Research On The Laser Beam Welding Behavior Of Fine-grained ZK60Magnesium Alloy

Magnes ium a lloys have recently become one o f the most important structurematerials due to their low density, high specifc strength, excellent castability andsuperior performance in ins ulating electro magnetic interference, absorbing vibrationand etc. Mg-Zn-Zr （ZK series） a lloys ha ve been considered as one of the mostexcellent co mmercia l magnesium allo ys, especia lly the ZK60ma gnes ium alloy, whic hperforms even better not only for its exce lle nt mec hanica l property but also for itshigh ability to restrict stress corrosion property. However, ZK60is diffic ult to bewelded by conventiona l arc weld ing methods, because cracking occurs as aconsequence of a very wide me lting temperature range above200℃and excess iveheat input, and this restricts its application in engineering structure. Thus, laser beamweld ing（LBW） becomes one of the preferred methods to weld ing the ZK60magnes ium a lloy owing to its low heat input. In this thesis, the CO₂laser has beenapplied to weld ing fine-gra ined ZK60ma gnes ium allo y sheets, and microstructuresand mechanica l properties of autogeno us ly we lded joints are investigated. On thesebases, the effects of initia l state of sheets and postwe ld heat treatme nt on themicrostructures and mecha nical properties of the welded joints are also discussed.The main results are listed as fo llows:Good mechanica l properties can be obtained as the heat input ranges fro m18J/mm to30J/mm and the weld ing speed ranges fro m2m/min to4m/min. When thelaser power is of1400W and weld ing speed at4m/min, optimum joints with ultimatetensile strength at300MPa and e longatio n of12%, whic h are up to92.5%and65%ofthe base meta l respective ly, can be obtained successfully. The organization in thefus ion zone changed fro m equiaxed dendrites into equiaxed crysta l with thedecreas ing heat inp ut. Under the optimum conditio n, the fus ion zone is characteristicof equiaxed gra ins with the size of8μm and the heat affected zone is narrow. Theorganizations near the fus ion line ha ve a go od uniformity. A favorable we ld withoutdistortio n and liq uatio n cracking in the PMZ can be obtained when the fine-gra inedsheets are adopted. The presented experimenta l results demo nstrate that liq uationcracking can be inhib ited effective ly attrib uting to the refining precip itates and grainsin the ZK60magnes ium a lloys.The eutectic phase in the weld ing jo int so lutio nizes into the matrix as well as theresidua l stresses in the weld ment are released during postweld heat treatment, as aresult, the mechanica l performance is improved apparently. When the joints stay 60mins at the te mperature of300℃, the ultimate tens ile strength and elongation canbe high up to324MPa and13%, which are respective ly c lose to99.7%and68.5%ofthe base metal.The grain and the precipitate s ize in the base meta l has no effect on theorganization in the fus io n zone, but has great influe nce on the size of grains in thePMZ and HAZ. The we lding performa nce of sheets in aged and in annea led is lowerthan that in as-rolled when the we lding speed is at4m/min and the laser power is of1400W. Fine dispersed precipitates in the base meta l can inhib it gra in growth in thePMZ and HAZ during welding, and the grain size is only6μm or so. When the size ofgrains in base metal is bigger than7μm, the grain size in the PMZ and HAZ will bebigger than12μm because of the welding heat cycle. Thus, orgnizations near thefus ion line in sheets of aged and annealed are nonuniform, and this will put annegative effect on the mechanica l property.