A Macro - and Micro - Structural Study of Laser Welds in D36 Ship Steel

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1 Tå í. ñïí. Åðéóô. êä. ÔÅÅ, V, ôåý Tech. Chron. Sci. J. TCG, V, No A Macro - and Micro - Structural Study of Laser Welds in D36 Ship Steel M. A. VLACHOGIANNIS Dipl. Ing. Mechanical and Industrial Engineer A. D. ZERVAKI Dipl. Ing. Metallurgist G. N. HAIDEMENOPOULOS Associate Professor of University of Thessaly Abstract A 1.7kW CO 2 laser was used for the study of bead-on-plate welds in D36 ship steel. The effect of welding heat input and focal point position on weld geometrical features and microstructure was determined. It was found that weld penetration as well as the width of weld pool and heat-affected-zone (HAZ) increase with heat input. Microstructure and hardness in the weld pool, Partial fusion zone and HAZ are also influenced by heat input. As the heat input is increased, the associated cooling rate is decreased, resulting in the formation of softer microstructures. Simple analytical models describing heat flow during welding were used to calculate the above mentioned geometrical features. The agreement between the calculated and experimentally determined weld penetration, as well as weld pool width, is sufficiently good and depends on weld heat input. The optimum experimental conditions defined from the bendon-plate study were applied to Laser welding of butt joints in 4mm D36 steel. 1. INTRODUCTION Laser welding has evolved in the past 15 years as a major welding process for numerous applications, from small spot welds to continuous single pass deep penetration welds. Reported advantages over welding with more conventional techniques, include among others, the small size of the weld pool and heat-affected zone (HAZ) combined with full penetration. In the welding of thin steel plates, e.g. in the shipbuilding industry, distortion associated with the high heat input of the conventional welding processes is often a serious problem. In this case, laser welding, with the associated size reduction of weld pool and HAZ, is considered as a promising alternative. It is, therefore, of great importance to characterize quantitatively the effect of laser welding parameters on the penetration, weld pool size, HAZ size and the associated microstructure of thin plate laser welds. To that end several efforts have been undertaken in the past. Breinan and Banas [1] determined that in HY-130 steel as well as grade B ship steel, laser welding is characterized by a Submitted: Aug. 14, 1997 Accepted: June 5, 1998 high depth to width ratio, while penetration depended mainly on focused power density and travel speed. Masumoto et al. [2] determined that laser welding of thin steel plates produced lower distortion than the conventional TIG welding. Moon [3] determined the effect of focal point on penetration in A36 steel laser welds. Metzbower et al. [4] as well as Strychor et al. [5] studied the effect of laser welding parameters on the microstructure and properties of A36 ship steel. More recently, Ducharme et al. [6] developed a mathematical model for laser welding of thin steel plates in order to predict the effect of welding parameters on weld pool dimensions and shape. The aim of the present work was to determine the effect of welding conditions (laser power, travel speed, focal point) on penetration, weld pool size, HAZ size and microstructure in bead-on-plate laser welds of D36 ship steel. The experimental results were compared with simple model predictions of heat flow during laser welding. The optimum experimental conditions were then applied to the welding of butt joints. 2. SYMBOLS T (z,t) is the temperature at depth z below the surface derived from applying a laser power for time t. q is the laser power. ë is the thermal conductivity of the steel. a is the thermal diffusivity of the steel. To is the initial temperature of specimens. A is the absorptivity of the surface. u is the travel speed of the moving specimen. The constant t o represents the time for heat to diffuse over a distance of beam radius r b. The length z o measures the distance over which heat can diffuse during the beam interaction time r b /u. V* is the volume which melts per second. z m is the depth below the surface up to which melting has occurred.

2 64 Tå í. ñïí. Åðéóô. êä. ÔÅÅ, V, ôåý Tech. Chron. Sci. J. TCG, V, No 1-2 q* is the energy that is available to raise the temperature of the remaining solid. L is the latent heat of fusion per unit volume of the material. x, y, z are the Cartesian co-ordinates. 3. EXPERIMENTAL PROCEDURE The material used in the present study was the controlledrolled fully-killed D36 (per ASTM) ship steel with chemical composition Fe - 1.7Mn Si Al C (mass contents in %). Plates of D36 steel were received with thickness 4mm. Specimens for the experimental study were cut with dimensions 500 x 200mm. The hardness of the asreceived material was HV. For the laser welding experiments, a 1.7KW CO 2 laser was used. A focusing lens of 27mm focal length was employed in order to increase the power density on the plate surface. Helium shielding gas was supplied co-axially to the laser beam through a nozzle of 4mm diameter. The range of the laser welding parameters was the following: laser power W, travel speed mm/min and focal point position +1, 0, -1 and -2mm from the plate surface. The beam diameter was between 0.9 and 1.5mm, depending on the focal point position. In all cases single pass bead-on-plate welds were performed. A total of 38 beads were produced. The laser welding parameters are summarized in Table 1. In order to determine the geometrical characteristics of the laser welds, macrostructural examination was performed in all laser weld beads. The examination involved the usual metallographic specimen preparation (mounting, grinding, polishing and etching in 10% Nital for 8sec). The geometrical parameters determined were the penetration depth, the width of the weld pool as well as the width of the HAZ (see Figure 1). Microstructural analysis was performed only on weld beads which exhibited full penetration. For this purpose the usual metallographic specimen preparation was employed, involving mounting, grinding, polishing and etching in 2% Nital for 10-20sec. Microhardness measurements were performed on the cross section of all full penetration welds using a microhardness tester with load 300gr. 4. RESULTS AND DISCUSSION 4.1. Geometrical characteristics The typical appearance of laser weld beads is depicted in Figure 2a (partial penetration) and Figure 2b (full penetration). Weld penetration was determined as a function of laser travel speed and focal point position relative to the plate surface. The results are shown in Figure 3 for laser power 1500W. Weld penetration generally decreases with laser travel speed. Regarding full penetration conditions, it can be Table 1: Summary of Laser welding parameters used in the beadon-plate experiments. Ðßíáêáò 1: Óýíïøç ôùí ðáñáìýôñùí ôçò óõãêüëëçóçò Laser ðïõ ñçóéìïðïéþèçêáí êáôü ôçí ðåéñáìáôéêþ äéáäéêáóßá. Figure 1: Geometrical characteristic sizes of laser weld beads determined by the microstructural analysis (1. Width of HAZ. 2. Width of Weld Pool. 3. Partial Fusion Zone. 4. Penetration 5. Plate Thickness). Ó Þìá 1: ÃåùìåôñéêÜ áñáêôçñéóôéêü ôçò äçìéïõñãïýìåíçò óõãêüëëçóçò Laser, õðïëïãéóìýíá áðü ôçí áíüëõóç ôçò ìéêñïäïìþò (1. ÐëÜôïò ôçò ÈÅÆ. 2. ÐëÜôïò ôçò ëßìíçò óõãêüëëçóçò. 3. Æþíç ÔÞîçò. 4. Äéåßóäõóç. 5. ÐÜ ïò åëüóìáôïò). seen that these depend on a combination of focal point position and travel speed. For example, when the focal point is set at -1mm (below the surface), full penetration is obtained for travel speeds of mm/min. When the F.P. position is set at -2mm, a travel speed of 200mm/min causes weld drop-out. In order to present the results in a more comprehensible form, the laser power Q and travel speed u, were combined in a single parameter, the heat input h, defined as h = Q/u (in J/mm). The variation of weld penetration with focal point position and heat input is given in Figure 4. Weld penetration is not affected substantially when the focal point position is below the plate surface, while it drops significantly when the focal point is above the plate surface. Therefore, in order to determine the effect of heat input on penetration the average penetration for focal point positions below the surface (0, -1, -2mm) was determined. The results are depicted in Figure 5. Initially the average weld penetration increases with heat

3 Tå í. ñïí. Åðéóô. êä. ÔÅÅ, V, ôåý Tech. Chron. Sci. J. TCG, V, No heat input is given in Figure 8. The width of the HAZ varies between 0.3 and 1.5mm. The average width of the HAZ is given as a function of heat input in Figure 9. (a) (b) Figure 2: Microstructure of laser weld beads (a) Partial penetration, specimen B12: 1140W, 400mm/min, FP at 0 (b) Full penetration, Specimen B28, 1680W, 600mm/min, FP at -2mm. Ó Þìá 2: ÌéêñïäïìÞ ôçò óõãêüëëçóçò Laser (a) Ìç ðëþñçò äéåßóäõóç, äïêßìéï B12: 1140W, 400mm/min, óçìåßï åóôßáóçò ðüíù óôçí åðéöüíåéá (b) ÐëÞñçò äéåßóäõóç, äïêßìéï B28, 1680W, 600mm/min, Ó.Å: -2mm. input up to 250 J/mm, where it stabilizes to a value corresponding to full penetration between 250 and 400 J/mm. For higher values of heat input, undercutting and drop-out phenomena were observed. The variation of the width of the weld pool with focal point position and heat input is depicted in Figure 6. As in the previous case, the weld pool width is virtually unaffected when the focal point is below the plate surface. The average weld pool width as a function of heat input is given in Figure 7. A linear dependence on heat input is observed. The dependence of the width of the HAZ on focal point position and 4.2. Microstructural analysis The microstructure of the laser weld beads was examined on cross sections of beads which exhibited full penetration. Since plate thickness and weld geometry were constant during the investigation, the resulting microstructure depended strongly on cooling rate, which in turn depends on the heat input employed. For each weld area (HAZ, Partial fusion zone and weld pool) the following microstructures were observed: Weld pool: For high heat input ( Joule/mm) the microstructure consisted mainly at the grain boundary of Widmanstatten ferrite and Primary Ferrite (PF(G)) and in the grain interior of acicular ferrite, Primary Ferrite (PF(I)) and pearlite (figure 10a) according to a nomenclature proposed by Lancaster [9]. This result is consistent with the lower cooling rate associated with the high heat input. For lower heat input values ( Joule/mm) the formation of Widmanstatten and acicular ferrite was suppressed in favour of bainite. The structure therefore consisted of primary ferrite and bainite, consistent with the high cooling rates associated with lower heat input (Figure 10b). Partial Fusion zone: The microstructure in the Partial fusion zone was martensitic for the entire heat input range investigated (figure 11). This behavior is consistent with the highest cooling rates exhibited in the Partial fusion zone, as it is the boundary between the weld pool and HAZ. Heat Affected Zone: For high heat input, the microstructure in the HAZ consisted mainly of a fine equiaxed ferrite/pearlite mixture in contrast to the banded ferrite/pearlite structure of the base plate. The breakdown of the banded structure in the HAZ is probably the result of the phase transformations occurring during the thermal cycle of the welding (Figure 12). This is consistent with the lower heating and cooling rate and, therefore, longer interaction time in the HAZ. For low heat input, the above behavior was not observed. Microhardness was determined on a cross section - in 2mm depth - of the laser weld beads. A characteristic microhardness profile is shown in Figure 13. It can be observed that maximum hardness appeared in the Partial fusion zone and is associated with the martensitic structure of this region. The average hardness of the weld pool, as well as the hardness of the Partial fusion zone, were determined as a function of heat input. The results are depicted in Figure 14. As the heat input increases, the hardness of both the weld pool and Partial fusion zone decrease. This is due to the corresponding decrease of the cooling rate, thus producing softer structures.

4 66 Tå í. ñïí. Åðéóô. êä. ÔÅÅ, V, ôåý Tech. Chron. Sci. J. TCG, V, No 1-2 Figure 3: Weld penetration as a function of laser travel speed for different focal point positions of the laser beam at a laser power of 1.5KW. Ó Þìá 3: Äéåßóäõóç óõãêüëëçóçò ùò óõíüñôçóç ôçò ôá ýôçôáò ôçò äýóìçò ãéá äéáöïñåôéêü óçìåßá åóôßáóçò ôçò äýóìçò ôïõ Laser. Ç éó ýò ôçò äýóìçò Þôáí ßóç ìå 1.5KW. Figure 5: Average weld penetration as a function of heat input (focal point positions 0, -1, -2mm). Ó Þìá 5: ÌÝóç ôéìþ ôçò äéåßóäõóçò óõãêüëëçóçò ùò óõíüñôçóç ôçò åéóñïþò èåñìüôçôáò (Ó.Å.: 0, -1, -2mm). Figure 4: Weld penetration as a function of focal point position and heat input. Ó Þìá 4: Äéåßóäõóç óõãêüëëçóçò ùò óõíüñôçóç ôïõ óçìåßïõ åóôßáóçò êáé ôçò åéóñïþò èåñìüôçôáò. Figure 6: Width of the weld pool as a function of focal point position and heat input. Ó Þìá 6: ÐëÜôïò ôçò ëßìíçò óõãêüëëçóçò ùò óõíüñôçóç ôïõ óçìåßïõ åóôßáóçò êáé ôçò åéóñïþò èåñìüôçôáò. In summarizing the above discussion the following comments can be made: Regarding the effect of heat input on the geometrical characteristics of the weld beads, it was shown that weld penetration, weld pool width and HAZ width increase with heat input. This is obviously associated with the higher thermal effect, i.e. higher temperature and longer interaction time, associated with high values of heat input. On the other hand, microstructure is mainly influenced by cooling rate. As the heat input is increased the cooling rate is decreased, resulting in softer structures in the weld pool and Partial fusion zone. Therefore, selection of heat input is very important as it determines the structure and properties of the welds. In the case of welding a 4mm thick plate for example, a heat input of the order of 250J/mm would result in full penetration without undercutting or drop-out defects. At the same time the weld pool width would be only 3.5mm and the HAZ would be limited to below 1mm. In contrast, conventional MIG welding of the same plate results in a weld pool width of 8-9mm and HAZ width of 2-3mm.

5 Tå í. ñïí. Åðéóô. êä. ÔÅÅ, V, ôåý Tech. Chron. Sci. J. TCG, V, No Figure 7: Average width of weld pool as a function of heat input (focal point positions 0, -1, -2mm). Ó Þìá 7: Ôï ìýóï ðëüôïò ôçò ëßìíçò óõãêüëëçóçò ùò óõíüñôçóç ôçò åéóñïþò èåñìüôçôáò (Ó.Å.: 0, -1, -2mm). Figure. 8: Width of HAZ as a function of focal point position and heat input. Ó Þìá 8: Ôï ðëüôïò ôçò È.Å.Æ. ùò óõíüñôçóç ôïõ óçìåßïõ åóôßáóçò êáé ôçò åéóñïþò èåñìüôçôáò Prediction of geometrical features from heat flow modeling Heat flow modeling during laser welding was employed in order to predict the weld penetration and the weld pool width. The geometry of heat flow depends on weld penetration. When the penetration is partial, the heat flow is threedimensional (3-D), while in full penetration heat flow approaches a two dimensional (2-D) geometry (see Figure 15). In order to bracket the behavior, two extreme cases were considered, corresponding to the implementation of 1-D and 3-D models respectively. The 1-D heat flow model was developed by Ashby and Easterling [7] and incorporates a moving finite heat source on a semi-infinite plate. According to this model, the temperature distribution T(z,t) in the plane perpendicular to the surface through the centerline of the beam can be expressed as: [( o )] ( z + z ) A q o T( z, t) = To + / exp 1 2 2π λ u t + t t 4 α (4.1) In Eq. (4.1), T(z,t) is the temperature at depth z below the surface derived from applying a laser power q for time t. The terms ë (48.7 W/m 0 C) and á (13.9X10-6 m 2 /sec) are the thermal conductivity and diffusivity of the steel, respectively; T o is the initial temperature of specimens, 15 0 C; A is the absorti-vity of the surface, 0.9; and u is the travel speed of the moving specimen. The constant t o represents the time for heat to diffuse over a distance of beam radius r b ( mm). The length z o measures the distance over which heat can diffuse during the beam interaction time r b /u. The model also takes into account the possibility of surface melting during the process. When the surface reaches the liquidus temperature, a portion of the laser energy is absorbed as latent heat of fusion. Although this amount of energy is released later, during solidification, it is temporarily removed from the input energy and is not available to melt more material. The volume which melts, per second, is equal to: 2 V* = 2 r b z m u (4.2) where z m is the depth below the surface up to which melting has occurred. So, the energy that is available to raise the temperature of the remaining solid is given by: Figure 9: Average width of HAZ as a function of heat input (focal point positions 0, -1, -2mm). Ó Þìá 9: Ôï ìýóï ðëüôïò ôçò È.Å.Æ. ùò óõíüñôçóç ôçò åéóñïþò èåñìüôçôáò (Ó.Å.: 0, -1, -2mm). q* = q - 2 r b z m u L (4.3) where L is the latent heat of fusion per unit volume of the material. The temperature field, in such a case, is calculated from equation (4.1) by replacing q with q*.

6 68 Tå í. ñïí. Åðéóô. êä. ÔÅÅ, V, ôåý Tech. Chron. Sci. J. TCG, V, No 1-2 (a) Figure 12: Microstructure of the HAZ of the weld. (Specimen No.B5, 1500W, 400mm/min, focal point -1). Ó Þìá 12: ÌéêñïäïìÞ ôçò È.Å.Æ. ôçò óõãêüëëçóçò. (Äïêßìéï Íï.Â5, 1500W, 400mm/min, focal point -1). The 3-D heat flow model was developed by Masubuchi [8] and incorporates a moving point source on a semi-infinite plate. According to this model, the temperature distribution can be expressed as: u ( ) R u q ( ) T (x,y,z,t) = T o + a w 2 a 2 e (4.4) 2 π λ R e where R = w + y + z (4.5) (b) Figure 10: Microstructure of weld pool at (a) high (450 Joule/mm) and (b) low heat input (125 Joule/mm). Ó Þìá 10: ÌéêñïäïìÞ ôçò ëßìíçò óõãêüëëçóçò óå (a) ÕøçëÞ (450 Joule/mm) êáé (b) áìçëþ åéóñïþ èåñìüôçôáò (125 Joule/mm). Figure 11: Microstructure of the Partial fusion zone of the weld. (Specimen No.B28, 1680W, 600mm/min, focal point -2). Ó Þìá 11: ÌéêñïäïìÞ ôçò Æþíçò ÔÞîçò ôçò óõãêüëëçóçò. (Äïêßìéï Íï.Â28, 1680W, 600mm/min, focal point -2). and w = x - ut (4.6) The meaning and the value of the various symbols of equations (4.1) through (4.6) are given in Section 2. Weld penetration was calculated as the depth z at which the temperature exceeded the liquidus temperature of the steel. The results are depicted as a function of heat input in Figure 16 and are compared with experimentally determined weld penetration. For low weld heat input, weld penetration is short and the heat flow is three-dimensional. Therefore, the calculated weld penetration according to the 3-D model (solid line) is in good agreement with the experimental results. As the heat input is increased, the weld penetration increases and heat flow approaches a two dimensional condition. The 3-D model fails to predict the weld penetration in this case. In contrast, the 1-D model, which incorporates a finite heat source, is in better agreement with the experimental results through-out the entire range of heat input. Weld pool width was calculated, as a function of heat input, with the 3-D model and the results are compared with

7 Tå í. ñïí. Åðéóô. êä. ÔÅÅ, V, ôåý Tech. Chron. Sci. J. TCG, V, No Figure 13: Microhardness profile of laser weld bead. (Specimen No.B14, 1500W, 200mm/min, focal point 0). Ó Þìá 13: Ðñïößë ìéêñïóêëçñüôçôáò ôçò óõãêüëëçóçò Laser. (Äïêßìéï Íï.Â14, 1500W, 200mm/min, focal point 0). Figure 14: Microhardness of weld pool and Partial fusion zone as a function of heat input. Ó Þìá 14: Ìéêñïóêëçñüôçôá ôçò ëßìíçò óõãêüëëçóçò êáé ôçò æþíçò ôþîçò ùò óõíüñôçóç ôçò åéóñïþò èåñìüôçôáò áíü ìïíüäá ìþêï õò. the experimentally determined width of the weld pool in Fig. 17. The agreement between calculated and experimental results is good for heat input above 200 J/mm, while at lower values of heat input the discrepancy increases. This can be explained by considering that the 3-D model incorporates a point heat source, meaning that the temperature distribution is substantially affected only in the region adjacent to the source. Therefore while the temperature can be predicted with accuracy away from the source, the inaccuracy is increased substantially close to the source. 5. LASER WELDING OF BUTT JOINTS Laser welding of butt joints was carried out for type D-36 steel plates using a 1.7 kw CO 2 laser facility. The welds were evaluated by both destructive and non-destructive tests. Figure 15: Geometry of heat flow: a) 3-D, partial penetration b) 2-D, full penetration. Ó Þìá 15: Ãåùìåôñßá ôçò ñïþò èåñìüôçôáò: a) 3-D, ìç ðëþñçò äéåßóäõóç b) 2-D, ðëþñçò äéåßóäõóç. Plates of D-36 steel with dimensions 300x150x4mm, were welded in butt joint geometry by applying the experimental conditions depicted in table 2. These conditions were determined from the previously presented bead-on-plate experiments. The welds were subjected to non-destructive quality control (radiography, liquid penetrants, magnetic particles), as well as to destructive testing (tensile and bend testing, macroexamination) The non-destructive testing revealed that specimens 1, 3 and 4 (see table 2) exhibited imperfections such as pores, incomplete penetration and misalignment, while specimen 2 showed almost complete penetration. The macrostructure of specimen No. 2 is shown in Figure 18. The tensile tests for specimens No2 and No3 were performed according to DIN specification and the results are presented in table 3. In the same table, the tensile properties of D36 steel, as well as tensile test results for conventional M.I.G. welds are presented for comparison. Compared to the conventional M.I.G. welding method, laser welding for the case of specimen No 2 shows an increase in both yield and tensile strength, indicating the superiority of the laser welding method. The high quality of laser butt welds is also indicated by bend test results. For example, specimen No 2, after being subjected to bend test, according to DIN specification, showed no microcracks in the face position. 6. CONCLUSIONS According to the proceeding discussion, the following conclusions can be made:

8 70 Tå í. ñïí. Åðéóô. êä. ÔÅÅ, V, ôåý Tech. Chron. Sci. J. TCG, V, No 1-2 Figure 18: Macrostructure of laser welded butt joint. Ó Þìá 18: ÌáêñïäïìÞ óõãêüëëçóçò laser. Figure 16: Weld penetration as a function of heat input. Comparison between experimental and calculated results with 1-D (dashed curve) and 3-D (solid curve) modeling. Ó Þìá 16: Äéåßóäõóç óõãêüëëçóçò ùò óõíüñôçóç ôçò åéóñïþò èå ñ- ìüôçôáò. Óýãêñéóç ìåôáîý ðåéñáìáôéêþí êáé èåùñçôéêþí áðï ôåëåóìüôùí ôïõ ìïíïäéüóôáôïõ êáé ôïõ ôñéäéüóôáôïõ ìïíôýëïõ. Table 2: Experimental conditions for butt joint welding. Ðßíáêáò 2: ÐåéñáìáôéêÝò óõíèþêåò óõãêïëëþóåùí óõìâïëþò. Table 3: Tensile test results and comparison between M.I.G. and Laser Welding. Ðßíáêáò 3: ÁðïôåëÝóìáôá ôïõ ôåóô åöåëêõóìïý êáé óýãêñéóç ôùí ìåèüäùí óõãêüëëçóçò M.I.G. êáé Laser. Figure 17: Weld pool width as a function of heat input. Comparison of experimental results with prediction of 3-D heat flow model. Ó Þìá 17: Ôï ðëüôïò ôçò ëßìíçò óõãêüëëçóçò ùò óõíüñôçóç ôçò åéóñïþò èåñìüôçôáò. Óýãêñéóç ôùí ðåéñáìáôéêþí áðïôåëåóìüôù í ìå ôçí ðñüâëåøç ôïõ ôñéäéüóôáôïõ ìïíôýëïõ ñïþò èåñìüôçôáò. (A) Weld penetration, width of weld pool as well as width of the heat affected zone, depend strongly on laser welding heat input. All the above geometrical characteristics increase with heat input. At the same time they are not affected substantially by the focal point position when the focal point of the laser beam is below the surface. (B) The resulting microstructures depend also on heat input. Measured microhardness decreases with increasing heat input due to the lower cooling rates involved. (C) Heat flow modeling can predict with sufficient accuracy the weld penetration when the heat input is low. For high heat input the 3-D model fails, while the 1-D model is in better agreement with the experimental results. The weld pool width can be predicted with sufficient accuracy with the 3-D model at high values of heat input. At low heat input the inaccuracy is increased due to the point heat source assumption incorporated in the model. (D) Successful laser welds of butt joints can be performed

9 Tå í. ñïí. Åðéóô. êä. ÔÅÅ, V, ôåý Tech. Chron. Sci. J. TCG, V, No by using the proper set-up and experimental conditions. The quality of these laser welds is superior to the conventional M.I.G. welding of the same material. ACKNOWLEDGEMENT This work has been partially supported by the Greek Secretariat of Research and Technology through the EPET II-170 project. The material used in this study was kindly offered by Hellenic Shipyards (Skaramanga Yard). REFERENCES 1. Breinan E.M., Banas C.M., Welding with High-Power Lasers, Proc. Conf. on Advances in Metal Processing, 25 th Sagamore Army Materials Research Conference, Lake George, N.Y., 1981, pp. 111/ Masumoto I., Shinoda T., Ishiyama H., Application of Laser Beam Welding to Thin Steel Sheet, Trans. Jpn. Weld. Soc., 1989, Vol. 20, pp. 50/ Moon D.W., Some Factors Affecting Penetration in Laser Welding, Materials Processing, 1985, Vol. 44, pp. 53/ Metzbower E.A., Hella R.A., Theodorski G., Laser Beam Welding at NIROP, A Navy Manufacturing Technology Program in Lasers in Material Processing, Los Angeles, California, ASM, 1983, pp. 266/ Strychor R., Moon D.W., Metzbower E.A., Microstructure of ASTM A-36 Steel Laser Beam Weldments, J. Metals, 1984, Vol. 36, pp. 59/ Ducharme R., Williams K., Kapadia P., Dowden J., Steen W., Glowacki M., The Laser Welding of Thin Metal Sheets: An Integrated Keyhole and Weld Pool Model with Supporting Experiments, J. Phys. D, Appl. Phys., 1994, Vol. 27, pp. 1619/ Ashby M.F., Easterling K.E., The Transformation Hardening of Steel Surfaces by Laser Beams-I. Hypo-Eutectoid Steels, Acta Metall., 1984, Vol. 32, pp. 1935/ K. Masubuchi, Analysis of welded structures, Pergamon Press, J. F. Lancaster, Metallurgy of Welding, Charman & Hall, Michael A. Vlachogiannis, Dipl. ing. mech. and industrial engineering, University of Thessaly, Pedion Areos, Volos. Anna D. Zervaki, Dipl. ing., Metallurgical Industrial Research and Technology Center, 1 st Industrial Area of Volos, Volos. Gregory N. Haidemenopoulos, Associate professor, Dept. of Mech. and Industrial Engineering, University of Thessaly, Pedion Areos, Volos.

10 72 Tå í. ñïí. Åðéóô. êä. ÔÅÅ, V, ôåý Tech. Chron. Sci. J. TCG, V, No 1-2 ÅêôåôáìÝíç ðåñßëçøç ÌåëÝôç ÌéêñïäïìÞò ÓõãêïëëÞóåùí Laser Íáõðçãéêïý Üëõâá D36 Ì. Á. Âëá ïãéüííçò Ìç /ãïò Ìç /êüò Âéïìç áíßáò Á. Ä. ÆåñâÜêç Ìåôáë/ãïò Ìç /êüò Ãñ. Í. áúäåìåíüðïõëïò Áíáðë. ÊáèçãçôÞò Ðáíåð. Èåóóáëßáò Ðåñßëçøç Ãéá ôç ìåëýôç ôùí óõãêïëëþóåùí ôïõ íáõðçãéêïý Üëõâá D36 ñçóéìïðïéþèçêå laser äéïîåéäßïõ ôïõ Üíèñáêá éó ýïò 1.7kW. Õðïëïãßóôçêå ç åðßäñáóç ôïõ óçìåßïõ åóôßáóçò êáé ôçò éó ýïò ôçò ä Ýóìçò óôá ãåùìåôñéêü áñáêôçñéóôéêü êáé ôç ìéêñïäïìþ ôçò äçìéïõñãïýìåíçò óõãêüëëçóçò. ÂñÝèçêå üôé ç äéåßóäõóç ôçò ëßìíçò óõãêüëëçó çò, ôï ðëüôïò ôçò ëßìíçò óõãêüëëçóçò êáé ôï ìýãåèïò ôçò ìåñéêþò æþíçò ôþîçò áõîüíïíôáé ìå ôçí áýîçóç ôçò éó ýïò ôçò äýóìçò. Ç ìéêñïäïìþ êáé ç óêëçñüôçôá ôçò ëßìíçò óõãêüëëçóçò, ôçò ìåñéêþò æþíçò ôþîçò êáé ôçò È.Å.Æ. åðçñåüæïíôáé áðü ôçí éó ý ôçò äýóìçò. Êáèþò ç éó ýò ôçò äýóìçò (êáé êáôü óõíýðåéá ç åéóñïþ èåñìüôçôáò) áõîüíïíôáé, ï ñõèìüò øýîçò ìåéþíåôáé, ìå áðïôýëåóìá íá äçìéïõñãïýíôáé ä éáöïñåôéêýò ìéêñïäïìýò. ñçóéìïðïéþèçêáí áðëü áíáëõôéêü ìïíôýëá ðïõ ðåñéãñüöïõí ôç ñïþ èåñìüôçôáò êáôü ôç äéüñêåéá ìéáò óõãêü ëëçóçò, ãéá ôïí õðïëïãéóìü ôùí ðñïáíáöåñèýíôùí ãåùìåôñéêþí áñáêôçñéóôéêþí. Ç óõìöùíßá ìåôáîý ôùí èåùñçôéêþí êáé ôùí ðåéñáìáô éêþí áðïôåëåóìüôùí åßíáé áñêåôü êáëþ, åîáñôþìåíç áðü ôï ìýãåèïò ôçò éó ýïò ôçò äýóìçò. Ïé âýëôéóôåò ðåéñáìáôéêýò óõíèþêåò ñç óéìïðïé- Þèçêáí ãéá ôç óõãêüëëçóç óõìâïëþò åëáóìüôùí ðü ïõò 4mm, áðü ôï ßäéï õëéêü. 1. ÐÅÉÑÁÌÁÔÉÊÇ ÄÉÁÄÉÊÁÓÉÁ Ôï õëéêü ðïõ ñçóéìïðïéþèçêå óôç ðáñïýóá åñãáóßá Þôáí ï íáõðçãéêüò Üëõâáò D36, ìå óêëçñüôçôá ìåôáîý HV êáé ìå çìéêþ óýíèåóç: Fe - 1.7Mn Si Al C (ê.â.%). ñçóéìïðïéþèçêáí åëüóìáôá ðü ïõò 4mm, ìå äéáóôüóåéò mm. Ç ðñáãìáôïðïßçóç ôùí «ñáöþí» (bead-on-plate) Ýãéíå ìå ôç âïþèåéá ìéá äýóìçò laser CO 2 1.7kW êáé äéáìýôñïõ ìåôáîý 0.9 êáé 1.5mm (åîáñôþìåíç áðü ôï óçìåßï åóôßáóþò ôçò). Ïé óõíèþêåò êáé ïé ðáñüìåôñïé ôçò óõãêüëëçóçò áðåéêïíßæïíôáé óôïí ðßíáêá 1. Ãéá ôç ìýôñçóç ôùí ãåùìåôñéêþí áñáêôçñéóôéêþí ôçò óõãêüëëçóçò (ó Þìá 1), Ýãéíå óôá äïêßìéá ç áðáñáßôçôç ìåôáëëïãñáöéêþ ðñïåôïéìáóßá (åãêéâùôéóìüò, ëåßáíóç, óôßëâùóç êáé çìéêþ ðñïóâïëþ ìå 10% Nital ãéá 8 sec) êáé ìå ôç âïþèåéá ôçò áíüëõóçò ôçò ìáêñïäïìþò õðïëïãßóôçêáí ïé ãåùìåôñéêýò ðáñüìåôñïé ðïõ Þôáí: ÕðïâëÞèçêå: ãéíå äåêôþ: ôï âüèïò äéåßóäõóçò êáé ôï ðëüôïò ôçò ëßìíçò óõãêüëëçóçò êáèþò êáé ôï ðëüôïò ôçò ìåñéêþò æþíçò ôþîçò. Ç ðñïáíáöåñèåßóá ìåôáëëïãñáöéêþ ðñïåôïéìáóßá ñçóéìïðïéþèçêå êáé ãéá ôçí áíüëõóç ôçò ìéêñïäïìþò ôùí äïêéìßùí ðëþñïõò äéåßóäõóçò. 2. ÁÐÏÔÅËÅÓÌÁÔÁ-ÓÕÌÐÅÑÁÓÌÁÔÁ 2.1. ÁíÜëõóç ìéêñïäïìþò - ÐáñáìåôñéêÞ áíüëõóç Óôá ó Þìáôá 2a êáé 2b áðåéêïíßæïíôáé äýï áñáêôçñéóôéêýò ìïñöýò ôçò ìáêñïäïìþò ôçò óõãêüëëçóçò laser ãéá ðëþñç êáé ìç ðëþñç äéåßóäõóç. Óôï ó Þìá 3 öáßíåôáé ç åîüñôçóç ôçò äéåßóäõóçò ôçò ëßìíçò óõãêüëëçóçò áðü ôçí ôá ýôçôá êáé ôï óçìåßï åóôßáóçò ôçò äýóìçò. Ç äéåßóäõóç ôçò ëßìíçò óõãêüëëçóçò ìåéþíåôáé, êáèþò áõîüíåôáé ç ôá ýôçôá ôçò äýóìçò laser. Ç áðáßôçóç ðëþñïõò äéåßóäõóçò ùò ðáñüãïíôá åðéôõ ïýò óõãêüëëçóçò áðïôåëåß ôï êñéôþñéï åðéëïãþò ôùí âýëôéóôùí óõíèçêþí óõãêüëëçóçò. ¼ðùò öáßíåôáé áðü ôï ó Þìá 3, õðüñ ïõí óõãêåêñéìýíïé óõíäõáóìïß óõíèçêþí ðïõ åðéöýñïõí ôï åðéèõìçôü áðïôýëåóìá. Ãéá ðáñüäåéãìá, üôáí ôï óçìåßï åóôßáóçò åßíáé 1mm êüôù áðü ôçí åðéöüíåéá ôïõ åëüóìáôïò, ðëþñçò äéåßóäõóç åðéôõã Üíåôáé, üôáí ç ôá ýôçôá âñßóêåôáé ìåôáîý mm/min. ÐïëëÝò öïñýò êáôü ôç óõãêüëëçóç ñçóéìïðïéåßôáé ç ðáñüìåôñïò h=q/u (J/mm), ãéá íá åêöñáóôïýí ôáõôü ñïíá ç éó ýò êáé ç ôá ýôçôá ôçò äýóìçò. Óôï ó Þìá 4 ðáñïõóéüæåôáé ç åðßäñáóç ôçò åéóñïþò èåñìüôçôáò h êáé ôïõ óçìåßïõ åóôßáóçò óôï ìýãåèïò ôçò äéåßóäõóçò. Ç äéåßóäõóç äåí åðçñåüæåôáé óçìáíôéêü áðü ôï óçìåßï åóôßáóçò, áñêåß áõôü íá åßíáé êüôù áðü ôçí åðéöüíåéá. Ç åðéññïþ ôçò åéóñïþò èåñìüôçôáò óõíïøßæåôáé óôï ó Þìá 5, üðïõ öáßíïíôáé ôá üñéá åðéôõ ßáò ðëþñïõò äéåßóäõóçò óå Ýëáóìá 4mm. Óôï ó Þìá 6 áðåéêïíßæåôáé ç åîüñôçóç ôïõ ðëüôïõò ôçò ëßìíçò óõãêüëëçóçò áðü ôï Óçìåßï Åóôßáóçò (Ó.Å.) êáé ôçí

11 Tå í. ñïí. Åðéóô. êä. ÔÅÅ, V, ôåý Tech. Chron. Sci. J. TCG, V, No åéóñïþ èåñìüôçôáò. Êáé ðüëé üôáí ôï óçìåßï åóôßáóçò åßíáé åíôüò ôïõ õëéêïý, ôï ðëüôïò ôçò ëßìíçò óõãêüëëçóçò ðáñáìýíåé áìåôüâëçôï (ãéá óôáèåñü h). Ç ìýóç ôéìþ ôïõ ðëüôïõò ôçò ëßìíçò óõãêüëëçóçò, ãéá ôá äéüöïñá óçìåßá åóôßáóçò, åßíáé ãñáììéêþ åîüñôçóç ôçò åéóñïþò èåñìüôçôáò h (ó Þìá 7). ÁíÜëïãá áðïôåëýóìáôá ðáñáôçñïýíôáé êáé ãéá ôï ðëüôïò ôçò ìåñéêþò æþíçò ôþîçò. Óôá ó Þìáôá 8 êáé 9 öáßíïíôáé ç åîüñôçóç ôïõ ðëüôïõò ôçò ìåñéêþò æþíçò ôþîçò áðü ôï óçìåßï åóôßáóçò êáèþò êáé ç ãñáììéêþ åîüñôçóç ôçò ìýóçò ôéìþò ôïõ ðëüôïõò áðü ôçí åéóñïþ èåñìüôçôáò. Ðáñáôçñåßôáé ìåãéóôïðïßçóç ôïõ ðëüôïõò ôçò ìåñéêþò æþíçò ôþîçò, üôáí ôï óçìåßï åóôßáóçò åßíáé -1mm. Ç áíüëõóç ôçò ìéêñïäïìþò åóôéüóôçêå óôá äïêßìéá ðëþñïõò äéåßóäõóçò. Ç äçìéïõñãïýìåíç ìéêñïäïìþ åîáñôüôáé áðü ôï ñõèìü øýîçò êáé êáôü óõíýðåéá áðü ôï ðïóü åéóñïþò èåñìüôçôáò. Ãéá êáèåìßá áðü ôéò ðñïáíáöåñèåßóåò ðåñéï Ýò ðáñáôçñþèçêáí ïé áêüëïõèåò ìéêñïäïìýò: Óôç ëßìíç óõãêüëëçóçò, ç ìéêñïäïìþ áðïôåëåßôáé áðü «Widmanstatten» öåññßôç, âåëïíïåéäþ öåññßôç êáé ðåñëßôç, üôáí ôï ðïóü åéóñïþò èåñìüôçôáò åßíáé ìåôáîý Joule/mm (ó Þìá 10a). Ç ðáñïõóßá ôùí ðáñáðüíù ìéêñïäïìþí ïöåßëåôáé óôïõò áìçëïýò ñõèìïýò øýîçò, ïé ïðïßïé ìå ôç óåéñü ôïõò ïöåßëïíôáé óôï õøçëü ðïóü åéóñïþò èåñìüôçôáò h. ¼ôáí ç ôéìþ ôïõ h êõìáßíåôáé ìåôáîý Joule/mm ( áìçëüôåñïé ñõèìïß øýîçò), ç äçìéïõñãßá ìðáéíßôç åßíáé åìöáíþò (ó Þìá 10b). Óôç ìåñéêþ æþíç ôþîçò, ïé õøçëïß ñõèìïß øýîçò åßíáé ç êýñéá áéôßá äçìéïõñãßáò ðëþñïõò ìáñôåíóéôéêþò äïìþò (ó Þìá 11). Óôç ÈåñìéêÜ Åðçñåáæüìåíç Æþíç (È.Å.Æ.) ç ìéêñïäïìþ áðïôåëåßôáé áðü éóïáîïíéêü öåññßôç êáé ðåñëßôç (equiaxed ferrite/perlite) óå áíôßèåóç ìå ôç äéáóôñùìáôùìýíç (banded) öåññéôïðåñëéôéêþ äïìþ ôïõ ìåôüëëïõ âüóçò (ó Þìá 12). íá ôõðéêü ðñïößë ìéêñïóêëçñüôçôáò áðåéêïíßæåôáé óôï ó Þìá 13, üðïõ äéá ùñßæïíôáé ïé ðñïáíáöåñèåßóåò ðåñéï Ýò. Ãéá ðáñüäåéãìá, ç õøçëþ óêëçñüôçôá ôçò ìåñéêþò æþíçò ôþîçò ïöåßëåôáé óôç äçìéïõñãßá ìáñôåíóéôéêþò äïìþò, ç ïðïßá ìå ôç óåéñü ôçò åßíáé áðïôýëåóìá ôùí õøçëþí ñõèìþí øýîçò. Ç åðßäñáóç ôïõ ñõèìïý øýîçò (ïõóéáóôéêü ôïõ ðïóïý åéóñïþò èåñìüôçôáò h) óôç ìýóç ôéìþ ôçò óêëçñüôçôáò ôçò ëßìíçò óõãêüëëçóçò êáé ôçò ìåñéêþò æþíçò ôþîçò öáßíåôáé óôï ó Þìá 14. Óõíïøßæïíôáò ôá óõìðåñüóìáôá ôçò ðáñáìåôñéêþò êáé ìéêñïóêïðéêþò áíüëõóçò, ðáñáôçñåßôáé áýîçóç ôçò äéåßóäõóçò, ôïõ ðëüôïõò ôçò ëßìíçò óõãêüëëçóçò êáé ôçò ìåñéêþò æþíçò ôþîçò, êáèþò ôï ðïóü åéóñïþò èåñìüôçôáò áõîüíåôáé. Áðü ôçí Üëëç ìåñéü, ç äçìéïõñãïýìåíç ìéêñïäïìþ åîáñôüôáé áðïêëåéóôéêü áðü ôïõò ñõèìïýò øýîçò. ¼ôáí áõîüíåôáé ï ñõèìüò øýîçò, ç ìéêñïäïìþ ôçò ìåñéêþò æþíçò ôþîçò êáé ôçò È.Å.Æ. áðïôåëåßôáé áðü ëåðôüêïêêï ìáñôåíóßôç êáé éóïáîïíéêü öåññßôç. Ìå ìåßùóç ôïõ ñõèìïý øýîçò ï ìáñôåíóßôçò ãßíåôáé ïíäñüêïêêïò êáé ãåíéêüôåñá äçìéïõñãïýíôáé ðéï ìáëáêýò äïìýò. Ãéá áëýâäéíï Ýëáóìá 4mm ç áðáßôçóç ðëþñïõò äéåßóäõóçò éêáíïðïéåßôáé, ìüíï üôáí ôï ðïóü åéóñïþò èåñìüôçôáò åßíáé ôïõëü éóôïí 250 Joule/mm. Ãé áõôü ôï ðïóü èåñìüôçôáò, ôï ðëüôïò ôçò ëßìíçò óõãêüëëçóçò åßíáé 3.5mm êáé ôçò È.Å.Æ. ðåñéïñßæåôáé êüôù áðü 1mm, óå áíôßèåóç ìå ôç ìýèïäï M.I.G., üðïõ ôá áíôßóôïé á ìåãýèç åßíáé 8,5mm êáé 2,5mm Ðñüâëåøç ôùí ãåùìåôñéêþí áñáêôçñéóôéêþí áðü ôá áíáëõôéêü ìïíôýëá ñïþò èåñìüôçôáò ñçóéìïðïéþèçêáí äýï ìïíôýëá ñïþò èåñìüôçôáò ãéá ôçí ðñüâëåøç ôçò äéåßóäõóçò êáé ôïõ ðëüôïõò ôçò ëßìíçò óõãêüëëçóçò. ¼ôáí ç äéåßóäõóç åßíáé ìç ðëþñçò, ç ñïþ èåñìüôçôáò åßíáé ôñéäéüóôáôç, åíþ, üôáí ç äéåßóäõóç åßíáé ðëþñçò, ç ñïþ èåñìüôçôáò åßíáé äéäéüóôáôç (ó Þìá 15). Ôï 1 ï ìïíôýëï ñïþò èåñìüôçôáò èåùñåß ìïíïäéüóôáôç ñïþ óå çìéüðåéñï óþìá, üðïõ ç êáôáíïìþ ôçò èåñìïêñáóßáò ðåñéãñüöåôáé áðü ôç ó. (4.1). Ôï ìïíôýëï ëáìâüíåé õðüøç ôçí ðéèáíüôçôá åðéöáíåéáêþò ôþîçò êáôü ôç äéüñêåéá ôçò óõãêüëëçóçò ìýóù ôçò ó. (4.3). Ôï 2 ï ìïíôýëï ñïþò èåñìüôçôáò èåùñåß ôñéäéüóôáôç ñïþ óå çìéüðåéñï óþìá êáé ðåñéãñüöåôáé áðü ôéò ó. (4.4)-(4.6). Ç äéåßóäõóç ïñßæåôáé ùò ôï âüèïò z, üðïõ ç èåñìïêñáóßá ðïõ õðïëïãßæåôáé îåðåñíü ôç èåñìïêñáóßá ôþîçò ôïõ õëéêïý. Ç óýãêñéóç ìåôáîý ôùí èåùñçôéêþí êáé ôùí ðåéñáìáôéêþí áðïôåëåóìüôùí áðåéêïíßæåôáé óôï ó Þìá 16, üðïõ ðáñéóôüíåôáé ç äéåßóäõóç ùò óõíüñôçóç ôçò åéóñïþò èåñìüôçôáò. Óå áìçëýò ôéìýò åéóñïþò èåñìüôçôáò ç äéåßóäõóç åßíáé ìç ðëþñçò êáé ôï ôñéäéüóôáôï ìïíôýëï óõìðßðôåé ðåñéóóüôåñï ìå ôá ðåéñáìáôéêü áðïôåëýóìáôá. Êáèþò áõîüíåôáé ç åéóñïþ èåñìüôçôáò, ðáñáôçñåßôáé áýîçóç ôçò äéåßóäõóçò êáé êáôü óõíýðåéá ç ñïþ èåñìüôçôáò åßíáé äéäéüóôáôç. Óõíåðþò, ôï ìïíïäéüóôáôï ìïíôýëï óõìöùíåß ðåñéóóüôåñï ìå ôá ðåéñáìáôéêü áðïôåëýóìáôá, üôáí áõîüíåôáé ç åéóñïþ èåñìüôçôáò. Ãéá ôï èåùñçôéêü õðïëïãéóìü ôïõ ðëüôïõò ôçò ëßìíçò óõãêüëëçóçò ñçóéìïðïéþèçêå ôï ôñéäéüóôáôï ìïíôýëï êáé ç óýãêñéóç ìå ôéò ðåéñáìáôéêýò ìåôñþóåéò öáßíåôáé óôï ó Þìá 17. ¼ôáí ç ôéìþ ôçò åéóñïþò èåñìüôçôáò åßíáé ìåãáëýôåñç áðü 200 Joule/mm, ç óõìöùíßá èåùñçôéêþí êáé ðåéñáìáôéêþí áðïôåëåóìüôùí åßíáé áñêåôü êáëþ. Áõôü ïöåßëåôáé óôçí ðáñáäï Þ åðßëõóçò ôùí åîéóþóåùí ñïþò èåñìüôçôáò (ãéá ôç äçìéïõñãßá ôïõ ôñéäéüóôáôïõ ìïíôýëïõ) üôé ç ðçãþ èåñìüôçôáò åßíáé óçìåéáêþ. ÔÝëïò, ðñáãìáôïðïéþèçêå óõãêüëëçóç óõìâïëþò óå äýï åëüóìáôá ðü ïõò 4mm. Ïé óõíèþêåò óõãêüëëçóçò êáé ôá áðïôåëýóìáôá ôçò äïêéìþò ôïõ ôåóô åöåëêõóìïý áðåéêïíßæï-

12 74 Tå í. ñïí. Åðéóô. êä. ÔÅÅ, V, ôåý Tech. Chron. Sci. J. TCG, V, No 1-2 íôáé óôïõò ðßíáêåò 2 êáé 3. Óõãêñßíïíôáò ôçí áíôï Þ åöåëêõóìïý êáé ôï üñéï äéáññïþò ìåôáîý ôùí ìåèüäùí laser êáé M.I.G., ðáñáôçñåßôáé ìéá óçìáíôéêþ áýîçóç ôùí éäéïôþôùí óôç óõãêüëëçóç laser. ÅîÜëëïõ, ïé ìç êáôáóôñïöéêïß Ýëåã- ïé Ýäåéîáí ôçí Ýëëåéøç ðüñùí êáé áôåëåéþí óôç äïìþ ôïõ åîåôáæüìåíïõ õëéêïý. Ìé Üëçò Á. Âëá ïãéüííçò, Ìç áíïëüãïò ìç áíéêüò âéïìç áíßáò, ÐáíåðéóôÞìéï Èåóóáëßáò, Ðåäßïí ñåùò, Âüëïò. ííá Ä. ÆåñâÜêç, Ìåôáëëïõñãüò ìç áíéêüò, ÅÂÅÔÁÌ Á.Å, 1 ç Âéïìç áíéêþ Ðåñéï Þ Âüëïõ, Âüëïò. Ãñ. Í. áàäåìåíüðïõëïò, ÁíáðëçñùôÞò êáèçãçôþò, ÔìÞìá Ìç áíïëüãùí Ìç áíéêþí Âéïìç áíßáò, ÐáíåðéóôÞìéï Èåóóáëßáò, Ðåäßïí ñåùò, Âüëïò.

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