A Comparative Study On Fundamentals Of Two Typical Laser Additive Manufacturing Technologies Of Metallic Components

Laser additive manufacturing (LAM) of high-performance metal parts and components has the features of complex-structure forming, high-precision forming and high-performance forming, it is one of the most promising technologies for the direct fabrication of complex precision metal parts and large-scale metal components. LAM is not only the supplement for the traditional technologies of casting, forging, welding and machining, but also open a new method for the fabrication of metal parts, it is of strategic significance to research. At present, LAM of high-performance metal components has two typical methods, one is Laser Cladding Deposition (LCD) based on the powder blowing technique, and another is Selective Laser Melting (SLM) based on the powder bed technique.However, due to the different processing methods and parameters, there are some big gaps in shape of molten pool (MP), cooing rate, microstructures and mechanical properties between LCD and SLM processes, so it is difficult to deeply understand the basic forming principle and application background of LAM. Therefore, a detailed comparative study on the two typical LAM technologies was presented in this thesis, and the main findings are as follows:Some single-track cladding samples were respectively deposited by broad-beam, middle-beam narrow-beam LCD and micro-beam SLM processing techniques from using 316L stainless steel alloy powders, and thus the LAM can be divided into four types of processing techniques. The shape coefficient and the penetrating coefficient are introduced to describe the change of the shape of MP. The shape and the size of MP are different in different processing techniques, which will lead to different heat conduction model and heat affected zone, and further influence the microstructure and performance of alloy. However, the change behavior of shape and size of MP with the normalized enthalpy in LCD and SLM are independent each other, showing the difference of the solidification between LCD and SLM.316L stainless steel block samples were also respectively deposited by broad-beam, middle-beam narrow-beam LCD and micro-beam SLM processing techniques, and the primary celluar arm spacings (PCAS) of the alloy are measured. By using the empirical relationship between PCAS and cooling rate, the cooing rate of the MP by different processing tehcniques are obtained. The results show that the difference of the cooling rate by different processing technique is above one order of magnitude. It is especially that the cooling rate of MP in broad-beam LCD is lareger than that in micro-beam SLM by 4 orders of magnitude. The change of energy input and the shape of MP are the mian reason for the decrease of cooling rate from SLM to LCD.Grain size of alloy in solidification is usually determined by cooling rate. In 316L stainless steel alloy deposited by LCD and SLM, the width, length and volume of a single columnar grain are all increases with increasing the energy input, and the aspect ration of columnar grain is also increased. However, the relationship between grain size and cooling rate in LCD is significantly different SLM. In LCD, the relationship between the grain size and the cooling rate is ?= a+b/(?)T (? is the grain size, T is the cooling rate, a and b are constants), which agrees well with the traditional solidification theory. In SLM, however, the relationship between the grain size and the reciprocal of square root of the cooling rate is a cubic function, which is different form the traditional solidification theory. In LCD, due to the lower cooling rate and supercooling degree, the ratio between nucleation rate and growth rae reduces, and with the reason of the unidirectionality of the heat conduction of MP, the columnar grain is more suited to growth in comparison with SLM.Due to the multiple thermal cycle in LCD and SLM process, the microstructure of alloy will be more complex. In 1Cr18Ni9Ti stainless steel alloy deposited by high-power SLM, with increasing the depth of MP, the frequency of thermal cycle in cladding layer increases, leading the cooling rate of MP and the supercooling degree decrease, which will result in a coarser grain based on the growth mechanism of columnar grain. In addition, multiple thermal cycle has an effect of solution treatment on the cladding layer, which will weaken the microsegregation of the elements where in the sub-grain boundary, and the distribution of the chemical composition is more uniformly in the alloy. Also, in IN 718 alloy deposited by narrow-beam LCD, with increasing the energy input, the temperature gradient of the liquid phase in MP increases and the constitutional supercooling decreases, leading a transition from dendrites to cellular grains. Additionally, the volume of laves phase in sub-grain boundary increases and the second precipitated phase in matrix decreases.Due to the difference of microstructures betweem LCD and SLM, the mechanical properties of the alloy deposited by SLM is superior to that by LCD. For 316L stainless steel alloy, the value of microhardness of the sample by SLM is 100 HV larger than that of the sample by LCD. The value of ultimate tensile strength of the sample is 180 MPa higher than that of the sample by LCD. Also, the value of yield strength of the sample by SLM is 2.5 times than that of the standard level of forging in ASTM, while the value of the yield strength of the sample by LCD is only 1.5 times than that of the standard level of forging. In 1Cr18Ni9Ti stainless steel alloy deposited by high-power SLM, with increasing the frequency of thermal cycle in cladding layer, due to the change of grain size and elementary composition, the tensile properties of the samples are nearly same. In IN 718 alloy deposited by narrow-beam LCD, with increasing the angle between the growth direction of the columnar grain and the depositing direction of the sample, the tensile property of the sample becomes worse. The tensile strength of the sample when the depositing direction of the sample is perpendicular to the tensile direction is 50 MPa larger than that when the depositing direction of the sample is parallel to the tensile direction.