【作者】 王刚；

【导师】 沈军；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2012， 博士

【摘要】 本文针对传统铸造法制备Ti基块体非晶合金时合金具有较低的玻璃形成能力的问题，采用不同的焊接方法将小尺寸的Ti块体非晶合金连接成大尺寸的Ti基块体非晶合金，对焊件的微观组织和力学性能进行了系统的研究，并对成功焊接Ti基块体非晶合金的机理进行了探讨。利用激光焊接技术快速加热和冷却的特点，对不同激光焊接参数下得到的焊件进行了微观组织和性能研究。实验结果表明，在激光功率为3.5kW，焊接速度10m/min时，可以获得完全非晶态的接头，并且该接头的拉伸强度可以达到母材的93%，体现良好的力学性能，断口表面分布大量脉状花样和熔滴。而在其它焊接参数下得到的焊件，其焊缝区和热影响区内均有晶体相析出。通过计算得出Ti₄₀Zr₂₅Ni₃Cu₁₂Be₂₀块体非晶合金在加热过程中保持非晶态特征的临界加热速度，该值远远小于激光焊接过程中样品的加热速度，因此，Ti₄₀Zr₂₅Ni₃Cu₁₂Be₂₀块体非晶合金在加热过程中不会发生晶化。进一步计算得出熔合线附近，靠近焊缝区获得非晶态合金的冷却速度约为780K/s，该值远远大于Ti基块体非晶合金的临界冷却速度，原因在于焊接过程不是在真空条件下进行，大气中的氧显著降低了非晶合金的玻璃形成能力。采用数值模拟的方法研究了Ti₄₀Zr₂₅Ni₃Cu₁₂Be₂₀块体非晶合金激光焊接过程中的温度场变化，进一步分析了Ti基非晶合金在激光焊接过程中保持非晶态结构的机理。以高斯面热源＋柱热源为热源模型，考虑对流、辐射的影响，测定了材料热物理参数随温度的变化情况。将模拟计算得到的结果与实验结果相对比，验证了模型的准确性。基于此模型，考察了不同焊接参数下Ti基非晶合金的热历史。结果表明，熔合线附近，靠近焊缝区得到的冷却速度与理论计算得到的值相一致；热影响区内，激光焊接过程中样品的加热速度亦远远大于临界加热速度。利用块体非晶合金在过冷液相区内具有超塑性的特点，研究了Ti₄₀Zr₂₅Ni₃Cu₁₂Be₂₀块体非晶合金的摩擦焊接行为。保持顶锻力和顶锻时间不变，在一定摩擦转速和摩擦时间条件下，可以获得完全非晶态的接头，并且完全非晶态的焊件接头拉伸强度可高达1580MPa，约为母材的90%。提高摩擦转速时，摩擦界面处有晶体相析出，晶化产物为β-（Ti, Zr）。研究了焊接参数对于飞边形成的影响，结果表明，飞边的厚度与摩擦压力、顶锻压力以及焊接时间无关，与转速呈正比关系，而飞边的体积与焊接时间成正比关系。对于接头保持非晶态的机理研究表明，非晶合金过冷液相区内粘度的急剧变化与摩擦转速之间可以相互调节以保持摩擦界面处的温度不至于过高。结合摩擦焊接界面的温度分布特征，得到一定转速下，界面处保持非晶态结构所需要的临界摩擦时间公式，计算得到不同摩擦转速下的临界摩擦时间，通过相关实验和模拟验证了该数据的正确性。结果表明，在一定的转速下，当施加的摩擦时间小于临界摩擦时间时界面可以获得完全非晶态结构。研究了Ti₄₀Zr₂₅Ni₃Cu₁₂Be₂₀块体非晶合金电阻点焊行为。在焊接电流5kA，焊接时间2cycles条件下，熔核区能够获得非晶态，然而，热影响区内有晶体相析出。另外，采用完全晶化的样品进行点焊，同样获得了完全非晶态的熔核区。对焊件进行拉剪测试，结果表明，焊件主要以三种断裂模式进行断裂：界面断裂、局部断裂以及纽扣式断裂，其中绝大多数为局部断裂模式。焊件的力学性能与点焊熔核尺寸以及接头处的晶化程度密切相关。对熔合线附近的冷却速度进行计算，得到获得完全非晶态接头所需要的临界冷却速度。更多还原

【Abstract】 Due to the limited glass formation ability, Ti-based alloy can be only cast into glassysamples with small size via conventional casting process. In the present dissertation,Ti₄₀Zr₂₅Ni₃Cu₁₂Be₂₀BMG samples with small-size have been successfully welded intolarge-size Ti-based BMG by different welding methods. The microstructure andmechanical behaviors of welded BMG samples have been systematically investigated.The underlying mechanism for the maintenance of a glassy phase in the welded BMGsample has been also discussed.The microstructures and mechanical properties of BMG samples welded by variousprocessing parameters have been investigated, based on the rapid heating and coolingcharacteristics of laser welding. The results indicated that a joint with fully amorphousfeature could be achieved using a welding speed of10m/min and a laser power of3.5kW. The sound joint exhibits a high tensile strength,93%of the base alloy,demonstrating a perfect bonding. Well-developed vein patterns, typical of the fracturefeature for amorphous metals, were observed over the whole fractured surfaces. For thecase of the samples welded from other welding parameters, crystalline phases wereobserved both in weld fusion zone and heat affected zone. The critical heating ratekeeping Ti₄₀Zr₂₅Ni₃Cu₁₂Be₂₀BMG without crystallization during the heating processwas calculated. The calculated critical heating rate was much lower than the heating rateexperienced during laser welding, suggesting that no devitrification occurs duringheating process for Ti₄₀Zr₂₅Ni₃Cu₁₂Be₂₀BMG. The cooling rate in the weld fusion zonehas been further calculated to be780K/s, much larger than the critical cooling rate ofcast BMG sample. This phenomenon can be attributed to the fact that that the laserwelding was carried out under atmosphere condition. Thus, the introduction of oxygencould seriously suppress the formation of amorphous phase, leading to the highercritical cooling rate for glass formation during laser welding.Temperature fields during laser welding have been studied by numericalsimulations, in order to further discuss the mechanism of successful welding of theTi₄₀Zr₂₅Ni₃Cu₁₂Be₂₀BMG. Heat source model with the cylinder and Gaussian wasselected. Material properties were detected as temperature-dependent functions, afterconsidering the effect of convection and radiation on heat transfer. Calculation resultsagree well with the experimental data through comparing of the weld width and poolshape, confirming the accuracy of heat source model. Based on the model, thetemperature fileds of the Ti-based BMG at different parameters were calculated. Thecooling rates measured around the fusion line, and near the weld fusion zone accordwell with the calculation values. For heat affected zone, the heating rate of sample was also much larger than the critical heating rate.Friction welding of Ti₄₀Zr₂₅Ni₃Cu₁₂Be₂₀BMG rods have been performed, utilizingthe unique superplastic deformation feature within supercooled liquid region of BMGs.Joint with fully amorphous structure can be obtained by selecting proper friction timeand rotational speed under constant upsetting force and upsetting time. The glassy jointobtained exhibits a high tensile strength （1580MPa）,90%of the base alloy. Withincreasing the rotational speed, numerous crystals embedded in the interface wereobserved under different friction times, the crystal phases are indentified as β-（Ti, Zr）.The influence of welding parameters on the formation of protrusion has been studied.The friction time and upsetting force exert no influence on the thickness of theprotrusion, whereas the rotational speed has great influence on the thickness of theprotrusion. However, the volume of protrusion is proportional to the friction time. Themechanism for the maintenance of a glassy phase in joint has been interpreted. Due tothe decrease of the viscosity resulting from the deformation of supercooled liquid andthe rotational speed, the temperature of the interface can be self-controlled so as not toover-heat. Based on the analysis of the temperature distribution during the frictionwelding, an equation mentioning the critical friction time for retaining amorphousstructure has been established. The experimental results varify the reasonability of theequation. Under constant rotational speed, the joint with fully amorphous structure canbe obtained when the friction time is lower than the critical friction time.Ti₄₀Zr₂₅Ni₃Cu₁₂Be₂₀metallic glass has been welded by using resistance spotwelding. Under5kA welding current and2welding cycles, the nugget zone was foundto retain its original amorphous structure. However, crystals embeded in the matrix wereobserved in heat affected zone. Moreover, fully crystallized Ti-based alloy sheets wereprepared and then welded under5kA welding current and2welding cycles. Amorphousfeature was also obtained from the WFZ of the crystallized Ti-based alloy sample,during the tensile-shearing tests, the fracture occurs following three distinctive fracturemodes: interfacial failure, partial failure, and button pullout failure. Meanwhile, thefracture mainly took place via partial failure mode. The mechanical performance of thejoint was closely related to the nugget size and crystallization volume fraction.Moreover, the critical cooling rate for preserving the amorphous phase has beenobtained by calculating the cooling rate around the weld line.更多还原