Research On Manufacturing Technique Of Single Crystal Of Cu-base SMA And Closed-Loop Fuzzy Control For DSCC Processing

Shape memory alloys (SMA) are a kind of important functional materials. Ti-Ni SMA had got the most widespread availability. However, Copper base SMA didn't get real application up to now, although Cu-base SMA has good shape memory performance and super elasticity. That was because that it's found that there were some questions such as bad ductility, easy intercrystalline failure and short fatigue life in the application process of the Cu-base SMA. Moreover, its cold-workability was very bad, and its fatigue strength was very low, too. The main reason of these questions was high anisotropic elastic behavior of the polycrystalline Cu-based SMA.In the paper a manufacturing technical route about preparing single crystal of Cu-base SMA, that is the directional solidification continuous casting (DSCC) technology, was presented. Based on the computer simulation study of the DSCC processing, the closed-loop control programme with correction of the disturbances in the DSCC processing was introduced. Solved the key skill craft in the manufacturing technique and completed the DSCC equipment's design and manufacture the question of Cu-base SMA single crystal industrialization product was settled at root. Thus a path to really actualize the engineering appliance of Cu-base SMA was provided.The experimental results showed the cooling-heating cyclic fatigue performance and recoverable deformation performance of Cu-Al series SMA were highly sensitive to grain boundaries in the material which is of high anisotropic elastic behavior. The single crystal of the Cu based SMA which is without grain boundaries had the best cooling-heating cyclic fatigue performance and recoverable deformation performance. For the single crystal samples of CuAlNiBe alloy, the strength of extension (σ_b) was 780MPa; the extensibility(ε%) was 17%; the break number of the cooling-heating cyclic fatigue was about 750 times and the maximum recoverable deformation was 10% being more than a match for the Ti-Ni alloys' 8～9%.However, a single crystal preparation way in laboratory was not fit for industrialization because of its low efficiency and high cost. Consequently, DSCC technology was the primary selection because of its satisfying requirement of product quality, production coefficient, yield and manufacturing cost.And a multichannel horizontal DS continuous casting equipment was designed referencing the model machine made before. The equipment's designs included following parts: melting part, insulation part, liquid-level controller part, continuous casting furnace part (this part was splitter box in multichannel equipment), casting mould part, cooling part, casting ingot pulling part, river diversion part, overflowing protect part and so on.The product experience and reseaches before showed that DSCC processing had strict requirements in technological parameters' control. And it also combined with continuous casting technique. Therefore, the key was to realize the closed-loop control in this process.In order to build the closed-loop control programme with correction of the disturbances in the DSCC processing a mathematical model of DSCC processing was built, which confirmed the location (Z) of the liquid-solid (L/S) interface as main control object and imported shape factor (X) as an assistant control object. With the model computer experiments to abtain deep acquaintance for the DSCC processing were underwent.Through analysing the simulation results, it was found , that the cooling distance (L), the pulling speed (V) and the inner temperature in the crystallizer (T_m) had obvious influence to the temperature distribution curve. The relationship functions between above three parameters and L/S interface location(Z) and shape factor (X) had abtained through computer simulation. The functions showed in following: 1) Functions between parameters and Z were: L: Z_E =-0.58 + 1.55λ_LV: Z_E = -0.58 + 0.5λ_V + 0.25λ_V²T_m: Z_E =-0.58 + 87.1λ_Tm-1199.3λ_Tm²2) Functions between parameters and X were: L: X = 0.25-0.17λ_LV: X = 0.25-0.06λ_VT_m: X = 0.25-4.32λ_TmThe simulation results also showed that the melt temperature Tb, the cooling water temperature Tw and the cooling water flow Q had not obvious influence to the temperature distribution curves. And the influences of the liquid surface level (h) mainly took place while L/S interface location (Z) at the out of crystallizer. Under this condication, if h was too high, its additional force couldn't be balanced by melt liquid surfacial force. Consquently, melt liquid leaked out and continuous casting process failed.The ultimate safe range of Z that DS continuous casting process was steadly on was from -4mm to 1.79mm from the simulation results.At the same time, the simulation results also indicated at first time that the relationship between Z and L or V was not simply plus, but a reinforcing relationship. The relationship function was shown as following:Z_E =-0.58+1.55λ_L +0.55λ_V +0.25λ_V² + 0.99λ_Lλ_V.From above functions, it could be found there was a reinforcing factor 0.99 between L and V.The difference between real measuring and computer simulation results of the temperature distributions was only about 20℃. That is to show the computer simulation had high reliability. Based on the analysis of the computer simulation results about the solidification principles and heat tansfer features of DSCC processing, the closed-loop control programme with correction of the disturbances in the DSCC processing was introduced. In the programme with correction of the disturbances in the DSCC processing the L/S interface location(Z) at the out of crystallizer was set as the object control variable and a ultimate safe range of Z also was confirmed. Then, L and V were set as the adjustable variables. And the inner temperature in the crystallizer (T_m), which disturbence was the most obvious, was set as the lash-up treating variable. That is to say, the T_m was adjusted while fluctuate of Z was too sharp or Z could not return back to the safe range in short time. The other parameters were keept to constant.With ANSYS software the fuzzy control system model with the temporal distribution functions between Z and L, V without any disturbance was set up and its control algorithms of nonlinear relation between object variable and adjustment variables in the DSCC processing were designedThe closed-loop fuzzy control processes of DSCC processing were simulated through MATLAB software. The simulation results proved the designed control system and the fuzzy control device satified the requirments of the control of the DSCC processing.Based on the technical features of the DSCC proceessing and the control program, following software and hardware system were also designed and prepared:Complete the following work: selecting a suitable computer, designing a constant temperature cooling water circulating system, a liquid level constant control system, a L/S interface location correction disturbance control system and an electric source anti-interference system.Select a commercial software as the control system development flatform. And confirm a configuration software as the main conrol application program. Adopting the blocking design method , base on configuration software feature, the whole project was partitioned to several parts. The direct-viewing man-machine interface was completed through designing every part. The results showed the interface was simplicity of operator and more fit for factory production.Through above works, the soft and hard parts constituted the control system used to the control of the DSCC processing, which became the base of real-time closed-loop control successful operation.However, although the design and manufacture of the DSCC equipment and the control system were completed, they were not put to use in the actual production process because of the limit of object condition. Therefore, the integrated correct judge about the designed equipment and control system could not be confirmed. We may believe that the technical route is feasible, but there is much more works to do before the designed equipment and control system put to use in actual production. And these works will be important in pushing development of Cu-base SMA single crystal preparation and the DSCC processing control system. And it also supplys a feasible way of the development of multi-factors control system in complex alloy single crystal continuous casting field, even in whole metallurgy industry.