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Problems Linked to Laser Welding of components having Large Thicknesses G. Caironi Dipartimento di Meccanica - Politecnico di Milano M. Coffetti consultant, Riva Techint Contact

ABSTRACT

There are a great many applications of laser sources in the field of welding, even in sectors where considerable difficulties of execution were previously verified.
The specific characteristics of heating by means of laser, with a high concentration of energy in localised areas and the simultaneous presence of restricted areas that have been thermally altered, represent special conditions as a whole and the material's answer to them opens a field of study that is of considerable interest in the metallurgical field.
The complex phenomena of a thermofluid mechanics nature, which are involved in the deep penetration welding technique, also involve a preliminary and accurate experimental set-up of the process in order to assure the high quality of welding that can be achieved with this technology.
The problems linked to the welding of components having large thicknesses were examined in this work while referring to the operative parameters and analysis of the types of defects, both internal and external, that were encountered when setting up and in the laser CO2 welding of these components.
Welding on thicknesses varying from 6 to 34 mm., with and without weld material, were taken into consideration in order to evaluate the modifications of the operative parameters and, in particular, the evolutions of the defects.
Head-bondings were analysed for transparency, individually and/or in several runs, even with unlike materials.
The work carried out pertained to the examination of samples that were characterised by the types of microscopic and macroscopic defects that can intervene in the laser welding process, in the view of ascertaining the origin and relations with the welding process in order to optimise the parameters of the process itself for each of the applications examined.

INTRODUCTION

Weldings with thicknesses of up to 34 mm for heavy steel structural work were taken into consideration in this work. Weldings made with and without filler wire in a single pass and in 2, 3 and 4 multi-passes were examined to this regard.
The work performed concerned the problems linked to the welding of components with large thicknesses, with reference made to the working parameters and analysis of the types of internal and external defects that may be encountered when setting up and performing laser welding on these components.
Joints with thicknesses varying between 10 and 34 mm, with and without filler wire, were examined for the purpose of evaluating the modifications of the working parameters and in particular the development of defects.
The research was conducted on welding samples made using two distinct CO2 Rofin Sinar laser sources that were operating in continuous mode and able to deliver maximum power capacities of 10 and 17 kW, respectively [1, 2, 3].

DESCRIPTION OF THE TRIALS

The samples that were examined belong to welding samples that gave rise to defects during the initial phase and during set-up of the welding process parameters [4].
Observations with an optical microscope and a scanning microscope were carried out for the purpose of:
• Analysing the microstructure of the most significant areas of the welding bead, namely the thermally altered area (ZTA), the melted area (Z.F.), the base metal and the contact areas between the passes that follow.
• Searching for macro and micro defects, such as cracks and inclusions of gas or non-metallic particles located in the ZTA and the Z.F.

Also microhardness variations in the various welding bead areas were evaluated, starting from the base metal of one edge and arriving at the base metal of the opposite edge.

CASES EXAMINED

The work carried out concerned examining the samples distinguished by the types of microscopic and macroscopic defects that can occur during the laser welding process. This was done to ascertain their origin and their relations with the welding process in order to optimise the parameters of the process itself for each of the applications examined. The parameters used for welding the same components without causing defects to arise are given in table 1 [5, 6].

	Parent metal	Thickness [mm]	Power [kW]	Welding speed [m/min]	Heat input [kJ/m]	Wire feed rate [m/min]
1	Fe430B	12	10.3	0.9	618	--
2	Fe430B	12	12.5	0.9	750	3.75
3	Fe430B	15	I - 14.0	0.84	1110	1.60
			II - 12.1	0.96	680	630
4	Fe430B	25	I - 16.0	0.6	1440	1.75
			II - 13.1	0.6	1180	7.90
			III - 12.2	0.72	916	7.90
5	Fe510C	34	I - 15.35	0.60	1381	1.25
			II - 15.0	0.60	1350	7.0
			III - 15.0	0.60	1350	7.9
			IV - 13.0	0.48	1462	7.9
Table 1: Welding parameters used in production.

1. WELDED JOINT WITHOUT FILLER WIRE
The sample was obtained from butt welding two sheets having a thickness of 12 mm with straight edges (fig. 1).

Fig 1: Macrograph of the joined sheets of 12 mm thickness without filler wire.

The junction was made in a single pass without using filler wire. The sample's specifications and the final parameters used for the welding are given in table 1 under point 1. The base metal, Fe 430 B, is characterised by a ferritic-pearlitic structure with 30-35% pearlite. Areas of bainite and martensite were detected in the melted area and at the same time there was a presence of porosity. The pearlite's dissolution with small areas of bainite and martensite is instead evident in the ZTA.
The joint's microhardness profiles are given in figure 2.

Fig 2: Microhardness in weld and in parent metal.

The increases of hardness in correspondence with the ZTA and Z.F. can be associated with the presence of martensitic and bainitic structures detected in the microstructural analysis.
The bead has the characteristic nail-head shape, which is typical of laser welding.
The Z.F. has dendrites with a columnar type of growth in a direction that is at a right angle to the axis itself. The size of the dendrites decreases while moving in the direction of the apex due to the increase of the cooling speed.
The welding has an underthickness defect caused by the lack of material and by excessive evaporation during its interaction with the laser beam. There is a longitudinal crack starting from the bead's head that is caused by the solid external mass's poor contribution of heat and excessive thermal removal, which brings about rapid solidification of the melted metal and a consequent formation of the crack.
There is a gas inclusion in the intermediate part. Its formation is due to the high viscosity of the bead that results in incorporating the gas bubbles present during the solidification phase. Their origin is attributable to the presence of substances that can be easily vaporised on the welding edges and to the air contained in the gaps between the edges. These defects can be reduced by thoroughly cleaning the edges, with proper presentation of the edges to be welded and by modifying the working parameters to reduce or improve the metallic bead-agitation.
The inclusion of S and Ca was observed in the Z.F. at the centre of the bead (figure 3).

Fig 3: Inclusion of S and Ca observed at the center of the bead.


Its presence is attributable to the base metal's quality (high sulphur contents). To remedy this, you should be particularly careful in selecting a quality base metal (low sulphur and phosphorous content) and use of filler metal in order to reduce the content of impurities in the melted bead.
Cracks were not observed in the ZTA. In conclusion, the presence of uniformly distributed porosity was detected in the Z.F.

2. WELDED JOINT WITH FILLER WIRE
The sample was obtained from the butt welding of two sheets having a thickness of 12 mm (fig. 4) with straight edges. In comparison with the previous case, the power capacity was increased and the gap size was decreased from 0 to 0.5 mm.
The junction was made in a single pass by using filler wire having a 1.2-mm diameter and in WA2 material to try to eliminate, during a preliminary research stage, the longitudinal crack that showed up in the previous sample.
The sample displays the presence of a gas cavity near the apex of the welding.
The Fe 430 B base metal presents the same microstructure of the previous sample. The sample's features and the final parameters used for the welding are given in table 1 under point 2.
Metallographic observation detected a structure that is similar to the previous case. In the ZTA, instead, dissolution of the pearlite with small areas of bainite and martensite is evident.
The sample presents a macroscopic defect that is attributable to gas inclusion when the melted bead is re-mixed. Analysis of the same defect permits us to show that it involves only one cavity coming to the surface in various places. Analysis of the SEM showed uniformly distributed porosity in the Z.F. It was probably caused by the evaporation of some elements in the alloy, of uncontrolled agitation and the bead's viscosity that may be associated with an incorrect position of the welding head, by the presence of polluting substances that can easily be vaporised on the welding edges and by air and/or moisture trapped inside the gaps on the edges. The presence of a non-metallic inclusion made up of calcium sulphide was observed.

3. WELDED JOINT IN THREE PASSES
The sample was obtained from the butt welding of sheets having a thickness of 25 mm and straight edges (fig. 5).
The junction was made with three overlapping passes and by using WA2 filler metal in 1.2-mm diameter wire. The base metal is Fe 430 B. The sample's features and the final parameters used for the welding are given in table 1 under point 4.
The Z.F.'s structure is made up of pearlite with the presence of bainite and martensite. Furthermore, a ferritic-pearlitic structure with some tempered martensite was observed in the intersecting area between the two passes. The heat treatment's effect caused by the following passes on top of the previous ones is obvious. The dendrites show up as being particularly coarse in the second and third pass. The passes show a satisfactory alignment, which is a sign of a fair centring of the beam. For welding components having a long length, centring of the beam is done by using a joint follower made with a low-power laser to keep the beam centred on the other passes. Even when using beam-centring systems, you can verify slight maladjustments of the passes due to the large thickness. The sample shows a defect of incomplete penetration of the first pass. Also a longitudinal macro crack, found on the third pass's Z.F. axis and caused by poor heat contribution, is evident. The Z.F. of each pass shows numerous non-metallic inclusions of sulphides.

Fig 4: Macrograph of the joined sheets of 12 mm thickness using filler wire.

Fig 5: Macrograph of the joined sheets of 25 mm thickness using filler wire with three overlapping passes.

Fig 6: Macrograph of the joined sheets of 34 mm thickness using filler wire with four overlapping passes.

4. WELDED JOINT IN FOUR PASSES
The sample was obtained from the head junction of sheets having a thickness of 34 mm (fig. 6).
The welding was performed in four passes with WA2 filler wire with a 1.2-mm diameter. The base metal is Fe 510 C. The sample's features and the final parameters used for the welding are given in table 1 under point 5.
Compared to the previous trial, the type of base metal, thickness, and power and speed values were changed. Furthermore, the number of passes was increased from 3 to 4 in order to give the welding a greater thickness.
The four passes' Z.F. show a ferritic-pearlitic structure with more or less marked traces of martensite in quantities that slightly vary according to the various passes. In the third pass's Z.F. we also detect the presence of tempered martensite. In the ZTA, dissolution of the pearlite is shown, with some martensite and upper bainite. Finally, we observed the presence of tempered martensite, caused by the previous pass's heat treatment, in the passes' overlapping area. The sample shows a cavity, attributable to a gas inclusion, in correspondence with the apex of the fourth pass. The dendrites are particularly coarse in the second, third and fourth passes due to the lower cooling speed. The passes display satisfactory adjustment, which is the sign of a fair centring of the beam. The sample presents small cavities in correspondence with the apex of the fourth pass, and this is attributable to gas inclusions. The presence of small manganese sulphide inclusions in the melted area were detected, and their origin is attributable to the base metal. Uniformly distributed porosity was detected in the melted areas of all four passes, and with a more marked distribution at the apex of the first pass. Observation of the ZTA did not reveal the presence of cracks. We verified the onset of longitudinal cracks that involve the entire bead quite often in the first two passes. The causes are associated with the high heat contribution and rapid heat disposal caused by the different melted bead volume compared to the high thickness of the surrounding metal. A problem linked to this type of defect is the inspection bodies' requirement that there be no defects after each pass. The execution of later passes causes remelting of the area affected by cracks and the consequent elimination of them.

REFERENCES

1. E.A. Metzbower - Penetration depth in laser beam welding - Welding Journal, 72 (1993) 403s.
2. G. Sayegh - Trends and needs in industrial laser welding systems - ECLAT '94
3. M. Balbi, G. Caironi, M. Coffetti - Filler wire in Laser Welding Technology - Proc. Int. Conf. "Welding and Joining Science and Technology" A.S.M., Madrid 1997
4. M. Balbi, G. Caironi, M. Coffetti - Saldatura Laser multipass: aspetti relativi alla qualità - Atti Conferenza nazionale sulle prove non distruttive monitoraggio diagnostica, Padova 1997.
5. M. Balbi, G. Caironi, M. Coffetti - Saldatura Laser CO2 per componenti in acciaio di grosso spessore o di geometria complessa - ATTI Convegno Materiali Ricerca e Prospettive Tecnologiche alle soglie del 2000, Milano 1997.
6. M. Balbi, G. Caironi, M. Coffetti - Laser Welding in Ship Construction" Proceedings of Conference on Trends in Welding Research '98 - Pine Mountain Georgia

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