A Novel Method of Supporting the Laser Welding Process with Mechanical Acoustic Vibrations

materials

Article A Novel Method of Supporting the Laser Welding Process with Mechanical Acoustic Vibrations

Arkadiusz Krajewski 1,* , Grzegorz Klekot 2 , Marcin Cybulak 1 and Paweł Kołodziejczak 1

1 Faculty of Production Engineering, Warsaw University of Technology, 02-524 Warsaw, Poland; [email protected] (M.C.); [email protected] (P.K.) 2 Faculty of Automotive and Construction Machinery Engineering, Warsaw University of Technology, 02-524 Warsaw, Poland; [email protected] * Correspondence: [email protected]; Tel.: +48-22-849-5811

Received: 14 July 2020; Accepted: 14 September 2020; Published: 20 September 2020

Abstract: The research described in this article presents a new contactless method of introducing mechanical vibrations into the base material during CO2 laser welding of low-carbon steel. The experimental procedure boiled down to subjecting a P235GH steel pipe with a 60 mm diameter, 3.2 mm wall thickness and 500 mm length to acoustic signals with a resonant frequency during the welding process. Acoustic vibrations with a frequency of 1385, 110 and 50 Hz were introduced into the pipe along the axis and transversely from the outer surface. The obtained welds were then subjected to structural tests and Vickers hardness measurements. The results of comparative tests show the impact of such introduced vibrations on the granular structure of the welds, as well as on their microhardness in specific areas, such as the face, penetration depth and the heat-affected zone. The effectiveness of the proposed method of introducing vibrations in the scope of grain size and shape as well as changes in the hardness distribution in the obtained welds is demonstrated.

Keywords: laser welding; mechanical acoustic vibrations; structure properties

1. Introduction In recent years, increased interest among researchers in finding new ways of improving the structure [1,2] and mechanical properties of welded joints or coatings [3] applied by welding methods [4–7], as well as improving the quality of metal alloys [8,9], can be observed. The issue of improving the micro and macro structural properties can generally be interpreted as improving metallurgical weldability, which has often been a priority [10]. A careful observer of the literature on the subject will notice that the majority of research in which the authors used mechanical vibrations focuses on the effects obtained, which undoubtedly leads in many cases to the fragmentation of grains [11], changing their shape into more equiaxial forms [12,13], improving mechanical properties such as strength, impact strength or hardness [14–16]. Few of these studies attached significant importance to technical problems associated with the effective introduction of vibrations and their form [2,17], and even more so with other parameters such as frequency, amplitude or phase [1,2,12,18]. Some researchers, such as [1], investigated ultrasonic-assisted laser welding on stainless steel, which was carried out to find out the effect of ultrasonic energy input on the microstructure and the strength of the weld metal. It is well known that the energy of ultrasonic vibrations depends mainly on the amplitude. It is also known that the effective transfer of vibration energy is not easy and depends on the rigidity of the attachment of the vibration system to the welded object. In this case, however, it would be difficult to expect a homogeneous distribution of energy, as shown by the laser vibrometer used for measurements [1]. As could be expected, the energy value (amplitude) strongly depended on the vibration phase in the object [2,18] and the obtained results were conditioned by the occurrence of

Materials 2020, 13, 4179; doi:10.3390/ma13184179 www.mdpi.com/journal/materials Materials 2020, 13, 4179 2 of 18 an arrow or vibration node. Therefore, the use of such methods of introducing vibrations whose energy varies with the distance of the source of vibrations from the liquid metal pool does not guarantee repeatable conditions of the technological process [1,2,5,10,12,18]. On the other hand, full control of mechanical vibrations is only possible when they are introduced by a well-tested and rigidly mounted vibrating system [2,9,13–15,18–20] or modulation of the electric arc [21–23] or laser beam [3]. The improvement of structural features and mechanical properties is obtained in various zones of welds or coatings, i.e., in the weld itself and in the heat-affected zone. Nothdurft in [24] proves during welding dissimilar materials the width of the unmixed zone between them was reduced due to the micro-turbulence formed near the fusion boundary caused by the cavitation effect. With the use of ultrasonic laser excitation, the main crystallography textures in the weld metal are shifted, and the ratio of misorientation was significantly improved. Quality improvement is achieved in many ways. These may be actions aiming at the optimization of technological parameters of the used welding methods or additional procedures using heat treatment, hybrid methods and innovative ways of affecting the liquid metal pool with various physical factors, such as an electromagnetic field or mechanical vibrations. Of these, it is worth paying attention to the latter, despite the fact that they do not occur very often and do not unconditionally guarantee success in every case of welding, surfacing or remelting. The use of mechanical vibrations carries a number of problems and limitations, depending on how they are introduced into the basic material, and raises some manufacturers’ fears, caused mostly by ignorance or lack of access to complicated vibration generating devices, but also technical difficulties related to ensuring steady contact between the basic material and the vibrating system [2,18]. All this means that the manufacturer of welded structures may not decide to use mechanical vibrations as the factor that, when properly used, will allow them to achieve the assumed quality improvement. The application of mechanical vibrations during welding processes shows that satisfactory results can be achieved without the need for direct and reliable attachment of the vibrating system to the base material [2,12,18,25]. Vibrations can be introduced, for example, by means of laser beam pulses [3,17], controlled modulation of the electric arc or impact methods using mass collision energy transfer [6,25]. An interesting technical solution was used by researchers [26] during TIG welding of aluminum alloys. They used an ultrasonic waveguide in the form of a roller and by rotating it were able to maintain a constant distance of the contact point from the molten metal pool. This resulted in an increase in hardness of over 8%, and an increase in tensile strength by almost 30%. The contact method of delivering ultrasonic vibrations in underwater welding in [27] also caused changes in mechanical properties. (increase in average hardness, toughness and tensile strength, grain refinement and reduction of the width of the boundaries between ferrite grains). This effect was achieved by the solid contact of the waveguide with the welded material. One of contactless methods is presented in this article. It is interesting because vibrations are introduced by the acoustic route, meaning that, first of all, this process is contactless, and secondly, it provides many possibilities of modification of the parameters characterizing mechanical vibrations, such as frequency, amplitude or distance from the welding heat source. Among the research works, there are various technical solutions with specific methods of introducing vibrations into objects subjected to welding processes. These include vibrations generated by tables on which the base material is mounted [7,16], or, during arc welding, vibrations that are carried by a consumable electrode [5,13,22]. All methods of supplying vibrations to the base material are worth attention, although not all of them can be used in any conditions of access to the material or when the distance from the heat source is too close to allow it. There are some original technical solutions regarding the introduction of vibrations by various methods, which are the subject of patent studies [6,17], but there are no such ones that provide the possibility of introducing vibrations without providing direct contact and fixing the vibrating system with the base material, which is a major impediment in industrial conditions. The impact of vibrations through carriers not related to the basic material gives a chance to avoid problems related to the proximity of the heat source and to simplify the technical process of Materials 2020, 13, 4179 3 of 18 introducingMaterials 2020, 13 vibrations., x FOR PEER Therefore, REVIEW it is important to propose such a method of introducing vibrations3 of 19 that does not require direct contact with the base material. One such method was used in the research the research described in this article. It allows for easy application in industry and for the economical described in this article. It allows for easy application in industry and for the economical conditioning conditioning of structures obtained in welding processes. of structures obtained in welding processes.

2. Materials and Methods The study study used used a a P235GH P235GH steel steel pipe pipe according according to toPN PN-EN‐EN 10217 10217-2‐2 with with a diameter a diameter of 60 of mm, 60 mm,wall wallthickness thickness 3.2 mm 3.2 and mm length and length 500 mm 500 (the mm composition (the composition and properties and properties of this steel of this are steellisted are in Table listed 1). in Table1). Table 1. Chemical composition and properties of P235GH steel according to PN‐EN 10204. Table 1. Chemical composition and properties of P235GH steel according to PN-EN 10204. C [%] Mn [%] Si [%] P [%] S [%] Cu [%] Cr [%] Ni [%] Al [%] Alsol [%]

0.11C [%] Mn0.42 [%] 0.016Si [%] 0.008P [%] 0.006S [%] Cu0.02 [%] Cr [%]0.02 Ni [%]0.01 Al0.03 [%] Alsol0.025[%] V [%]0.11 Mo0.42 [%] Nb0.016 [%] Co0.008 [%] Ti0.006 [%] As0.02 [%] 0.02N2 [%] 0.01Ca [%] Pb0.03 [%] 0.025Sn [%] V [%] Mo [%] Nb [%] Co [%] Ti [%] As [%] N2 [%] Ca [%] Pb [%] Sn [%] 0.0010.001 0.0020.002 ‐ - 0.0020.002 0.0010.001 0.0010.001 0.0070.007 0.00260.0026 0.0010.001 0.0010.001 Zn [%] W [%] B [%] Zr [%] CEV C[%]EV Re [MPa]Re Rm [MPa]Rm A5 [%] OO [%] [%] - HH22 [%][%] - Zn [%] W [%] B [%] Zr [%] A5 [%] 0.0012 0.001 0.0002 0.0006 0.19[%] 311[MPa] [MPa]408 45.6 ‐ ‐ 0.0012 0.001 0.0002 0.0006 45.6 0.19 311 408 Before the welding tests were performed, the resonant frequency of the pipe was determined Before the welding tests were performed, the resonant frequency of the pipe was determined along its axis (x) and in the transverse direction (y). During the measurement, the pipe was hung, along its axis (x) and in the transverse direction (y). During the measurement, the pipe was hung, and and the measuring sensors were attached halfway along its length and at one of its ends. A hammer the measuring sensors were attached halfway along its length and at one of its ends. A hammer drill, drill, three piezoelectric accelerometers, and PULSE analyzer environment with the Bruel and Kjaer three piezoelectric accelerometers, and PULSE analyzer environment with the Bruel and Kjaer type type 3050-B-060 module (Bruel and Kjaer, Naerum, Denmark) were used for the tests (Figure1). 3050‐B‐060 module (Bruel and Kjaer, Naerum, Denmark) were used for the tests (Figure 1).

Figure 1. TheThe measuring measuring circuit circuit used used to determine to determine the theresonant resonant frequency frequency of the of pipe: the pipe:1—pipe 1—pipe (PM), (PM),2—Bruel 2—Bruel and Kjaer and 3050 Kjaer‐B 3050-B-060‐060 module, module, 3—Bruel 3—Bruel and Kjaer and PULSE Kjaer PULSE Labshop, Labshop, 4—hammer 4—hammer drill, 5–7 drill,— 5–7—vibrationvibration sensors, sensors, 8—two 8—two suspension suspension ropes ropes..

In order to test the natural frequency, thethe pipe was hung looselyloosely on a thin rope, i.e., in conditions of the minimum number of ties, in isolation from friction (Figure1 1),), and and was was stimulated stimulated by by impulse impulse stroke. Then, Then, the the pipe pipe vibrations vibrations were were recorded recorded in two mutually perpendicular directions in the frequency rangerange fromfrom 00 to 6 kHz. The The methodology methodology of vibration generation using the acoustic method and thethe devices, devices, sensors sensors and and software software used used are describedare described in more in detailmore indetail [28,29 in]. In[28,29]. order toIn eliminateorder to theeliminate instability the instability of most arc of weldingmost arc methodswelding methods during vibration during vibration process, process, it was decided it was decided to use a to laser use source,a laser source, which, which, by its nature, by its nature, guarantees guarantees the stable the andstable repeatable and repeatable welding welding of the of substrates the substrates in these in conditions.these conditions. Welding Welding tests were tests carried were outcarried in a ﬂatout position in a flat parallel position to theparallel pipe axis.to the Weld pipe penetrations axis. Weld werepenetrations obtained: were Without obtained: the participationWithout the ofparticipation mechanical vibrations,of mechanical and vibrations, with the participation and with the of participation of acoustically generated mechanical vibrations. A CO2 laser with fast longitudinal flow of VFA2500 type from Wegmann–Baasel (Aschheim, Germany) was used for welding. It can work in pulse or constant mode in the power range from 100 to 2500 W. The radiation wave generated by the

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acoustically generated mechanical vibrations. A CO2 laser with fast longitudinal ﬂow of VFA2500 type from Wegmann–Baasel (Aschheim, Germany) was used for welding. It can work in pulse or constant mode in the power range from 100 to 2500 W. The radiation wave generated by the laser is 10.6 µm long. A mixture of CO2,N2 and He gases was used as protection. In the experiments, a helium blast in the beam axis was used with a capacity of 10 L/min through a nozzle 4 mm in diameter spaced from the parent material surface by 5 mm. A beam of coherent light was used, which was 1 mm in diameter on the surface of the workpiece and in the TEM 10 mode. As the focus (d = 0.4 mm) of the beam fell above the surface, it was impossible to obtain a steam channel. This was dictated by the desire to obtain Materials 2020, 13, x FOR PEER REVIEW 5 of 19 a larger size of the metal pool, thus allowing for easier observation of changes in its structure after crystallization.Table 2. Welding As a result, parameters linear and welds specification with a length regarding of about the 150introduced mm arranged vibrations: parallel f—acoustic to the pipe axis onfrequency its outer [Hz], surface P— werepower obtained: of the laser Weld [W], No. v— 1,welding with vibrations speed [m/min], at a frequency direction of 50of Hzvibrations introduced transverselyintroduction to the according axis and to perpendicularFigure 2a–c. to the surface of the pipe; weld No. 2, with vibrations at a frequency of 50 Hz introduced along the pipe axis to its interior; weld No. 3, with vibrations at a frequencyWeld of 1385 No. Hz introducedF [Hz] alongP [W] the pipeV [m/min] axis to its interior;Direction weld of No. Vibration 4, reached without vibrations; and1 weld No. 5, with50 vibrations1150 at a frequency0.5 of 110 Hz introducedtransverse along the pipe axis to its interior. Figures2 3 and 4 show50 the1150 two ways of introducing0.5 acousticlongitudinal vibrations—transversely and longitudinally—in3 relation1385 to the pipe1150 axis, respectively.0.5 Laser weldinglongitudinal parameters and speciﬁcations regarding the introduced4 ‐ vibrations are1150 presented in0.5 Table ‐ 2. 5 110 1150 0.5 longitudinal

(a) (b)

(c)

Figure 2. Introduction of acoustic vibrations transversely to the axis of the pipe, perpendicular to its surface ( a),), along along the the axis axis of of the the pipe, pipe, into into its its interior interior (b), (b area), area of welding of welding (c): (1c—):pipe 1—pipe dimensions, dimensions, 2— 2—BruelBruel and and Kjaer Kjaer type type 1023 1023 harmonics harmonics generator, generator, 3— 3—thethe 2734 2734 amplifier, ampliﬁer, 4 4—Bruel—Bruel and and Kjaer Kjaer type 4295 sound source, 5—Bruel5—Bruel andand KjaerKjaer typetype 22362236 m,m, 6—laser6—laser head.head.

For the generation of acoustic vibrations, the Bruel and Kjaer type 10231023 harmonicsharmonics generator with the 2734 ampliﬁeramplifier (Bruel and Kjaer, Naerum,Naerum, Denmark)Denmark) waswas used,used, as well as the Bruel and Kjaer type 4295 sound source. The Bruel and Kjaer type 2236 m was used to measure the sound pressure level. In welding tests, sound signals with a constant sound pressure level of approximately 100 dB (e(effectiveﬀective valuevalue ofof thethe acousticacoustic pressurepressure amplitudeamplitude isis aboutabout 2 2 Pa) Pa) were were used. used. After performing experimental welding tests, five longitudinal welds were obtained, which were subjected to comparative metallographic tests and Vickers hardness measurements at a load of 100 g. Metallographic tests of the samples were carried out with an OLYMPUS BX51M optical microscope (Olympus Optical Co., Ltd., Tokyo, Japan) with Stream Essential v. 2.3 software (, Olympus Optical Co. Ltd, Tokyo, Japan) and an electron microscope JEOL JSM‐7600F (JEOL Ltd., Tokyo, Japan) to obtain images on the cross‐section of welds and the surface distribution of elements.

3. Results Tests of the geometry, structure and properties of the obtained welds were preceded by determining the resonance frequency of the welded object, which was the pipe. Then, optical and electron microscopy was performed, and granular structure analysis, weld geometry measurements

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Table 2. Welding parameters and speciﬁcation regarding the introduced vibrations: f—acoustic frequency [Hz], P—power of the laser [W], v—welding speed [m/min], direction of vibrations introduction according to Figure2a–c.

Weld No. F [Hz] P [W] V [m/min] Direction of Vibration 1 50 1150 0.5 transverse 2 50 1150 0.5 longitudinal 3 1385 1150 0.5 longitudinal 4 - 1150 0.5 - 5 110 1150 0.5 longitudinal

After performing experimental welding tests, ﬁve longitudinal welds were obtained, which were subjected to comparative metallographic tests and Vickers hardness measurements at a load of 100 g. Metallographic tests of the samples were carried out with an OLYMPUS BX51M optical microscope (Olympus Optical Co., Ltd., Tokyo, Japan) with Stream Essential v. 2.3 software (Olympus Optical Co. Ltd, Tokyo, Japan) and an electron microscope JEOL JSM-7600F (JEOL Ltd., Tokyo, Japan) to obtain images on the cross-section of welds and the surface distribution of elements.

3. Results

MaterialsTests 2020 of, 13 the, x geometry,FOR PEER REVIEW structure and properties of the obtained welds were preceded by determining6 of 19 the resonance frequency of the welded object, which was the pipe. Then, optical and electron andmicroscopy elemental was concentration performed, distribution and granular were structure examined. analysis, The results weld geometryof these studies measurements are presented and below.elemental concentration distribution were examined. The results of these studies are presented below.

3.1. Resonance Resonance Frequency Frequency Determining The ﬁrstfirst important important research research step step was towas determine to determine the resonance the resonance frequency frequency of the pipe of undergoing the pipe undergoingthe welding the process. welding Stimulation process. withStimulation vibrations with in vi thebrations resonance in the area resonance results in area a high results amplitude in a high of amplitudevibrations, aof particular vibrations, gain a occursparticular when gain the forcedoccurs frequency when the is equalforced to frequency the characteristic is equal resonance to the characteristicfrequency of the resonance object. Infrequency the case ofof welding,the object. the In e ﬀthecte case of using of welding, vibrations the witheffect a of resonant using vibrations frequency withmay nota resonant give the bestfrequency result inmay terms not of give structure the best and result mechanical in terms properties. of structure However, and determiningmechanical properties.the resonance However, frequency determining will be treated the resonance as a reference frequency point in will these be studies.treated as The a obtainedreference results point in of thesethe experiment studies. The carried obtained out to results determine of the the experime naturalnt vibrations carried out are to presented determine by the the natural vibrations vibrations spectra arein Figure presented3a,b. by the vibrations spectra in Figure 3a,b.

(a) (b)

Figure 3. The spectrum of vibration acceleration: (a) x-axis and (b) y-axis depending on the frequency.

Analyzing the course of the graphs of the natural vibrations amplitude for the vibrations frequency value, it can be clearly seen that the resonant frequency determined on the basis of vibrations in the x- and y-axis is about 1385 Hz.

3.2. Shape Factors of Welds

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3.2. Shape Factors of Welds Geometry parameters were performed on cross-sections of the obtained weld penetrations in half of their length. A magnification of 200 was used to show significant differences in the × microstructure of individual joint penetration areas. Metallographic images of welds allowed us to assess the width of the face B, depth of penetration H and allowed to calculate the shape factor B/H. All listed geometrical features were determined based on three samples obtained with a specific set of technological parameters and are presented in Table2, while shape parameters are presented in Table3 and Figure4. The results regarding the shape factors are the arithmetic mean of the three cross sections. In the course of metallographic tests, the differences in the measured values were within the range of +/ 10 µm − Table 3. Parameters during tests. f—vibrations frequency, P—power of the laser, Direction—direction of vibrations introduction, B—face width, H—depth of penetration, B/H—weld shape factor.

Weld No. F [Hz] P [W] Direction B [µm] H [µm] B/H Materials 2020, 13, x FOR PEER REVIEW 7 of 19 1 50 1150 transverse 619 201 3.08 2 50 1150 longitudinal 505 238 2.12 33 1385 1385 1150 1150 longitudinal longitudinal 603603 196196 3.083.08 44 ‐ -1150 1150 ‐ - 762762 162162 4.704.70 5 110 1150 longitudinal 568 216 2.63 5 110 1150 longitudinal 568 216 2.63

(a) (b) (c)

Figure 4. Geometrical factorsfactors of of ﬁve five obtained obtained welds: welds: (a )(a width) width of faceof faceB,( Bb,) ( depthb) depth of weld of weldH,(c H) shape, (c) shape factorfactor B/H.

ItIt is noteworthy that that in general, with the the use of vibrations, the the width of the face decreases in relation toto weldingwelding attempts attempts without without the the use use of of vibrations. vibrations. This This eﬀ ecteffect is most is most clearly clearly visible visible in the in case the caseof vibrations of vibrations with with a frequency a frequency of 50 of Hz 50 introduced Hz introduced along along the axis the of axis the of pipe the (weld pipe No.(weld 2). No. The 2). depth The depthof penetration of penetration in this in case this increases case increases the most. the Onemost. can One also can observe also observe a correlation a correlation with the withB/ Htheshape B/H shapefactor, factor, which reacheswhich reaches the minimum the minimum in the test in No. the 2. test An No. interesting 2. An interesting observation observation is that the introduction is that the introductionof vibrations of with vibrations a frequency with ofa frequency 50 Hz transversely of 50 Hz transversely to the axis of to the the pipe, axis whichof the tookpipe, place which in took test placeNo. 1, in caused test No. an 1, increase caused an in Bincreaseand a decrease in B and a in decreaseH in relation in H in to relation weld No. to 2.weld This No. indicates 2. This indicates a greater ae ﬃgreaterciency efficiency of vibrations of vibrations introduced introduced longitudinally longitudinally to the welding to the direction. welding direction.

3.3. Optical Microcopy, Grain Analyses and Hardness Distribution The structures of the obtained welds were compared at 200 × magnification, which allowed us to evaluate the specifics of the grain structure, their directivity and size in accordance with ASTM E 112‐13. Figure 5a,c,e,g,i,k contain photos of representative structures of the basic material and those obtained after welding using a laser with the effect of acoustic vibrations of different frequencies and without the use of vibrations. As a result of using the Stream Essential v. 2.3 software, the distribution of grains of a given size was determined and the results are shown in Figure 5b,d,f,h,j,l.

(a) (b)

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3 1385 1150 longitudinal 603 196 3.08 3.3. Optical Microcopy, Grain Analyses and Hardness Distribution 4 ‐ 1150 ‐ 762 162 4.70 The structures of the obtained welds were compared at 200 magnification, which allowed us 5 110 1150 longitudinal 568× 216 2.63 to evaluate the specifics of the grain structure, their directivity and size in accordance with ASTM E 112-13. Figure5a,c,e,g,i,k contain photos of representative structures of the basic material and those obtained after welding using a laser with the effect of acoustic vibrations of different frequencies and without the use of vibrations. As a result of using the Stream Essential v. 2.3 software, the distribution of grains of a given size was determined and the results are shown in Figure5b,d,f,h,j,l. The grain size test was carried out near the face of the obtained joints in the area of a rectangle with sides 105/90 µm, which was marked in the micrographs in Figure5. The maximum of the grain size curve in the case of weld No. 2 (obtained as a result of the impact of vibrations with a frequency of 50 Hz introduced(a) longitudinally into the inside of(b the) tube) is shifted towards the smaller(c) grains, and thereforeFigure the 4. largerGeometrical grain factors size number. of five obtained The course welds: of the(a) width remaining of face curves B, (b) obtaineddepth of weld for welds H, (c) No.shape 1 and 5factor in Figure B/H. 5m is very similar and no major di fferences can be found. Hardness was tested on the cross-sections of welds using the Vickers method at a load of 100 g and, It is noteworthy that in general, with the use of vibrations, the width of the face decreases in as was the case with shape parameters before, one point on the graph was obtained as the arithmetic relation to welding attempts without the use of vibrations. This effect is most clearly visible in the mean of three measurements. The results obtained are shown in Figure6. case of vibrations with a frequency of 50 Hz introduced along the axis of the pipe (weld No. 2). The In the case of weld No. 4 (Figure6a) obtained without vibration support, a significant increase in depth of penetration in this case increases the most. One can also observe a correlation with the B/H hardness can be observed in the area of the heat-affected zone at a depth of 50 µm (approximately shape factor, which reaches the minimum in the test No. 2. An interesting observation is that the 325HV0.1) and a slight increase in the axis of weld metal (250HV0.1). The results of the hardness introduction of vibrations with a frequency of 50 Hz transversely to the axis of the pipe, which took measurement along the weld axis (Figure6b) at a depth of about 100 µm showed a hardness of about place in test No. 1, caused an increase in B and a decrease in H in relation to weld No. 2. This indicates 300HV0.1. a greater efficiency of vibrations introduced longitudinally to the welding direction. Weld No. 1 (Figure6c), which was obtained with a vibration of 50 Hz that was introduced transversely in the heat-affected zones at a depth of 50 µm, did not exceed 240HV0.1, and in the weld 3.3. Optical Microcopy, Grain Analyses and Hardness Distribution metal center, the hardness oscillates around the value of 200HV0.1. Measurements along the weld axis (FigureThe6d) structures at a depth of of the approximately obtained welds 200 wereµm showcompared an increase at 200 to× magnification, 350HV0.1. which allowed us to evaluateWeld No. the 2specifics (Figure 6ofe), the obtained grain structure, with the usetheir of directivity a 50 Hz vibration and size introduced in accordance longitudinally with ASTM in E 112the‐ heat-a13. Figureffected 5a,c,e,g,i,k zones at acontain depth photos of 50 µm, of oscillatesrepresentative around structures 245HV0.1, of andthe basic in the material weld metal and center, those obtainedthe hardness after does welding not exceed using a 270HV0.1. laser with Measurementsthe effect of acoustic along vibrations the weld axis of different (Figure 6frequenciesf) at a depth and of withoutapproximately the use 200 of vibrations.µm showed As an a result increase of using to 350HV0.1. the Stream Essential v. 2.3 software, the distribution of grains of a given size was determined and the results are shown in Figure 5b,d,f,h,j,l.

(a) (b)

Figure 5. Cont.

Materials 2020, 13, 4179 8 of 18 Materials 2020, 13, x FOR PEER REVIEW 8 of 19

(e) (f)

(g) (h)

(i) (j)

Figure 5. Cont. Materials 2020, 13, 4179 9 of 18 Materials 2020, 13, x FOR PEER REVIEW 9 of 19

(k) (l)

(m)

Figure 5. Representative structurestructure and and percentage percentage grain grain distribution distribution of of the the rectangle rectangle area area 05 /05/9090 µm μ of:m (of:a,b ()a, parentb) parent material—average material—average grain grain size size 11.58, 11.58, (c ,(dc,)d face) face of of weld weld No. No. 4 4 without without vibration—averagevibration—average grain sizesize numbernumber 11.02, 11.02, ( e(,ef,)f face) face of of weld weld No. No. 1 (501 (50 Hz Hz transverse)—average transverse)—average grain grain size size 11.16, 11.16, (g,h )( faceg,h) offace weld of weld No. No. 2 (50 2 (50 Hz Hz longitudinal)—average longitudinal)—average grain grain size size 11.26, 11.26, (i ,(ji),j face) face of of weld weld No. No. 3(13853(1385 Hz longitudinal)—average grain sizesize 11.12, ((k,,ll)) faceface of weld No. 5 (110 Hz longitudinal)—average grain size 10.80, (m) summary chart.

WeldThe grain No. 3size (Figure test was6g), carried obtained out with near the the use face of of 1385 the Hz obtained vibrations joints introduced in the area longitudinally of a rectangle inwith the sides heat-a 105/90ffected μm, zones which at was a depth marked of 50 in µthem, micrographs reached about in Figure 300HV0.1, 5. The while maximum in the weldof the metalgrain centersize curve (Figure in the6h), case the of hardness weld No. changed 2 (obtained quite as significantly a result of fromthe impact about of 160HV0.1 vibrations at awith depth a frequency of 50 µm to,of 50 at Hz a distance introduced of 120 longitudinallyµm from the into face, the approximately inside of the tube) 110HV0.1, is shifted and towards then at the 200 smallerµm it reached grains, approximatelyand therefore the 245HV0.1. larger grain size number. The course of the remaining curves obtained for welds No. 1In and the 5 case in Figure of weld 5m No. is very 5 (Figure similar6 i),and obtained no major with differences the use can of vibrations be found. with a frequency of 110 HzHardness introduced was longitudinallytested on the cross in the‐sections heat-aff ofected welds zones using at athe depth Vickers of 50 methodµm, it oscillatedat a load of around 100 g 250HV0.1,and, as was while the incase the with weld shape metal axisparameters (Figure 6before,j), the hardness one point fluctuated on the graph quite significantly was obtained from as thethe valuearithmetic of about mean 200HV0.1 of three atmeasurements. a depth of 50 µThem to results approximately obtained are 110HV0.1 shown at in a Figure distance 6. of 200 µm from the face, and then to approximately 280HV0.1 at 200 µm. In the case of the weld No. 1, which was obtained with the assistance of vibrations with a frequency of 50 Hz supplied from the outside, perpendicular to the pipe surface, the highest hardness value was recorded at a depth of 200 µm (350HV0.1). This may be the result of the direct impact of cyclic changes in acoustic pressure on the molten metal pool. This process could increase the cooling rate and affect the argon shield at the welding site.

Materials 2020, 13, 4179 10 of 18 Materials 2020, 13, x FOR PEER REVIEW 10 of 19

(a) (b)

(e) (f)

(g) (h)

Figure 6. Cont.

Materials 2020, 13, 4179 11 of 18 Materials 2020, 13, x FOR PEER REVIEW 11 of 19

(i) (j)

FigureFigure 6. 6. HardnessHardness distribution: distribution: Close Close to the to the weld weld face face line line (a,c, (ea,g,c,,ie, ,andg,i), in and the in center the center of weld of at weld depth at directiondepth direction (b,d,f,h (b,j).,d ,f,h,j).

3.4. SEMIn the and case EDX of weld Examination No. 4 (Figure 6a) obtained without vibration support, a significant increase in hardnessIn an attempt can be observed to elucidate in the the area changes of the inheat structure‐affected due zone to at the a depth impact of 50 of μ acousticm (approximately vibrations, 325HV0.1)SEM and EDX and testsa slight were increase conducted. in the The axis most of interestingweld metal results (250HV0.1). are presented The results in Figure of the7. hardness measurementInteresting along changes the weld in the axis structure (Figure 6b) were at a observed depth of about by SEM 100 images μm showed taken a of hardness the face of area about of 300HV0.1.the weld center (Figure7). At a magnification of 2500 times, the biggest di fference can be seen in the faceWeld zone’s No. 1 structure (Figure between6c), which the was weld obtained reached with within a vibration vibration of and 50 the Hz corresponding that was introduced area in transverselythe welds obtained in the heat with‐affected the support zones of at acoustica depth of vibrations 50 μm, did with not a exceed frequency 240HV0.1, of 1385 and Hz in introduced the weld metalaxially. center, In other the cases, hardness the di oscillatesfferences around are less noticeable,the value of although 200HV0.1. they Measurements do occur. It is clearalong here the thatweld a axisdistinctive (Figure area 6d) at resembling a depth of an approximately imbalanced structure 200 μm show was createdan increase that to does 350HV0.1. not occur in other cases. The SEMWeld studies No. 2 (Figure in Figure 6e),7 a,c,e,g,iobtained show with the the structures use of a 50 in Hz weld vibration center introduced areas, and longitudinally those in the face in thezone heat are‐affected shown inzones Figure at a7 b,d,f,h,j.depth of 50 It can μm, be oscillates noticed around here that 245HV0.1, when using and acousticin the weld vibrations metal center, with thea resonance hardness frequencydoes not exceed of 1385 270HV0.1. Hz during Measurements welding, the along crystallites the weld are axis clearly (Figure arranged 6f) at in a depth different of approximatelydirections. The 200 same μm happens showed an when increase using to vibrations 350HV0.1. with a lower frequency of 50 Hz introduced transverselyWeld No. and 3 (Figure axially. 6g), When obtained using with of acoustic the use vibrations of 1385 Hz introduced vibrations axially introduced with longitudinally a frequency of in110 the Hz, heat the‐affected change is zones less pronounced. at a depth of 50 μm, reached about 300HV0.1, while in the weld metal centerAs (Figure a result 6h), of the laser hardness welding changed of the pipe, quite a significantly thin layer of from iron, about silicon, 160HV0.1 aluminum at a and depth manganese, of 50 μm  to,about at a 10 distanceµm thick, of forms120 μ onm from the surface. the face, These approximately are probably 110HV0.1, separations and of alloythen componentsat 200 m it of reached melted approximatelysteel. In Figure 8245HV0.1., you can see a comparison of the structure of the obtained layers in different samples. It is clearIn the that case the of faceweld of No. weld 5 (Figure No. 4 (Figure6i), obtained8a), reached with the with use a of vibration-free vibrations with laser, a frequency is characterized of 110 Hzby aintroduced lack of structure longitudinally orientation, in the unlike heat‐affected the other zones cases. at Ina depth the case of 50 of μ weldm, it No. oscillated 1 (Figure around8b), 250HV0.1,fused with while vibration in the with weld a frequency metal axis of (Figure 50 Hz introduced 6j), the hardness transversely, fluctuated and inquite the casesignificantly of weld No.from 5, theobtained value withof about the longitudinal 200HV0.1 at introduction a depth of 50 of μ vibrationsm to approximately with a frequency 110HV0.1 of 110 at Hz, a distance a clear decreaseof 200 μm in fromthe silicon the face, concentration and then to in approximately the face can be 280HV0.1 observed. at The 200 case μm. of weld No. 2 obtained with a vibration of 50In Hz the (Figure case 8ofc) the introduced weld No. longitudinally 1, which was shows obtained that wewith are the dealing assistance with aof clear vibrations fragmentation with a frequencyof precipitates of 50 withHz supplied a visible from orientation the outside, here. perpendicular This orientation to the of pipe the precipitatessurface, the highest perpendicular hardness to value was recorded at a depth of 200 μm (350HV0.1). This may be the result of the direct impact of the surface of the pipe is most apparent in the case of weld No. 3, obtained with the introduction of cyclic changes in acoustic pressure on the molten metal pool. This process could increase the cooling vibrations with a resonance frequency of 1385 Hz (Figure8e). rate and affect the argon shield at the welding site.

3.4. SEM and EDX Examination In an attempt to elucidate the changes in structure due to the impact of acoustic vibrations, SEM and EDX tests were conducted. The most interesting results are presented in Figure 7.

Materials 2020, 13, x FOR PEER REVIEW 12 of 19

Materials 2020, 13, 4179 12 of 18

10 μm 10 μm

(a) (b)

10 μm 10 μm

(e) (f)

Figure 7. Cont.

Materials 2020, 13, 4179 13 of 18 Materials 2020, 13, x FOR PEER REVIEW 13 of 19

10 m 10 m

(g) (h)

10 m 10 m

(i) (j)

FigureFigure 7. 7. SEMSEM images images (2500 (2500 ×): ):in in the the face face zone—( zone—(a) aweld) weld No. No. 4; ( 4;c) (weldc) weld No. No. 1; (e 1;) weld (e) weld No. No. 2; (g 2;) × weld(g) weld No. No. 3; (i 3;) weld (i) weld No. No. 5 and 5 and in the in thecenter center of welds—( of welds—(b) weldb) weld No. No. 4; (d 4;) (weldd) weld No. No. 1; (f 1;) weld (f) weld No. No. 2; (2;h) ( hweld) weld No. No. 3; ( 3;j) (weldj) weld No. No. 5. 5.

InterestingThe results changes of the SEM in the and structure EDX examination were observed of the by surface SEM images layer oftaken the of obtained the face welds area of show the weldspeciﬁc center diﬀ (Figureerences 7). in At the a morphologymagnification of of the 2500 joints times, fused the biggest with a difference laser and can subjected be seen to in acousticthe face zone’svibrations. structure This between is shown the in Figure weld reached8. within vibration and the corresponding area in the welds obtained with the support of acoustic vibrations with a frequency of 1385 Hz introduced axially. In other cases, the differences are less noticeable, although they do occur. It is clear here that a distinctive area resembling an imbalanced structure was created that does not occur in other cases. The SEM studies in Figure 7a,c,e,g,i show the structures in weld center areas, and those in the face zone are shown in Figure 7b,d,f,h,j. It can be noticed here that when using acoustic vibrations with a resonance frequency of 1385 Hz during welding, the crystallites are clearly arranged in different directions. The same happens when using vibrations with a lower frequency of 50 Hz introduced transversely and axially. When using of acoustic vibrations introduced axially with a frequency of 110 Hz, the change is less pronounced. As a result of laser welding of the pipe, a thin layer of iron, silicon, aluminum and manganese, about 10 m thick, forms on the surface. These are probably separations of alloy components of melted steel. In Figure 8, you can see a comparison of the structure of the obtained layers in different samples. It is clear that the face of weld No. 4 (Figure 8a), reached with a vibration‐free laser, is characterized by a lack of structure orientation, unlike the other cases. In the case of weld No. 1 (Figure 8b), fused with vibration with a frequency of 50 Hz introduced transversely, and in the case of weld No. 5, obtained with the longitudinal introduction of vibrations with a frequency of 110 Hz, a clear decrease in the silicon concentration in the face can be observed. The case of weld No. 2

Materials 2020, 13, x FOR PEER REVIEW 14 of 19 obtained with a vibration of 50 Hz (Figure 8c) introduced longitudinally shows that we are dealing with a clear fragmentation of precipitates with a visible orientation here. This orientation of the precipitates perpendicular to the surface of the pipe is most apparent in the case of weld No. 3, obtained with the introduction of vibrations with a resonance frequency of 1385 Hz (Figure 8e). The results of the SEM and EDX examination of the surface layer of the obtained welds show specificMaterials 2020differences, 13, 4179 in the morphology of the joints fused with a laser and subjected to acoustic14 of 18 vibrations. This is shown in Figure 8.

(a) (b)

(e)

Figure 8. SEMSEM and and surface surface distribution distribution of Si Si in in the the face face of weld; ( a)) weld No. 4 4 made made without without acoustic acoustic vibration, ( b) weld No. 1 1 (50 (50 Hz Hz with with transverse transverse propagation of acoustic vibration), (c) weld No. 2 (50 (50 Hz with longitudinal propagation of acoustic vibration), ( d)) weld No. 3 3 (1385 (1385 Hz with longitudinal propagation of acoustic vibration), ( e) weld No. 5 5 (110 (110 Hz with longitudinal propagation of acoustic vibration).

4. Discussion Discussion The presented results show that the acoustic vibrations input can affect aﬀect both the geometry of the weld and its face’s microstructure.microstructure. The The scale scale of of these these changes changes is is not not comparable comparable with with eﬀ effectsects reached reached in inother other methods methods that that directly directly introduce introduce vibrations vibrations [1,2, 5[1,2,5,10,18,25],,10,18,25], but theybut they give thegive opportunity the opportunity to reach to reachconstant constant conditions conditions in the in welding the welding place usingplace using contactless contactless supplying supplying vibration, vibration, as in [ 16as, 25in ].[16,25]. As a a result result of of the the conducted conducted tests, tests, the the obtained obtained welds welds did not did clearly not clearly indicate indicate any tendency. any tendency. Each oneEach of one them of themdeserves deserves attention. attention. Therefore, Therefore, the support the support with withvibrations vibrations of 50 of Hz 50 supplied Hz supplied from from the outsidethe outside perpendicular perpendicular to the to pipe the pipecontributed contributed to the to achievement the achievement of the of highest the highest hardness hardness in the in weld the No.weld 1. No. The 1. reason The reason could couldbe increased be increased cooling cooling and interaction and interaction with the with shielding the shielding gas. gas. The maximum depth of penetration and, at the same time, the minimum width of the face were obtained in the case of a weld treated to 50 Hz vibrations spreading axially inside the pipe (weld No. 2). The reasons can be found in the transverse stresses generated by the cyclic change of the sound pressure inside the pipe. The grain size decreased the most when exposed to vibrations of 50 Hz, propagating along the pipe axis (weld No. 2). Materials 2020, 13, 4179 15 of 18

SEM tests, on the other hand, showed the greatest differences in the structure of the top layer for a weld treated with vibrations of resonance frequency. Practically all vibration assisted welds differ in this area from the vibration-free welds. It can be see the directional arrangement of crystallites perpendicular to the surface. It can be seen that the lower vibration frequency permits the higher weld penetration depth. The direction of the vibrations introduction is important-when they are introduced transversely to the axis of the weld or axially, a much lower penetration can be observed than in the case of a longitudinal direction. Supporting the laser welding method with the introduction of acoustic vibrations has a particularly strong impact around the fusion line. Without vibrations, the fusion line is blurred, and it is difficult to determine it clearly; there is also a wide heat-affected zone. With the support of vibrations, the fusion line becomes clearly visible and the heat-affected zone decreases noticeably. The grain size of the weld supported with vibrations is reduced. The smallest grains were observed in the case of longitudinal introduction of acoustic vibrations with a frequency of 110 Hz. Hardness measurements showed that the longitudinal introduction of acoustic vibrations generally reduced the hardness value (by approximately 50HV0.1) in the heat-affected zone, regardless of the vibration frequency used. At a resonance frequency of 1385 Hz, the decrease in hardness in relation to the weld obtained without vibrations was small and amounted to about 25HV0.1. When transverse acoustic vibrations were applied, the hardness also decreased by about 50HV0.1, but at 200 µm below the face, it increased to 350HV0.1. This may indicate a shift deeper the in equipotential surface associated with the heat-affected zone. SEM investigations have shown significant differences between the structure of areas adjacent to the weld face obtained with the support of acoustic vibrations with a resonance frequency of 1385 Hz and that obtained with a non-vibration weld. The use of lower frequency vibrations led to these significant differences no longer being observed. Acoustic vibrations with a resonant frequency seem to bring about the largest changes in the structure obtained in the face area. Other deeper zones, such as the center, fusing lines or heat-affected zone do not show any greater differences. The fragmentation of the structure can be achieved by the longitudinal introduction of vibrations inside the pipe, which explains the uniform transverse stresses in the basic pipe material. The most pronounced directionality of the weld face crystal structure occurs when using vibrations with a resonant frequency, where the direction of crystal growth is perpendicular to the axis of the pipe. The relationship of the vibration amplitude on the position of the sample in relation to the excitation source was not the subject of research here, although it may have an impact on the obtained effects. However, taking into account the relatively low frequency of acoustic vibrations, their length is many times greater than the distance between the source of vibrations and the welding site used in the experiment. Only in the case of a resonance frequency of 1385 Hz, the wavelength estimated from the simple c/f relationship in the air is approximately 0.24 m. Assuming that the wave leaves the source (transducer, loudspeaker) with the maximum amplitude, it is at a distance of approximately 0.24 m from the source it has only a slightly lower amplitude due to the damping effect (0.07–4 dB/km). Taking into account the much lower frequency of 50 and 110 Hz, we obtain a wavelength in the air of 6.7 and 3 m. It will be different, of course, when the vibrations penetrate into the metal (steel) because the velocity of the longitudinal wave in this case is approximately 5200 m/s and the distances here are key problem. According to the authors of item [16], who used vibrations with a frequency of 150 to 300 Hz, the main effect on the changes in mechanical properties is their frequency. On the other hand, the problem of oscillation of the molten metal pool formed by laser welding is undoubtedly very important. Due to the fact that the melt pool vibration tests were not carried out in this case, it is definitely worth referring to what the available publications write on this subject. In one of them [30], model experiments and their comparison with experience were carried out. It has been shown that even very small fluctuations in laser power can lead to strong oscillations in the Materials 2020, 13, 4179 16 of 18 molten metal pool which is dominated by the natural frequency response. Since the occurring dynamic phenomena are non-linear, bifurcation effects can occur, which tend to expand the natural frequency peaks in the vibration spectrum of the pool. The determined Fourier spectra are in good agreement with the corresponding measurements in the ultraviolet and near infrared ranges during industrial CO2 laser welding. The presented model is limited to low beam velocity rates, native materials of low thickness and a small diameter laser beam. This fits well with the experimental conditions used in our article. It can therefore be assumed that the radius of the molten metal pool is small compared to the thickness of the workpiece, as is the case here. Due to the small damping factor of the pool’s radial oscillations, slight laser power fluctuations like 1% at the resonance frequency are sufficient to produce amplitudes large enough for the keyhole to collapse, which can result in a welding imperfections. The main resonant regions of the pool in a 1 mm thick iron element are between 500 and 3500 Hz, depending on the radius of the laser beam and the absorbed laser power. Due to non-linearity, resonance effects can be doubled, or more moderate, at half the natural frequency if the amplitudes are large enough. In addition, each resonance area is extended even by several 100 Hz due to the bifurcation phenomenon. In the case of the TM10 beam mode used, there is an oscillation of the pool, which resembles an ellipse that extends along the beam’s feed direction and shrinks in the perpendicular direction. Although in the quoted item there is no analysis for TEM 10, it can be noticed that assuming the beam speed of 1 m/min, the oscillation frequency of the pool for TEM00 is approximately 230 Hz and for TEM20 approximately 60 Hz. In another publication [31], it is mentioned that the pool vibration frequency is greater than 1 kHz observed both during spot welding and during welding with a moving beam. In the case of spot welding with a 20 ms laser pulse, oscillations of the weld pool with a frequency in the range of 200–500 Hz were observed, which remained in the first 5 ms after the laser pulse. In the time interval starting at 25 ms and ending about 40 ms from the start of the laser pulse, the oscillation frequency increased to 1.3 kHz. The clotting time was found to be approximately equal to the pulse duration for the spot welding. In the third position [32], the pool vibration frequency is also close to the value of 200 Hz. It is also indicated that the evolution of the face hump is not a mass movement but a propagation of the phase front which is determined by the alloy ejection towards the sides of the molten metal pool. In our research, the values of acoustic frequencies of 50, 110 and 1385 Hz were used. Two of them are close to the values quoted in the first and third publications, while one (1385 Hz), being the resonant frequency of the entire laser remelted element, is closer to the value suggested in the second publication. Certainly, it is worth trying to use more frequency values in the 50–2000 Hz range in future research, it would be possible to obtain more complete research. It would be even better to determine numerically, analytically or experimentally the natural frequency of the pool oscillation for a specific TEM mode, power, velocity rate and beam diameter, taking into account other important parameters such as gas flow, material factors, etc. However, this is an issue for a separate publication and it is possible that such attempts will be made. To sum up, the frequency values used in our research described in this article are in the area close to what were suggested. In the results obtained here, such dependencies can be noticed, although it is not easy to establish an indisputable trend at this stage of the research. In the future, it is planned to perform research with the use of a greater number of frequency values, so as to capture important relationships.

5. Conclusions A new non-contact method was presented, introducing vibrations into the welded area. Its signiﬁcant advantages include ensuring constant and repeatable vibration conditions at the welding site and there being no need to ensure a solid assembly of the vibrating system, as opposed to other contact methods of vibration introduction. Materials 2020, 13, 4179 17 of 18

SEM and EDX and metallographic testing showed the impact of vibration frequency and direction of their introduction on structures obtained in the surface layer during laser processing. The largest structure changes occurred in the face area of the welds. The weld dimensions obtained in these experiments were relatively small. In the future, greater beam power should be considered to make the observed changes clearer. Based on the research, it is not yet possible to clearly determine which direction of vibration or their frequency is the most favorable, because it depends on the expectations. However, we here have a chance for easier application in industrial conditions. Additional tests in the future in which the frequency and acoustic vibration input angle change more often would allow us to determine the "welding window". However, based on the obtained results, it can be concluded that the proper selection of the direction of vibration introduction and their frequency enables control of the surface layer structure and distribution of alloying elements, which directly aﬀects the properties of the obtained surface layer produced by laser processing with the participation of acoustic vibrations.

Author Contributions: Conceptualization, A.K.; methodology, G.K. and P.K.; software, G.K.; validation, A.K. G.K. and M.C.; formal analysis, A.K. and G.K.; investigation, A.K., G.K., M.C. and P.K.; resources, A.K.; data curation, M.C.; writing–original draft preparation, A.K.; writing–review and editing, A.K.; visualization, M.C. G.K.; supervision, P.K.; project administration, A.K.; funding acquisition, A.K. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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