design. For seismic design,
deformation capacity is essential to dissipate the energy caused by the earthquake motion. In such
cases, the joints of a steel structure are required to show ductile behaviour.

However, if there is a small HAZ in a welded joint, where the ultimate tensile strength is less than the
yield stress in the adjacent unaffected steel, the HAZ will fracture before significant plastic
deformations occur near the joint. This renders the structure unsuited for plastic design or seismic
applications. A previous investigation [3, 4] examining the suitability of portal frame knee joints for
use in a plastically designed structure constructed from cold-formed rectangular hollow sections
(RHS), found that under opening bending moment, the connection fractured in the HAZ before large
plastic deformation occurred. It should be noted that the connection displayed adequate strength, as
opposed to ductility, which means that it was still suitable for use in elastic design.

This paper describes the initial portion of a research project examining the strength and ductility of
joints constructed from in-line galvanised cold-formed steel. The aim is to quantify the effect of
welding on the mechanical properties of cold-formed steel.

2      TEST SPECIMENS

A typical portal frame and possible knee joint details are shown in Figure 1. Two possible connections
are shown. The first is an unstiffened connection in which adjoining RHS are butt welded directly
together. The second detail is a stiffened connection, in which a plate is inserted between the two
RHS legs. Each RHS is either butt welded or fillet welded to the stiffening plate. These two
connection details are suggested by CIDECT [5]. This type of mitred welded connection could also
be used for open steel sections.

Rather than testing a large and expensive connection, it is possible to test a small component of such
a system. Hence, two flat plates were butt welded together to simulate a portion of a more
complicated connection, as shown in Figure 2. The plates would then be tested in tension.




                                                   2
        Knee Joint
                                                     Stiffening plate welded              Two legs welded directly
                                                        between two legs                         together




                                              (a) Stiffened welded connection   (b) Unstiffened welded connection


                           Figure 1: Types of connections in a portal frame




                             Steel strip




                              Butt weld




                             Figure 2: Butt weld between 2 DuraGal flats


2.1    Steel Properties

DuraGal flats in nominal thicknesses of 3.8 mm and 8.0 mm were chosen. The 3.8 mm sections were
Grade C400 (fyn = 400 MPa, fun = 450 MPa), and the 8.0 mm flats were Grade C350 (fyn = 350 MPa,
fun = 400 MPa). Currently, there is no Australian Standard applicable to the manufacture of cold-
formed open profiles, and hence the sections are manufactured to an internal BHPSPP Specification,
TS100. The carbon equivalent of this steel is typically CE = 0.24 6 0.30 [1].

2.2    Weld Metal Properties

Two types of welding wire were used in the GMAW process. Autocraft LW1 (fyn = 390 MPa,
fun = 500 MPa) and Autocraft Mn-Mo (fyn = 530 MPa, fun = 630 MPa), to AS/NZS 2717.1 [6] were used.
More details on the wire properties can be found in [7].

2.3    Typical Welding Parameters

Two methods of GMAW were employed, the “dip-transfer” and “spray-transfer” modes. Generally,
the spray transfer method requires a higher wire speed and higher current, and consequently a higher
heat input. It is not possible to include the full details of all welding procedures in this paper, due to
length requirements, however some typical welding parameters are given in Table 1.




                                                      3
 Welding Machine                         Transmig 250, with Transmatic 62 wirefeeder
 Gas                                     Argoshield 50; 23 % CO2, 77 % Ar; 25 L/min
 Electrode                               Autocraft LW1 or Autocraft MnMo
 Dip Method                              Potential: 18 - 19 V
 (range of values)                       Current: 120 - 135 A
                                         Wire speed: 3700 mm/min
                                         Welding speed: 260 - 400 mm/min
                                         Heat input: 0.334 - 0.494 kJ/mm
 Spray Method                           Potential: 26 V
 (range of values)                      Current: 205 - 210 A
                                        Wire speed: 9700 mm/min
                                        Welding speed: 450 - 920 mm/min
                                        Heat input: 0.412 - 0.728 kJ/mm
                                     Table 1: Typical welding parameters

2.4    Joint Preparation

Square butt welds, and single-V butt welds, welded both sides were used, as shown in Figures 3
and 4.


                                                                               1

                                                Plate
                                                                               2
                                             thickness, t


                Gap, G                                            Either 1 or 2 welding runs

               Typical dimensions:
         t = 3.8 mm, G = 0.0 or 0.9 mm
                     Figure 3: Typical Joint Preparation for 3.8 mm DuraGal Flat
             (Type B-C 1a or Type B-C 1b from Table 4.4(A) of AS/NZS 1554.1: 1995 [8])


                θ                                                          2       3
                                                                               1
                                                Plate                          4
                                             thickness, t

                                                                  Either 4 or 5 welding runs
                Gap, G   Root face, Fr

               Typical dimensions:
        t = 8.0 mm, Fr = 1.0 mm, θ = 90E
                G = 0.0 or 0.9 mm
                      Figure 4: Typical Joint Preparation for 8.0 mm DuraGal Flat
                     (Type B-C 2a from Table 4.4(A) of AS/NZS 1554.1: 1995 [8])




                                                            4
3       TEST PROCEDURE

Two 150 mm long plates were butt welded together. The sections were either 100 mm wide (3.8 mm
thick specimens) or 150 mm wide (8 mm thick sections). Different tests were performed in
accordance with AS 2205.1 [9].

A tensile coupon was cut longitudinally from the plate in accordance with AS 2205.2.1 [10] as shown
in Figure 5. The butt weld was located transversely at the middle of the coupon. The tensile coupons
were prepared and tested to AS 1391 [11]. An extensometer was used to measure strain. The
coupons were tested in a 300 kN capacity SINTECH Testing Machine with friction grips to apply the
loading as shown in Figure 6. A constant strain rate of approximately 1.0 × 10-3 s-1 was used. In some
cases the weld reinforcement was removed so that a completely flat coupon was tested. In the
remaining cases the weld reinforcement remained.

The properties of the weld metal itself were obtained by performing an all-weld-metal tensile test to
AS 2205.2.2 [12]. Properties of the unwelded steel were determined to AS 1391.

Macro specimens were also cut from the specimens. However the results of the macro section
examination and Vickers Hardness tests are not presented in this paper.


                  25
                                        Separate
                                         flatbar
           45                           sections
                               12.5
                                            Butt
                                            weld

      180 90                   65
                       12.5
                                                        Extensometer
                               12.5                     (connected to
                                                        computer data                       Test specimen
                                                           logger)
          45                                                                                  Butt weld



    Figure 5: Dimensions and location of tensile
            coupon within welded plate
           (all dimensions in millimetres)                                                 MTS Sintech grip




                                                       Figure 6: Schematic details of tensile testing




                                                   5
4                                       RESULTS

From each test it is possible to obtain stress strain curves such as the one shown in Figure 7.
Figure 7 illustrates how the tensile behaviour changes from the unwelded material, to the welded
sample and weld metal only. The stress is calculated as the load divided by the flat area of the
coupon, and similarly the strain is the ratio of the extension of the specimen measured by the
extensometer divided by the original 50 mm gauge length of the extensometer. However, in the cases
of the specimens where the reinforcement was not removed, the stress and strain are not uniform
along the entire 50 mm gauge length, since the extensometer straddles the reinforced region.

The values of yield stress and ultimate stress are given in Table 2, and can also be seen in Figures 8
to 11. Since the steel is cold-formed there is no well-defined yield stress, and the yield stress quoted
is the dynamic 0.2% proof stress. The term dynamic is used since the stress was determined while
the testing machine was loading at a constant rate of stroke.

Figures 12 and 13 show a selection of fractured specimens.


                                                                                                                       800
                                                                                                                       700
                                                                                                                       600
                                                                                                        Stress (MPa)




                                                                                                                       500
                                                                                                                       400
                                                                                                                                                               Mn Mo Weld Metal
                                                                                                                       300
                                                                                                                                                               3.8 mm Parent Metal
                                                                                                                       200                                     MnMo 3.8 mm spray no gap
                                                                                                                       100                                     MnMo 3.8 mm dip 0.9 gap
                                                                                                                         0
                                                                                                                             0        0.02         0.04        0.06       0.08                   0.1             0.12            0.14
                                                                                                                                                                      Strain



                                                                                                                        Figure 7: Typical stress strain curves

                   700                                                                                                                                                                     600
                   600                                                                                                                                                                     500
                   500
                                                                                                                                                                            Stress (M
    Stress (MPa)




                                                                                                                                                                                           400
                   400
                                                                                                                                                                                           300
                   300
                                                                                                                                                                                           200                                                               Ultimate Stress (fu)
                   200                                                                                                                                                                                      8 mm DuraGal Flatbar
                                                        3.8 mm DuraGal Flatbar                                           Ultimate Stress (fu)                                                                 Butt Welded LW1
                                                                                                                                                                                           100                                                               Yield Stress (fy)
                   100                                     Butt Welded LW1                                               Yield Stress (fy)                                                                                                                                           reinforced
                                                                                                                                                                                                                                                Spray no




                                                                                                                                                                                             0
                                                                                                                                                                                                                        Parent




                                                                                                                                                                                                                                                                         Dip gap




                           0
                                                                                                                                                                                                                                                            Dip no
                                                                                                                                                                                                       Nominal




                                                                                                                                                                                                                                                                                      Dip gap
                                                                                                                                                                                                                                        Weld
                                        Nominal




                                                                                                                             Dip no
                                                            Parent




                                                                                           Spray gap




                                                                                                                                        Dip gap
                                                                                                        reinforced




                                                                                                                                                  reinforced
                                                                                Spray no
                                                                         Weld




                                                                                                        Spray gap




                                                                                                                                                                                                                                                             gap
                                                                                                                                                   Dip gap




                                                                                                                                                                                                                                                  gap
                                                                                                                              gap
                                                                                  gap




                                   Figure 8: Results - 3.8 mm steel, LW1                                                                                                                         Figure 9: Results - 8 mm steel, LW1

                                  800                                                                                                                                                      800
                                  700                                                         3.8 mm DuraGal Flatbar                                                                       700                                                             8 mm DuraGal Flatbar
                                                                                                Butt Welded MnMo                                                                                                                                            Butt Welded MnMo
                                  600                                                                                                                                                      600
                                                                                                                                                                            Stress (MPa)
                   Stress (MPa)




                                  500                                                                                                                                                      500
                                  400                                                                                                                                                      400

                                  300                                                                                                                                                      300
                                                                                                       Ultimate Stress (fu)                                                                200                                                             Ultimate Stress (fu)
                                  200
                                                                                                       Yield Stress (fy)                                                                   100                                                             Yield Stress (fy)
                                  100
                                                                                                                                                                                             0
                                    0
                                                                                                                                                                                                       Nominal



                                                                                                                                                                                                                        Parent




                                                                                                                                                                                                                                               Spray no



                                                                                                                                                                                                                                                           Spray no




                                                                                                                                                                                                                                                                       Dip gap




                                                                                                                                                                                                                                                                                    reinforced
                                                                                                                                                                                                                                        Weld




                                                                                                                                                                                                                                                                       MnMo
                                                                                                                                                                                                                                                            gap reo
                                                                                                                                                                                                                                                MnMo



                                                                                                                                                                                                                                                            MnMo




                                                                                                                                                                                                                                                                                     Dip gap

                                                                                                                                                                                                                                                                                      MnMo
                                                  Nominal



                                                                     Parent




                                                                                            Spray no



                                                                                                                       Spray no
                                                                                   Weld




                                                                                                                                      Dip gap



                                                                                                                                                  Dip gap
                                                                                                                                      MnMo




                                                                                                                                                  MnMo
                                                                                                                        gap reo




                                                                                                                                                                                                                                                 gap
                                                                                             MnMo



                                                                                                                        MnMo




                                                                                                                                                    reo
                                                                                              gap




                      Figure 10: Results - 3.8 mm steel, Mn-Mo                                                                                                                             Figure 11: Results - 8 mm steel, Mn-Mo



                                                                                                                                                                      6
    Material           Weld Type         Reinforcement   Electrode Type            Heat Inputa             Yield    Ultimate   Failure locationd
                                                                                    (kJ/mm)               Stressb   Stressc
                                                                                                          (MPa)      (MPa)
  All weld metal   Nominal propertiese        -              LW1                                           390        500            -
  All weld metal   Measured properties        -              LW1                                           419        521            -
  All weld metal   Nominal propertiese        -              MnMo                                          530        630            -
  All weld metal   Measured properties        -              MnMo                                          526        699            -
 3.8 mm DuraGal    Nominal properties         -                -                    Unwelded               400        450            -
 3.8 mm DuraGal    Measured properties        -                -                    Unwelded               525        601            -
 3.8 mm DuraGal        Dip - Gap             No              LW1                   0.365/0.365             392        497           Weld
 3.8 mm DuraGal        Dip - Gap             Yes             LW1                   0.365/0.365             457        549           HAZ
 3.8 mm DuraGal       Dip - No Gap           No              LW1                   0.365/0.365             395        490           Weld
 3.8 mm DuraGal       Spray - Gap            No              LW1                      0.533                369        502           HAZ
 3.8 mm DuraGal       Spray - Gap            Yes             LW1                      0.533                422        539           HAZ
 3.8 mm DuraGal      Spray - No Gap          No              LW1                   0.533/0.348             344        487           HAZ
 3.8 mm DuraGal        Dip - Gap             No              MnMo                  0.351/0.351             435        546           HAZ
 3.8 mm DuraGal        Dip - Gap             Yes             MnMo                  0.351/0.351             490        570           HAZ
 3.8 mm DuraGal      Spray - No Gap          No              MnMo                  0.426/0.426             424        535           HAZ
 3.8 mm DuraGal      Spray - No Gap          Yes             MnMo                  0.426/0.426             440        540           HAZ
  8 mm DuraGal     Nominal properties         -                -                    Unwelded               350        400            -
  8 mm DuraGal     Measured properties        -                -                    Unwelded               399        507            -
  8 mm DuraGal         Dip - Gap             No              LW1             0.494/0.494/0.494/0.412       401        486          Parent
  8 mm DuraGal         Dip - Gap             Yes             LW1             0.494/0.494/0.494/0.412       406        493           HAZ
  8 mm DuraGal        Dip - No Gap           No              LW1             0.498/0.360/0.432/0.360       400        494           HAZ
  8 mm DuraGal       Spray - No Gap          No              LW1             0.728/0.494/0.494/0.412       387        474           HAZ
  8 mm DuraGal         Dip - Gap             No              MnMo         0.411/0.411/0.411/0.411/0.334    411        496          Parent
  8 mm DuraGal         Dip - Gap             Yes             MnMo         0.411/0.411/0.411/0.411/0.334    413        496          Parent
  8 mm DuraGal       Spray - No Gap          No              MnMo            0.690/0.585/0.585/0.585       384        473           HAZ
  8 mm DuraGal       Spray - No Gap          Yes             MnMo            0.690/0.585/0.585/0.585       386        487           HAZ
Notes: a)      For cases of multiple weld runs, the heat input for each welding run is given.
       b)      Yield stress is the dynamic 0.2% proof stress.
       c)      Ultimate stress is the dynamic ultimate stress.
       d)      HAZ was defined as the region of zinc removal caused by welding, rather than by micro or macroscopic observation.
       e)      Nominal properties obtained from reference [7].

                                                                  Table 2: Summary of results


                                                                               7
      Figure 12: Failed 8.0 mm specimens               Figure 13: Failed 3.8 mm specimens
(The weld is at the centre of each specimen. The extent of zinc removal has been indicated with a
                           permanent marker on either side of the weld.)



5      DISCUSSION

Several observations can be made from the test results.

There is considerably more variation in the results of the 3.8 mm steel compared to the 8.0 mm steel.
There is a statistically significant drop in yield and ultimate stresses in the welded 3.8 mm steel,
compared to the unwelded material. The change in properties for the 8 mm steel is small.

The measured properties of the unwelded 3.8 mm steel are significantly higher than the nominal
properties. This is very common, but as a result, the strength of the 3.8 mm steel is higher than that
of the commonly used welding wire, Autocraft LW1. It is usual practice to match the strength of the
welding consumable to that of the parent metal. In two instances (3.8 mm, dip method, LW1), fracture
occurred in the weld rather than in the parent metal. For the corresponding case using the spray
method (higher heat input), failure occurred in the HAZ, indicating that the higher heat input had
reduced the strength of the HAZ by a greater amount compared to the dip method.

Welding produces a greater percentage reduction in strength for the 3.8 mm steel, compared to the
8 mm steel. The 3.8 mm steel is more heavily cold-worked to produce its higher nominal strength
compared to the 8 mm steel. Consequently, there is greater scope for strength reduction in the HAZ.

The higher strength electrode (Mn-Mo) produced greater capacity in the 3.8 mm sections compared
to the LW1 electrode despite a similar heat input. The Mn-Mo electrode had an almost negligible
effect on the 8 mm steel, compared to the results of the LW1 electrode.

The higher heat input method of spray transfer compared to dip transfer produces a larger reduction
in yield and ultimate stresses.

There are several instances in which the ultimate strength of the welded 3.8 mm specimens drops
below the yield stress of the parent material. Consequently, a welded connection of this type would
not be able to provide the amount of ductility required for seismic or plastic design applications. The
ultimate strength of the welded 8 mm samples did not fall below the yield stress of the parent material.




                                                   8
It should be noted that to utilise the available feedstock most efficiently, BHPSPP use virgin strip with
yield stress fyn = 360 MPa for the 3.8 mm flat bar, and a different strip with yield stress fyn = 300 MPa
for the 8.0 mm flat bar. This is the most likely cause of the 3.8 mm steel exhibiting strength
considerably higher than the nominal properties. It is possible that BHPSPP may change the
feedstock, so that all DuraGal flatbars are produced from the 300 MPa strip. It is possible that flat bar
made from this material will not experience as significant changes in the strength of the HAZ,
compared to the product tested.

This paper has considered the preliminary results of the initial stage of this project. Future
examinations will consider microhardness determination, macro cross section examination, and fillet
welded specimens.

6      SUMMARY

This paper has described tensile tests to examine the effect of various welding procedures on the
mechanical properties of in-line galvanised cold-formed steel. Sections of cold-formed flats were butt
welded together using either the dip transfer method or the spray transfer method using either LW1
or Mn-Mo electrode. In most cases, the tensile specimens failed in the heat affected zone. There was
a small reduction in the yield and ultimate stresses in the welded 8.0 mm steel compared to the
unwelded steel. The 3.8 mm samples displayed a more significant drop in yield and ultimate stresses
when welded, and the drop in strength was greater when the higher heat input spray method was
used. The 3.8 mm steel had a higher nominal strength than the 8.0 mm steel due to more cold
working in the manufacturing process, so it is not unexpected that this steel experienced a greater
drop in strength when welded. Significantly, there were some occasions in which the ultimate strength
of the welded 3.8 mm specimens dropped below the yield stress of the parent material

7      REFERENCES

[1]    BHPSPP, (1999), “Structural Cold Formed Hollow Sections and Profiles”, Technical
       Information, BHP Structural and Pipeline Products, Mayfield, Newcastle. Australia.

[2]    American Welding Society, (1976), Welding Handbook, Vol 1, Fundamentals of Welding, 7th
       Edition, (Weisman, C., editor), AWS, Miami, Florida, United States.

[3]    Wilkinson T. and Hancock G. J., (1998), “Tests of Knee Joints in Cold-Formed Rectangular
       Hollow Sections”, Research Report, No R779, Department of Civil Engineering, The University
       of Sydney, Sydney, Australia.

[4]    Wilkinson T. and Hancock G. J., (2000), “Tests to examine the plastic behaviour of knee joints
       in cold-formed RHS”, Journal of Structural Engineering, American Society of Civil Engineers,
       Vol 126, No 3, March 2000, pp 297-305.

[5]    Packer, J. A., Wardenier, J., Kurobane, Y., Dutta, D, and Yeomans, N., (1992), Design Guide
       for Rectangular Hollow Section (RHS) Joints under Predominantly Static Loading, CIDECT
       Design Guide No 3, Verlag TÜV Rheinland GmbH, Köln, Germany.

[6]    Standards Australia/Standards New Zealand, (1996), Australian Standard AS/NZS2717.1
       Welding - Electrodes - Gas metal arc, Part 1: Ferritic steel electrodes, Standards Australia,
       Sydney, Australia.

[7]    CIGWELD, (1993), Welding Consumable Guide, Part No. WCGUIDE, Preston, Victoria,
       Australia.




                                                   9
[8]    Standards Australia / Standards New Zealand, (1995), Australian / New Zealand Standard
       AS/NZS 1554.1 Structural Steel Welding, Part 1: Welding of Steel Structures, Standards
       Australia, Sydney, Australia.

[9]    Standards Australia, (1997), Australian Standard AS 2205.1 Methods for destructive testing
       of welds in metal; Method 1: General requirements for tests, Standards Australia, Sydney,
       Australia.

[10]   Standards Australia, (1997), Australian Standard AS 2205.2.1 Methods for destructive testing
       of welds in metal; Method 2.1: Transverse butt tensile test, Standards Australia, Sydney,
       Australia.

[11]   Standards Australia, (1991), Australian Standard AS 1391 Methods for tensile testing of
       metals, Standards Australia, Sydney, Australia.

[12]   Standards Australia, (1997), Australian Standard AS 2205.2.2 Methods for destructive testing
       of welds in metal; Method 2.2: All-weld-metal tensile test, Standards Australia, Sydney,
       Australia.

8      NOTATION

The following symbols are used in this paper:

fu     Ultimate tensile strength
fun    Nominal ultimate tensile strength
fy     Yield stress
fyn    Nominal yield stress

9      ACKNOWLEDGEMENTS

This paper forms part of a research project entitled “Strength and fracture of steel connections”, jointly
funded under the SPIRT scheme by the Australian Research Council and BHP Structural and Pipeline
Products. The experiments were carried out in the J. W. Roderick Laboratory for Materials and
Structures, Department of Civil Engineering, The University of Sydney. The authors are grateful for
the work of Mr Grant Holgate, in the manufacture of the test specimens, and the advice given by Mr
Paul Grace, WTIA, with respect to welding procedures.




                                                   10

				
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Description: Effect of GMAW on the Mechanical Properties of In-Line Galvanised