Study On The Strengthening Mechanism Of Molybdenum Wire Doped With Si,Al,K

Molybdenum wire is often used in incandescent lamp industry for coiling mandrels, filament support wires, foil seals and other parts because of its excellent electricity and high temperature performance. Whereas, the recrystallization temperature of pure molybdenum is a little low. It begins to recrystallize at the temperature of 800℃or so, and then, its high temperature mechanics performance decrease. Therefore, we can increase the recrystallization temperature and high temperature mechanics performance of molybdenum wire by means of Si,Al,K doping. Up to now, the research has been mature on the recrystallization temperature increasing and the high temperature performance improving of molybdenum wire in order to prolong its life and broaden its use range by Si,Al,K doping in home and overseas. The Si,Al,K doped molybdenum wire has even been put to manufacturing in some regions. But, most of the studies focus on the recrystallization temperature increasing and the high temperature performance improving. Few scholars have been devoted to the strengthening mechanism research of molybdenum wire doped with Si,Al,K. Most of researchers apply the strengthening mechanism of Si,Al,K doped tungsten wire to explain that of Si,Al,K doped molybdenum wire indiscriminately. In addition, there is almost no report about the content change of doped elements Si,Al,K during the preparation of doped molybdenum wire, the existence form of Si,Al,K and their evolution after each process. All problems mentioned above have been investigated in this artilce. The strengthening mechanism of molybdenum wire doped with Si,Al,K has also been analyzed and discussed. In this paper, Atomic Absorption Spectroscopy(AAS), Scanning Electron Microscopy(SEM), Energy Dispersive Spectroscopy(EDS), Differential Scanning Calorimetry(DSC), Thermal Simulation, Transmission Electron Microscopy(TEM) and other methods have been used to analyze the molybdenum wire doped with Si,Al,K. Experiment results are listed briefly as follows: ①The content of Si,Al,K falls dramatically after reduction and sintering and there is little left at last. The more the initial doped content of Si,Al,K, the more the residual content of them. Namely, the residual content of Si,Al,K is positively related to their initial doped content. ②The higher the initial doped content of element K, the lower the return rate after reduction and sintering. In other words, the return rate of element K is negatively relate to its initial doped content. Whereas, there lies no such rule for Si and Al. ③The density of sintering molybdenum ingot decreases with the increase of Si,Al,K content doped initially. ④The high temperature mechanics performance of molybdenum wire has been improved markedly by Si,Al,K doping. ⑤The recrystallization temperature of molybdenum wire has been increased by 550～600℃via Si,Al,K doping which generates a coarse and interlocked grain structure . The recrystallization temperature increases with the Si,Al,K doped content. ⑥The doped elements Si,Al,K exist in form of potassium aluminosilicate and aluminosilicate particles on the surface or in the surface holes of molybdenum powder after reduction. ⑦During the low temperature sintering period, the aluminosilicate dopant particles decompose fully, but potassium aluminosilicate is relatively stable. During the high temperature sintering period, the potassium aluminosilicate dopant particles are also to decompose. Most of the decomposed elements Si,Al or their oxides diffuse along grain boundaries into surface-connected porosity and out of the ingot, the residual dissolves into the molybdenum matrix. Because there is no solubility in the molybdenum matrix for K, most of the decomposed element K is removed from the ingot in element form after grain boundary diffusion along grain boundaries into surface-connected porosity. Other decomposed element K generates K bubbles along grain boundary. The sintering temperature of molybdenum is only 2000℃around, so the potassium aluminosilicate dopant particles can not decompose fully under this condition and there must be potassium aluminosilicate left. Therefore, the element K exists in two modes, K bubble and the potassium aluminosilicate dopant particles. K bubble is the main fashion in existence. ⑧K bubble, dopant element-containing pores and the potassium aluminosilicate dopant particles are elongated into ellipsoids in working direction during the swaging and wire drawing processing. When the aspect ratio of bubble exceeds a critical value, these deformed bubbles break up into rows of bubbles due to Rayleigh instabilities during high temperature annealing. The bubble rows and dopant particles distribute not only along the grain boundaries but also in the elongated grains. ⑨The strengthening phase of Si,Al,K doped molybdenum wire is not only the K bubble, dopant particles , such as potassium aluminosilicate, should also be included.We can't apply the strengthening mechanism of Si,Al,K doped tungsten wire to explain that of Si,Al,K doped molybdenum wire indiscriminately.