| Due to the acceleration of global population ageing,health is becoming a major concern in the world.In health care industry,implantable medical devices and components for helping elderly people,for example pacemaker,the most common such devices,have attracted much attention.Usually in pacemaker,the power adopts lithium-iodine battery,after the electrolyte injection battery from the hole,need for sealing the hole.At present,the tin sealing the hole and then laser welding cover sheet is most widely.The reliability of this method is lower,and affect the lifetime of the battery.With efforts to realize the laser hole sealing,and improve the sealing effect of the hole.In this paper,0.40mm-thick commercially pure grade 1 titanium with 0.1mm diameter holes was sealed using low power N d: YAG pulse laser.Respectively from the laser power,spot diameter and pulse width to study the influences of process parameters on the weld geometry of the sealing joints.The dynamic changes of the weld pool during the laser hole-sealing process were observed by high speed photography,and the sealing process was analyzed by combining the shape of sealing zone and the stress of the molten metal.The results show that full seal welded joint can be achieved when the laser power is 20.8W,pulse width is 6.0ms and spot diameter is 0.4mm.Nugget size and surface hardness of sealing joint increases with the increase of laser power.The micro hardness of the weld and HAZ is higher than the base metal,and the maximum micro hardness value is 239 HV.The microstructure of the sealing joint consists of acicular alpha,serrated alpha,platelet alpha and fine acicular alpha.Besides,with the increase of the spot diameter,the surface melting area of the sealing joint is basically unchanged,but the sealing mode is changed from the keyhole to the heat-conduction type,and the heat-conduction model is more suitable for the hole sealing.The theoretical maximum effective sealing thickness is about 386μm.Further,calculated laser energy of sealing area shown that there was a positive proportion between the theoretical energy value and laser spot size,but the theoretical analysis does not match with the actual situation.The reason may be that the molten metal beside the hole flow into the hole under the effect of gravity and other forces,which cause theincreasing of actual absorption of laser energy.In addition,the sealing thickness increases with the increase of pulse width too,but when the pulse width is not less than 7ms,the bottom of sealing area with the central bulges and both sides concaves will formation.The analysis results show that the time of maintain molten state increases with the increase of pulse width,and the central molten metal continue to flow downward and the surrounding molten metal moved upward by Marangoni flow when the time of maintain melting state increased.Through high-speed photography to observe the dynamic behaviors of the weld pool surface during laser hole sealing show that the molten pool with laser power is17.6W,20.8W and 22.4W from the convex shape gradually transits to smooth shape as time goes on.The weld pool is changed from bright to dark and then turn bright,and the volume of the weld pool surface is decrease before the light turns on,and the center of the molten pool can be seen hole shadow.The liquid level of the molten pool begins to decline after the molten pool transits to steady.In the laser hole sealing process,the hole is not filled in the laser action phase,and the molten metal begins to fill the hole after the laser disappears.Then,the weld pool is frozen after the hole is filled.The dynamic change of the weld pool is a process of expanding,filling the hole,and solidifying contraction in the whole laser hole sealing process.In formation of sealing joints can be divided into the following stages.First,the local melting of metal around holes during the early period of laser action.Secondly,with the laser irradiation time increases,the molten metal expansion to metal interior and the holes periphery.Again,after laser disappearance,the liquid metal begins to fill the hole due to the surface tension and other forces,and liquid metal contact with each other in the center of the hole.Fourth,the liquid metal begins to move up and down after the liquid metal convergence the hole center.Fifth,liquid metal filling hole completed,and the metal bath surface roughly the same as the upper surface of pure titanium.Finally,the liquid level of the molten pool begins to decline,and entering the stage of solidification contraction.By estimating the physical model of the dynamic change of the weld pool,it is directly reflect the laser hole sealing process,and the formation of Y shape,V shape and X shape characteristic of sealing area can be illustrated. |
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