WELDING MOBILE PLATFORMS ELEMENTS MADE OF AHSS STEEL IN VIEW OF NEW REQUIREMENTS IN CIVIL ENGINEERING AND TRANSPORT

The developing automotive industry has prompted an increase in the demand for new welding technologies in civil engineering and transport that reduce the mass of mobile platforms mounted on vehicles and increase their operational range at the same time. The AHSS steels are the materials most commonly adapted for the production of motor vehicles due to their high tensile strength. Nonetheless, the joints created with their use are less strong from the native material. This article examined whether the application of micro-jet cooling would alter the mechanical properties of the welds. Based on the performed tests, joints with better quality that passed positively in the hardness and tensile strength evaluations were obtained. The presented research proves that the use of additional elements in the welding process significantly affects the quality of the obtained welds and brings satisfactory results with a greater repeatability than before.


INTRODUCTION
New trends prevailing in civil engineering, architecture and urban planning postulate spatial limitations of cities, creation of buildings in an appropriate scale and shaping of a strong urban centre. In accordance with these trends and with the ever-shrinking space available for development, constructors are obliged to design and erect higher and higher buildings. Their construction requires the use of modernised mobile platforms with an expanded operational range and a weight that remains unchanged [1,2].
The technology of constructing mobile platforms mounted on vehicles is constantly adapting in terms of meeting changing safety standards. The revised regulations on exhaust emissions in automotive vehicles require the application of additional components such as particulate filters, exhaust catalysts, elements of exhaust systems, for example, ejectors or assisting mufflers that significantly increase their total mass. In trucks, the difference may reach up to 200 kg. Requirements imposed upon mobile platforms are presented in Figure. 1. Fig. 1. Requirements imposed upon mobile platforms [3] Manufacturers of mobile platforms mounted on vehicles are constantly looking for new solutions that enable the reduction of the extra weight of these additional elements and elevation of the working range of these platforms. In order to increase the properties of platform structures, another goal is to reinforce their tensile strength [3][4][5][6][7][8].
The article aims to verify whether the use of modernised technology will allow the achievement of the above-mentioned objectives. It is assumed that the use of micro-jet cooling during the welding of mobile platform elements made of Advanced High Strength Steel (AHSS) will increase the mechanical properties of the joint [9, 10].

MATERIALS AND METHODS
The AHSS steels are repeatedly applied in civil engineering and transport due to their high tensile strength, max. up to 1700 MPa, high yield strength, about 1300 MPa and high 211. elongation point up to 20%. AHSS steels are used mainly for thin-walled constructions as their strength allows for the reduction of the overall weight of the structure [11,12]. The basic welding problem of this group of steels is that the strength of the produced joint is much lower from the native material, despite the steel's good technological properties such as weldability and machinability [13,14]. Steels from the AHSS group are considered as difficult to weld since the heat-affected zone (HAZ - Figure 2) is susceptible to welding cracks even when preheated to max. 120°C [1,11]. This is the effect of the martensitic structure of these steels as well as the great hardness of their material. Table 2 presents the chemical composition of some of the AHSS steels used for the construction of mobile platforms mounted on vehicles.  During the welding process of AHSS steels, a decrease of mechanical properties in HAZ, as well as an increased susceptibility to delayed cracking, may be observed. It is recommended to limit the linear energy during welding to the level of 5 kJ/cm. In order to improve the weldability of steel, it is proposed to introduce cooling which promotes martensitic transformation.
The best effects when welding AHSS steel lifts, and in particular thin-walled elements, are achieved in processes where shielding gases or mixtures based on argon and carbon dioxide are used. For MAG (Metal Active Gas) welding, CO2 or a mixture of Ar-CO2 are used. For TIG (Tungsten Inert Gas) welding with pure argon is adopted.
In order to improve the mechanical properties of welded joints made of AHSS steel, we decided to use MAG welding and the technology of micro-jet cooling. During the welding of mobile platform elements, the following micro-jet cooling parameters were adopted: -number of cooling nozzles: 1, -type of cooling medium: Ar, -pressure of the cooling medium: 0.6 MPa, -diameter of micro stream: 70 m, -distance of the micro-jet nozzle from the welded surface: 20 mm.
For tests, the DOCOL 1200M martensitic steel from the AHSS high strength steel group was selected due to its high strength properties. DOCOL 1200M is a material recommended for the construction of mobile platforms mounted on vehicles. It was expected to achieve an increased weld strength, better relative elongation as well as an increased content of finegrained ferrite and martensite without the separation of bainite. For this purpose, the MAG process in combination with micro-jet cooling was applied on the face side of the weld.
To obtain the most favourable structure and an optimal juncture of the selected material (DOCOL 1200M from the AHSS group) using the MAG process, the following elements were selected accordingly: electrode wires, shielding gases as well as micro-jet cooling parameters.
In order to assess the quality of welded joints visual tests, analysis of tensile strength, as well as structure verification, were applied.
During the tests the following parameters were adapted: -two types of shielding gases: CO2 and a mixture of 82% Ar + 18% CO2, -two electrode wires: Welding parameters for both electrode wires and both mixtures were identical: the diameter of the electrode wire was 1.0 mm, arc voltage 19 V, current of the welding 115 A, welding speed 300 mm/min, source of a direct current (+) at the electrode and the single-stitch weld. In all cases, the parameters of micro-jet cooling were at the same level: micro-jet gasargon, stream diameter 70 m and a gas pressure of 0.6 MPa.

RESULTS AND DISCUSSION
The first welding of mobile platform elements mounted on vehicles was completed without the use of micro-jet cooling. Figure 3 shows the method of constructing the joint. To assess further weldability of the mobile platform, elements shielding gases 82% Ar + 18% CO2 as well as both electrode wires (UNION X90 and UNION X96) were selected. The welding process was performed using a ceramic washer. The results of the produced connections are shown in Table 3  The use of micro-jet cooling delivered much better results. The method produced a better quality of welds, without any visible bumps. Afterwards, the welded elements of the platform were inspected for stretching. The strength of the connections was tested using the INSTRON 3369 testing machine. The following analyses were carried out: -strength of DOCOL 1200M steel without the use of micro-jet cooling, -strength of DOCOL 1200M steel with the use of micro-jet cooling.
Each test was repeated 3 times (samples 1, 2, 3). The results of the strength tests are presented in Figures 7 and 8 and in Tables 5 and 6.  The average Re value of the MAG welded joint was 449 MPa and the average Rm value was 746 MPa. As a result of micro-jet cooling used during the welding process, Re and Rm values increased accordingly to Re =552 MPa and Rm =890 MPa.
Thereafter, the microstructure analysis was performed. The microstructure of the crosssection of the weld with a visible martensitic structure is shown in Figures 9 and 10.  Figure 9 presents visible martensite, coarse ferrite and bainite. This is an unfavourable structure causing inferior plastic properties (relative elongation is at the level of 4%). Martensite and fine-grained ferrite are visible in Figure 10. This structure is more advantageous: allows obtaining of better plastic properties (with the relative elongation at the level of 6%), and elimination of cracks in the joint and in the heat affected zone.

CONCLUSIONS
Civil engineering and transport designate new requirements for the constructors of mobile platforms. A need for new welding technologies that significantly reduce the weight of mobile platforms mounted on vehicles and increase their operating range and lifting capacity at the same time is constantly growing.
Material that is increasingly used for mobile platforms manufacturing is the hard-to-weld AHSS steel. Its high strength is almost double from the strength of the welded joint. The relative elongation obtained in the previously used processes is low, which indicates poor plastic properties of the joint. Therefore, new solutions that aim to improve the weldability of thin-walled AHSS steel structures, increase the tensile strength and expand the relative elongation at the same time are being sought.
The increase of the operational parameters (such as load capacity and range) while maintaining the mass of mobile platforms mounted on vehicles can be obtained by modifying the AHSS steel welding technology. The research presented in this article proves that the use of micro-jet cooling enables production of better quality welds with much higher repeatability than before. This technology requires, however, further development and additional research. With the appropriate selection of technological parameters of MAG welding and micro-jet cooling (type of micro-jet gas), it will be possible to determine the welding criteria for various steel grades used in the construction of mobile platforms.