Effect of beam oscillation and focusing on the electron beam welded 1100M high strength structural steel joint

The development of ultra-high-strength steels (UHSS) is revolutionizing its need in the diverse field of application, and the main credit going to the new technological advancement in the manufacturing of steels using special heat treatment like the thermomechanical rolled process to produce UHSS with up to 1300 MPa tensile strength in the thicker plate. The steel provides higher strength, toughness, and extensive reduced weight, significant in structural components and higher load lifting vehicles (e.g., mobile cranes, trailers, concrete pump trucks, etc.) with lower fuel consumption. However, welding of UHSS is also a challenge due to their higher crack susceptibility and hardness reduction in the heat-affected zone (HAZ). The study’s main objective is to analyse the effect of beam focus on the weld geometry and the different beam oscillation on the weld microstructure and hardness behaviour of the 1100M EB-welded joint. The S1100M steel samples of the thickness of 15 mm were butt-welded by autogenous electron beam welding (EBW). The study finds that UHSS can be satisfactorily welded with EBW, improving the weld geometry with beam focus. The observed results show that beam oscillation’s microstructural and hardness behaviour was attributed to developing a more uniform microstructure.


Introduction
Because of growing environmental concern and sustainable development, the demand for lightweight materials has increased over the last decade, and steel manufacturers have shifted their focus to higher strength steels (HSS) and lighter weights with higher safety. Various kinds of high-strength structural steels (HSSS) have been developed through different technological and heat treatment processes [1]. In the case of HSSS, the available maximum strength steel with thicker plate is up to 1300 MPa. However, as tensile strengths increase, so does the complexity of the joining process, which leads welding experts and manufacturers to produce more efficient, productive, adaptable, smart, and novel welding technologies [2]. Therefore, it is very important to understand the weldability of these steels with different new innovative welding processes to understand the microstructural change and mechanical behaviour of the welded joints and heat-affected zone (HAZ). The HSSS with a higher tensile strength hardens more in the fusion zone (FZ) and especially in the coarse-grained heat-affected zone (CGHAZ).
The beam welding process like electron beam welding (EBW) technology finds more efficient methods in welding thicker high strength steel plates in a single pass rather than conventional arc welding processes which leads in reduction in welding time, quality, strength etc. [3,4]. Furthermore, it is essential to comprehend the beam welding process and its effect on the welded joint and HAZ in terms of its various characteristics (accelerating voltages Ua; beam current, Ib, and so on), as well as dynamic effects on the beam (beam oscillations, beam focus position etc.) etc. [5]. We often use static beam in the EBW process with optimal parameters for welding the different materials. However, the use of beam with various dynamic effects in the case HSSS changes the geometry of the weld and HAZ, proves many benefits in terms of microstructural and mechanical properties of the welded joints [6]. The use of dynamic effects aids in improving mechanical properties, obtaining uniform structures, particularly in the case of dissimilar materials welded joints, and avoiding defects such as porosity, cold and hot cracks, and so on [3,7,8]. The different beam oscillation methods were like circular, ellipsoidal, parallel etc. and the different focusses like sharp focus, over focussed, under focussed [7]. Several studies have been reported before on the effect of beam oscillation on the welded joint of different materials and various alloys, however, a very little or no research has been found with the HSSS.
In this paper, the main focus is given on the comparison of the effect of different beam oscillations and beam focusses on the weld geometry, weld microstructure and hardness behaviour of the S1100M.

Material properties
The base material (BM) used in the study was S1100M, thermomechanically rolled steel with thickness of 15 mm. The plates with the dimension of 150 × 50 mm were cut and the joining faces of the plates were machined precisely by milling machine to the maximum allowable airgaps of 0.20 mm along the weld joint to be produced to secure precise fit for the welding. The mechanical and chemical properties of the investigated BM are provided in the table 1 and table 2 respectively.

Experimental set-up
The welding was performed by using the electron beam welding robotized device PZ EZ 30 STU complex, (First Welding Company, Bratislava, Slovakia) equipped with two electron beam guns with the power of 30 kW for each gun, a vacuum chamber with dimensions of 1800 × 2360 × 3150 mm and volume of 13.4 m 3 and the pumping system allowing reaching the vacuum of 5 × 10 -2 Pa within 25 minutes, and the vacuum in an electron gun was 10 -5 Pa. The vacuum pressure in the chamber was 9.8 × 10 -5 Pa. The samples were cleaned in acetone and dried before performing the welding. Then the samples were placed on a workbench in the vacuum chamber. Jigs were used to prevent deformation. The work distance (WD) was 200 mm. The EBW trials were conducted to obtain the optimal parameters with the full depth penetration for the static beam. Then the experiments were done with different focuses and oscillation while the accelerating voltage (Ua) in kV beam current (Ib) in mA and welding velocity (v) in mm/s were kept constant. The calculated welding linear heat input was 0.462 kJ/mm. After chemical etching, the specimens were examined under a microscope to determine weld profile characteristics such as weld width, HAZ, and weld depth, etc. This was done to understand the effect of various process parameters on weld bead geometry. The hardness tests were performed according to EN  The hardness of the BM is 383 ±3. Figure 1 showed the macrohardness graphs for different beam oscillation, the macrohardness at the center of the FZ with beam circular oscillation (403 HV10) have nearly similar macrohardness in comparison with no oscillation (399 HV10) while with elliptical oscillation it was 410 HV10. The elliptical beam oscillation (sample 9) showed highest macrohardness in HAZ 450 HV10, no oscillation (sample 1) showed 441 HV10 while circular oscillation measured 436 HV10. The average FZ and HAZ macrohardness values are presented in the table 4. The use of circular beam oscillation in EBW of S1100M steel joints caused the nearly similar average hardness in FZ and slightly higher in HAZ compared to no oscillation.
The macroscopic examination of the welded cross section with various beam oscillations and without oscillation revealed that the width of FZ was greatest with elliptical oscillation (sample 9) and narrowest with circular oscillation (sample 6), with full depth of penetration achieved in all cases. Figure 2 shows a smoother transition from the top of the weld seam to the end of it with the circular oscillation (sample     Figure 3, S-1, without beam oscillation, depicts a typical very fine pine leaves pattern at the weld's centre. The influence of the circular beam oscillation pattern in the FZ microstructure is clearly shown in figure 3, S-6. The step solidification of the weld zone induced by the circular beam oscillation pattern increased the cooling rate. The weld zone hardened in the steps as the beam oscillation repeatedly melted the weld pool. Nucleation and grain growth began separately at each stage of solidification. Furthermore, when circular oscillation was used, the width of the parting zone in the centre of the welded joint was greater than when the joint was welded without beam oscillation. In case of elliptical oscillation pattern, the width of the weld across the thickness was not uniform but bulged at one side of the parting line and seems zigzag which can be seen from figure 2, S-9. Also, figure 2 shows weld faces and reverse beads formed without undercuts in S-1, S-6, and S-9. The fast cooling caused by the EBW process resulted in the FZ having an entirely martensitic structure.

Beam focus
The beam focus plays a significant role in the welding of thicker plate since it influences the forces in the molten pool. Incorrect selection of the focal points leads to the creation of undercuts on weld faces and reverse beads, also cavities in the weld seam. The optimal focus position in EBW in terms of maximizing weld depth is controlled by varying the focusing current, If [6]. The investigation of the focus position is made without oscillations. The focusing was on the surface and the focusing current (If) means the diameter of the beam for 900 -910 mA was 2 mm and for 920 -940 mA was 3 mm. It can be observed from the figure 5, that the optimum focus position for maximum weld depth and defect-  Table 5 shows the welding parameters used with various beam focus positions.    EBW is a specialised welding process technology with high energy power beam applications to obtain thicker welds without using filler materials in a single pass. The lower linear heat input in the process helps with a very narrow HAZ and FZ. As a result, the welded joint typically experiences very high heat transfer. Thus, due to this, the high peak temperatures do not show much effect on the grain size of the HAZ microstructure, and the very fine HAZ microstructure in UHSS usually increases the 6 hardness in excess due to the formation of martensite structure [9]. Therefore, it is very imperative to understand the hardness behaviour of the UHSS in HAZ and FZ with different dynamic effects of the EB-welding process. The measured weld bead geometry features in mm and macrohardness with different beam focusses are shown in table 6. Macrohardness results showed that FZ average hardness of the sharp focus (410 HV10), under focus (413 HV10) and over focus (412 HV10) sample are similar while the average hardness of HAZ is increasing from sharp focus (406 HV10) to over focus (428 HV10). It is worth noting that hardness values along the depth of the weld are varying and higher in the root for both sharp focus (S-1) and over focus (S-5) while it is decreasing in the case of under focus (S-3) and lowest at the root. It shows the inhomogeneous microstructure of the welded joint due to the non-uniform properties.
From the figure 5 and table 6, we can clearly observe that the sharp focus (S-1) has full depth of penetration, width of the HAZ and FZ is less than the under focused (S-3). However, the penetration of the under focused sample, S-3 is about 70% and sample, S-4 is about 86% of the complete thickness of the welded joint. This implies that the under focused are not giving full penetration while we are increasing the focusing current (can be seen from S-3 to S-4 in table 5) in under focused condition towards sharp focus (S-1), HAZ and FZ width decreasing and penetration increasing. In case of under focus, from sample S-3 and S-4, it can be observed that the one side of the top face bead showed undercut and cavity formation due to the process instability. However, in the case of over focus (S-5), full penetration has been achieved, while the width of top FZ is higher and width of top HAZ is lower compared to the sharp focus (S-1) but overall width is lower than the both sharp focus (S-1) and under focus (S-3 and S-4). The cross section of the weld profile is observed like funnel shaped and not smooth transition from top to middle (figure 5, S-5). The microstructure of the welds with different beam focus current are shown in Figure 6. The microstructural study of the welds clearly showed that welds have non-uniform microstructure which in conformance with variation in the hardness along the width and the depth of the weld. The width of the parting zone for over focus (S-5) along the centre line of the welded joint is greater than when the joint was welded with sharp focus and under focus current.

Conclusion
In this study, we observed that effect of beam oscillation, no oscillation, and different beam focus on the weld profile, weld bead geometry especially on width of HAZ and FZ, and depth of penetration for the UHSS EB-welded joint. Also, how it affects the microstructure of the FZ, hardness in the FZ and HAZ. The use of circular beam oscillation in EBW of S1100M steel joints caused the nearly similar average hardness in FZ and slightly higher in HAZ compared to no oscillation but overall width of the FZ and HAZ are narrower compared to the no oscillation because of step solidification which enhanced the cooling rate. Variation in beam focus affects the penetration depth in the weld, sharp focus and over focus showed full penetration while under focused showed partial penetration. The overall width of the HAZ and FZ for the over focus (S-5) is lowest among others. So, it can be concluded that the focusing affects the final weld geometry to the greater extent. The weld cross section geometry of the over focus shows a characteristic funnel shaped. The average hardness along the width of FZ are nearly similar for sharp focus, under focus and over focus while the HAZ average hardness increasing from sharp focus to over focus.