| Ocean Drilling Platform is the main equipment of marine operation whose level of design and construction reflects the strength of marine equipment capacity to some extent.Leg rack is the most core component of the platform which can directly affect lifting safety and the using effect of the platform.At present,the main way for welding of leg rack is normally done by manual welding technique.However,taking the multiple welding procedure,long working hours and low efficiency of manual welding into account,it is with great significance to develop the efficient welding method.Based on the reasons above,a new efficient method–robotic double-side arc weave vertical welding was proposed in this paper based on the welding of racks in ocean drilling platform leg.In this method,the backing weld is asymmetric while the filling weld is symmetric.And also in the welding procedure,the back chipping part is avoided which can greatly approve efficiency.Influenced by gravity,the action of drop transition process becomes more complicated and weld control becomes more difficult in the process of vertical welding with weaving for thick plate.Thus,the defects of incomplete fusion on side wall and lack of penetration in the root can easily appear.Therefore,this paper developed the research on the relationship among the action of drop transition process,optimization of welding parameters,microstructure and mechanical properties of joint in the process of vertical welding with weaving for thick plate.Through comparative analysis of the droplet transfer process with or without arc weave,this paper deeply studied the transfer characteristics of welding droplet in vertical welding with weaving.Thus it can achieve the stable control of droplet transfer.Based on the theory of static balance,the effect mechanism of weaving inertia force on droplet transfer is interpreted perfectly.The results indicated that weave of arc could accelerate the fall off of the melting drop,promote the drop transition and increase the stability of drop transition process.When dip angle is around 45°,the droplet transfer appears in the form of one pulse more drops with faster transfer frequency.On the other hand,when dip angle is around 85°,the droplet transfer appears in the form of one pulse one drop with stable transfer process.As the weaving amplitude increases,the frequency of droplet transfer in the root reduced gradually while transfer in the sidewall increased gradually.The average diameter of droplet almost stays the same while droplet in the root increases slightly and that in the sidewall decreases slightly.The average diameter of droplet and transition frequency only changed little when the weaving frequency is around 0.6Hz or 1.2Hz.However,when the weaving frequency increases to1.8Hz,the frequency of droplet transfer increased extraordinarily and the diameter of droplet becomes smaller.The transition mode and fusion status of side section are mainly influenced by dwell-time.Increasing the dwell-time would lead the transition frequency to a trend of increase with decrease after the first.However,the standing time has little influence on the diameter of droplet.The fusion of side section is mainly influenced by swing amplitude and standing time.Frequency of weaving is the key factor to the landing position of welding drop.Adjusting this parameter can change the forces of droplet and lead to the stable transfer of droplet,thus solving the problems of lack penetration in the root and incomplete fusion in the side section.In this research,the droplet transfer process stays the most stably with dip angle 85,frequency of weaving 1,2Hz,weaving ranges1.5mm and standing time 0.5 seconds.Double ellipsoid heat source model for weave arc welding was established through local coordinate transform,and the program for subroutine moving heat source was written by FORTRAN.The numerical simulation of the temperature and stress field of DSAW was realized by ABAQUS.The results show that the stress value of the back welding is larger than that of the other filling passes.Stress concentration is more likely to appear in the root.The back pass of DSAW is the key in multi-pass welding process.Consequently,effect of arc distance,welding heat input and preheat temperature on the temperature and stress field of the back pass of DSAW were studied.Results indicate that arc type and residual stress do not meet the linear relationship.Lower stress and angular distortion can be obtained with 40mm arc distance.While the total heat input remains unchanged,to enlarge the fore arc power and reduce the rear arc power can obtain lower stress and angular distortion.The effective results show that the values of t8/5 and t8/3/3 of no preheating DSAW are larger than single arc welding under the preheat temperature 100?.It shows that diffusible hydrogen has more time to escape from the weld area when using the DSAW method.Experiments were done on double-side arc weave vertical welding.This research also studied how weave parameters influence the shaping of welding.The method of support vector machine SVM is used to make the prediction model for weld shapes with different swing parameters.Besides,particle swarm optimization PSO algorithm is used to optimize the process parameters.Results show that as the swing increases,the reinforcement decreases and then increases.By the same time,the average penetration in side section presents a substantially increasing trend.Results also show that as swing frequency increases,the weld reinforcement decreases and then increases as opposed to the average penetration in side section.With the increasing of residence time,both the weld reinforcement and average penetration in side section increase gradually.The best weld shape and optimal welding parameters can be obtained through optimal model based on SVM prediction model and particle swarm PSO algorithm.The reinforcement in 0.754mm and average penetration in side section in 2.663mm can be obtained when the swing goes1.523mm,swing frequency 1.302Hz and standing time 0.541s.This group of parameters can just lead to the minimum of objective function HF?x?.In order to obtain double-side double-arc swing vertical welded joints with good forming and no welding defects,research was done on the bead arrangement of multi-pass for thick plate.This research was based on the optimal process parameters and also based on the characteristics of robotic swing vertical welding.Robotic double-side arc weave vertical welding is used in the welding of 50mm thick low-alloy high-strength steel plates in which each side has 4 layer with 6 welding bead and joints show“waist”characteristics.Due to different welding thermal cycles in different areas of welding,microstructure and geometry of backing weld and filling weld are different consequently influence the mechanical properties of different areas.Backing weld is asymmetrical double-side arc welding and the joint area experiencing effects from both side,shows double-arc characteristics with deeper penetration and smaller width called double-arc function area.Filling weld is symmetrical double-side arc welding.Due to influences of backing welding bead,thermal effect of both side decreases as the layers build up,then the filling weld shows single-arc characteristics with shallower depth and bigger width called single-arc function area.Heat affect zone?HAZ?has the highest hardness and weld metal is the second to HAZ while base metal has the lowest hardness.Weld metal is harder than re-melt area among the layer and single-arc function area has higher hardness than double-arc function area.Average tensile strength is 920.28MPa higher than 908.90 MPa of single-arc function area.The percentage elongation of double-arc function area is 18.8%which was also higher than 17.7%of single-arc function area.Impact energy at low temperature of double-arc function area is 2.25?m smaller than 2.72?m of single-arc function area.Percentage of high-angle boundary?>15??of double-arc function area is 41.06%higher than 38.79%of single-arc function area.Results of research show that the grain size and high percentage of big-angle boundary?>15??are the main reasons for higher impact toughness in double-arc function area than single-arc function area. |
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