New Technology Of Preheating And Pressing Assisted Laser Beam Welding And Design Of Device With Resistance For Predeating And Pressing

Laser beam welding is regarded as the most potential welding method because of its high speed, low distortion, precision and flexibility. However, there exist several problems which restrict the range of its engineering utility, such as the power rank of laser, the assembly conditions (clearance, stagger margin and different thickness, etc), the process controlling (focus fluctuating, beam centralizing, etc), the reflectivity of materials (Aluminum, Magnesium and Copper, etc) and so on. In order to broaden the field of laser beam welding, compounded welding methods utilizing other welding methods to remedy the drawbacks of laser beam welding become the research emphasis in advanced countries.My supervisor firstly puts forward the new concept of LB—RSW (Laser Beam—Resistance Seam Welding) in 2003, and has accomplished principium experiment and applied invention patent. This article is an important part of this assignment. In order to validate the feasibility of LB—RSW, this article study the effect of preheating temperature and press on weld formation starting with laser beam welding under the conditions of preheating and press.This article firstly analyses the weld formation of laser beam welding in combination with preheating and press on aluminum alloys, based on the reseach on weld surface under different preheating temperatures, it indicates that the welding process is unstable under room temperature, splash is much, weld wave is uniform, and there exists large concave hole. Under the preheating temperature of 250℃, the welding process becomes stable as the temperature increases, splash decreases gradually, the scale lines of weld wave become tidy and fine, and concave holes evidently reduce. The best weld surface formation is obtained at 250℃, and there are least welding splash between 250℃and 300℃. However, when preheating temperature varies from 300℃up to 400℃, welding splash increases evidently, scale lines of weld wave become crassitude and unequal, continuous concave holes are formed on weld surface. Through the cross section figure of weld in laser deep penetration welding under different preheating temperatures, we can conclude that the weld pool width decreases suddenly on the connect position of the two plates. That is because the joint surface between aluminum plates, whose oxidated film is cleared before welding, is oxidated again and a bed of Al2O3 film is formed while preheating the aluminum plates. As the high melting point and low heat conductivity of the oxidated film lead to the weld pool fluid flow and temperature fields severe aberrance, merely can the part near weld centre on top surface obtain adequate heat to melt and form a weld, while non-fusion flaw appears on both sides which results in incontinuous weld pool width between the two plates. In addition, the problem becomes severe especiallywhen the preheating temperature reaches 400℃. According to experimental analysis on the effect of press to weld formation, weld burl is obtained between the plates, concave holes become deeper, and the formation becomes poor as a result of molten metal flowing between the clearance in weld pool with the clearance increasing. However, the problem is completely avoided as laser lap welding in combination with press is carried out on the plates.The trend of weld microstructure is revealed in this article according to the study on microstructure figure under different preheating temperatures (20℃, 200℃, 300℃, 400℃): Little change of the microstructure is discovered between 20℃and 200℃; the microstructure of weld own a trend of becoming coarse in evidence with the preheating temperature increasing above 300℃; the weld microstructure is maken up of thick branch crystal when the preheating temperature reaches 400℃. When the preheating temperature keeps at a low level, there exists a fairly large temperature gradient between weld pool and base metal as well as within weld pool. Thus, as the temperature of weld pool reduces, epitaxial solidification comes out in fusion line and strong directioinal columnar grain grows. Large composition supercooling is formed in the weld center with columnar grain growing and weld pool cooling, and nucleation rate is high which leads to quantities of fine equiaxed grain forming. With the preheating temperature increasing, temperature gradient between weld pool and base metal as well as within weld pool decreases, which results in slow cooling rate, low supercooling gradient and nucleation rate, long solidification time, and therefore microstructure becomes crassitude.The analysis of joint microhardness is also carried out in this paper, which reveals that weld zone owns the highest hardness. Moreover, the hardness of weld which is preheated is lower than that of weld which has not been preheated, because the energy of welding process is great while the element Magnesium of weld is lost partially.According to the observation on macrophoto of welded workpiece, pore, whose distribution state is of daisy chain and dense or isolated, is a common problem in laser welding of aluminum alloys. In addition, the quantity of pores inside weld is determined by process parameters, surface state and circumstance factor. There exist two types of pores: hydrogen pores and pores acquired by destroyed keyhole. Besides, the oxidated film on surface of aluminum alloys tends to absorb surrounding moisture, while solubility of hydrogen in molten aluminum alloy is 20 times that in solid aluminum alloys. In the course of melting and cooling, hydrogen tends to break away from the liquid and form hydrogen gas, but the cooling rate of laser welding is quicker than that of other welding methods which increases the feasibility of pores formation. Generally speaking, hydrogen pores have routine shape, their dimension is larger than that of branch crystal, and solidification style of branch crystal can be observed on the inner surface. The phenomena that keyhole caves in to form pores is achieved by kinetics factors in keyhole.Furthermore, the problem of pores formation is difficult to be avoided because the formation of laser beam welding has properties of large ratio of weldpool depth to width, high welding and cooling velocity.This paper has also analysized the crack of workpieces, which concludes that no crack in the weld is obtained under the condition of preheating, compared to the condition of room temperature. The cracks are mostly induced by fluidifying and solidification, and they belong to thermal crack.Several types of preheating and pressing assisted welding methods are studied deeply in this paper, and the most reasonable device with resistance for preheating and pressing is chosen. Refering to all types of standards and requests concerning welding machine in welding handbook, the integral configuration of frame as well as water, electricity and gas system are designed systemically. What's more, the machine has been completed and calibrated, and tests of calibrating the machine indicate that it can satisfy the requirement of the new technology of preheating and pressing assisted laser beam welding.