| The copper is a metal that the human being made use of at the earliest stage, and also one of the most important metal materials in industry. It has very excellent electrical conductivity and thermal conductivity, normal temperature and low temperature plasticity, corrosion resistance, so it is the metal material which is popular with lots of fields, such as electronics and automobile, etc. Along with the increasing demand of the copper material, the lacking situation of the copper resources is also more and more serious. Lightening weight and decreasing volume is the most important development trend currently; however the welds of shin copper plate are the most important limiting factor. Presently, there are lots of experts who research the problem of welding shin copper plate and adopt brazing, electron-beam welding and friction stir welding, etc. But, because the copper material has higher thermal conductivity and is easily oxidized, shin copper plate is easy to engender burning through, distortion, pore, crackle in the process of welding. Laser welding technology is recognized as an advanced manufacturing technology which conforms to sustainable development stratagem in the 21 century. It has the characteristics such as energy concentration and high welding speed, small heat-affected zone and high welding strength, easiness to realize automation, etc, is very suitable for the welding of thin metal sheets. Therefore, it is necessary, urgent and strategic that surrounding the welding of shin sheet copper to launch the technical research of laser welding.First, we make a theoretical study of the laser welding, including describing the physical process of laser thermal conduction welding, deducing the metals material biggest melt depth before attaining boiling point, studying the relation of depth of fusion and the power density, analyzing the influence of several processing parameter (power density, pulse width, pulse frequency, weld speed and defocusing amount) of laser welding process on welding quality by reference literature.Based on the theoretical analysis, the trial research was done on shin copper plate welding with YAG laser. In order to observe weld effect expediently, the experiment adopts butt-weld mode. In order to prevent sheet copper from occurring misalignment and thermal deformation during welding process, we designed a special sheets welding fixture for this experiment. Through checkout, the fixture can reach the requirement of assembly tolerance and transverse direction collimation degree. The reflectivity of the copper versus laser is very high. If welding directly, it will need more power, and it is easily that the laser breaks off lens through the medium of reflection. This text used two kinds of method: graphite coating and welding directly. Through the contrast, we find out that graphite coating can make the laser absorptivity of copper raise to above 65%, and weld process are also more stable. Besides the processing parameter and coating, the accuracy of the laser focus modulation and the power stability of the machine outputs is also an important factor that influences welds result. As weld quality comes into prodigious contact with weld width, we scale the weld result by observing weld surface quality and measuring weld width firstly.Through a lot of experiments, we built up weld mathematical model according to the principle of experiment optimality. This model describes the quantitative relation between the pulse work current, pulse width, pulse frequency three and the weld width respectively. By checkout, the model curve and test data matched very well. Making use of the model, we analyzed the single factor influence. First, we fixed two factors in the model on certain level, and then researched the relation between another factor and weld width. Through analysis, in three influence factors, the current has the most influence on the weld width, the pulse frequency takes second place, and the pulse width influences least. In addition, we analyzed the relation between pulse frequency and weld width as well as between pulse width and weld width in different currents. At last, we came to the conclusion that in the shin copper plate welding experiment, we should first choose pulse frequency according to the request of welding speed, then select the pulse work current according to the pulse frequency, finally attain the best weld result through slightly adjusting the pulse width and defocusing amount. The pulse width is generally chosen at 2.5-3ms, defocusing amount at 0.5-1mm.In order to realize and check up the performance of welding joint and weld quality, we made some experiments and analysis from surface topography, metallographic structure, micro hardness, tensile strength and so on. Through the checkout and analysis, we draw some conclusion as follows:(1) The laser welding has the function of decontamination to the shin copper plate, the weld surface is smoother, impurity is very less and heat-affected zone is very narrow; existent welding defect is primarily pinhole and crackle; The prime reasons of the welding defect are that, at liquid state, the activity copper versus oxygenic is higher, hydrogen solubility larger, and during the welding process it's easy to bring strong stress and distortion. We can reduce pore and crack through cleaning the weld position and strictly checking the weld heat input.(2) In the welding process, for welding seam and heat-affected zone undergo strong hot effect, grains grow up obviously. As a result, the joint intensity and extensibility decline. But compared with traditional welding manner, the grain of joint is more exiguous, which is one important aspect of laser welding superior to traditional welding manner.(3) After welding, on the sheet copper, weld zone hardness is the least, heat-affected zone takes the second place, and mother metal's rigidity is the biggest. The weld hardness of adopting graphite coating is less than the one of the welding without coating. The prime reason is that the graphite impurities in the welding seam impact the welding effect. The welding speed affects the welding seam's rigidity too. The faster welding speed is, the narrower the welding seam is, the higher the hardness is, but the welding stability fluctuates greatly, whereas when the welding speed become slow, the weld hardness is slightly low, and the welding stability fluctuate lightly.(4) The tensile performance of the welding seam is lower than one of mother metal: the tensile strength of the welding seam can achieve above 65% of the mother metal's, but the extensibility is only 1/2 of the mother metal's. The prime reason is that there oxidize copper to the cuprous oxide at the welding process, which will impair the intermetallic combining capacity. In addition, there isn't phase transition in the brazing process. The welding seam and heat-affected zone undergo strong heat effects, which result in the grain growth, sequentially lead to the decline of the joint intensity and extensibility, especially the extensibility and cold bending performance decline more obviously.In this paper, we proved the feasibility of jointing sheet copper with laser welding, and found out the best processing parameter of welding sheet copper. As the science and technology are booming, along with the laser machining technology continually mature and the research on sheet copper's laser welding more profound, we believe that the application of copper will much more extensive, it will keep on impelling the research of welding technique to ultrathin sheets of other materials.
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