【作者】 方超；

【导师】 乐启炽；

【作者基本信息】 东北大学 ， 材料加工工程， 2011， 硕士

【摘要】 大量研究表明,超声预处理可显著细化镁合金、铝合金以及Sn-Pb等低熔点合金的凝固组织。超声细化技术的本质在于声空化及声流作用导致的非线型效应导致类合金凝固行为的变化,从而实现组织细化。金属合金凝固行为包括形核和长大两个过程,对凝固行为的影响应该与熔体微观结构的变化相关。电阻是金属熔体结构的敏感性物理性质,可以反映金属熔体的结构变化。为理解超声是否对熔体结构产生影响从而导致凝固行为变化最终导致凝固组织细化,因此,为国家自然基金项目“超声致镁合金熔体凝固行为与熔体电阻率变化的相关性研究”的一部分,本文对超声处理条件对Sn-20%Pb合金熔体电阻的影响进行系统测量,并对相应的凝固组织进行观察,试图建立超声导致的熔体电阻变化与凝固组织之间的相关性,从而揭示金属凝固组织超声细化的微观机理。研究取得如下主要结论：(1)通过对超声处理前后Sn-20%Pb合金熔体电阻的实时连续检测,超声处理时合金熔体电阻显著降低(称为瞬时电阻变化量|ΔR|),超声停止后仍然保持一个较小的电阻降(称为残留电阻变化量|Δr|),且此电阻降需在经历一段时间(Δt2)方可回复至超声处理前状态。研究表明,|ΔR|反映了超声作用时声空化过程和声流效应对电阻的影响,而|Δr|反映了超声空化效应对熔体微观结构的改变程度,Δt2反映的是超声处理的有效时间；(2)超声功率和超声处理温度对Sn-20%Pb合金熔体电阻的|ΔR|和|Δr|影响显著,提高超声功率或提高处理温度,|ΔR|和|Δr|均增加；而处理时间对|ΔR|影响很小适当延长处理时间,|Δr|有所增加,但处理时间过长则不再有明显变化；(3)通过对超声处理的有效时间Δt2的比较表明,提高超声功率或处理温度以及延长超声处理时间,Sn-20%Pb合金的Δt2均显著延长,其中330℃熔体经800W超声处理5min后的At2达到155min；(4)通过对不同组成的Pb-Sn合金熔体电阻的测量表明,Sn含量增加相同超声处理条件所致的熔体电阻变化值|ΔR|和|Δrl均显著增加,这表明超声处理高Sn的Pb-Sn合金熔体时超声空化易于发生且其对熔体结构的改变也较容易；(5)通过对Sn-20%Pb合金熔体降温过程熔体电阻的连续检测表明,凝固过程施加超声,合金的初始凝固温度发生改变,初始凝固温度随超声功率增加先增加后降低；(6)通过组织观察及与|Δr|的关联表明,超声预处理显著细化Sn-20%Pb合金熔体的凝固组织,且|Δr|与凝固组织晶粒尺寸d之间存在函数关系,表明|Δr|可以良好表征熔体结构变化,而超声导致的熔体结构是凝固组织细化的根本原因。因此,在电阻变化数据充分积累后将可通过|Δrl预测熔体的凝固组织,这对超声预处理的熔体凝固组织控制至关重要。更多还原

【Abstract】 A mass of researches indicate that ultrasonic pretreatment can significantly refine the solidification structure of the low melting point alloy, such as Mg, Al, and Sn-Pb alloys. The essence of ultrasound refining technique is the Nonlinear effect caused by sound cavitation and sound flow Metal alloy solidification behavior include two process of nucleation and growth. The influence of solidification behavior should be related to the microstructural changes. Resistance, a sensitive physical quantity to the micro structural, can reflect the structural changes. So we can understand the effects of ultrasound on the solidification behavior of the mechanism by observing the changes of resistance. In this article, as a part of National Science Foundation project "the relational research between ultrasound on solidification behavior of Mg alloy melt and melt resistivity changes", the resistance changes of Sn-20%Pb under ultrasound processing conditions were measured comprehensively and the corresponding solidifying structure was observed, influence of the system and the corresponding solidification organization, trying to build up to observe the ultrasonic melt resistance changes and solidified the correlation between the organization, and the refinement of mechanism of metal solidification organization after ultrasound trentment are revealed The main conclusions are as follows:(1) The melt resistance significantly reduced(AR) under Ultrasound treatment, remains small resistance drop(Ar) after ultrasonic stop and need time (Δt2) back to the original state before ltrasound treatment. Research shows that |ΔR| reflects the influence of the cavitation and sound flow effect to resistance under ultrasonic treatment,|Δr| reflects the melt micro structural change caused by ultrasonic cavitation effect,Δt2 reflects the valid time under ultrasound treatment;(2) Ultrasonic power and ultrasonic processing temperature influence significantly AR andΔr (?)f Sn-20%Pb alloy melt resistance. Improving ultrasound power or increasing processing temperature makes|ΔR| and |Δr increased; And the influence of processing time to |ΔR| is very small. Extending the processing time increased |Δr| a little that no longer changes after a period of time. (3) Through the comparison of the ultrasonic treated valid timeΔt2, improving ultrasound power or processing temperature and extending the ultrasound processing time, madeΔt2 of Sn-20% Pb alloy significantly extend, of which theΔt2 was up to 155 min at 330℃and ultrasound treated 5 min at 800 W;(4) The resistance measurements of different Sn-Pb alloy melt composition show that the increase of Sn content made |ΔR| and |Δr| increased significantly. In other words, under the ultrasonic treatment on high Sn alloy melt,ultrasonic cavitation occurred easily and the melt structure could be changed easily as well;(5) Continuous melt resistance measurement of Sn-20%Pb alloy at cooling process show that, The initial solidification temperature of alloy changed on the process of solidification,the initial solidification temperature went up and down with ultrasonic power increase;(6) The observation of relationship between organization and |Δr| in the heating process show that ultrasonic pretreatment could refine significantly coagulation organization of Sn-20% Pb alloy melt. In the heat preservation process |Δr| and solidified organization grain size had a function relation. The melt structure change caused by Ultrasound is the root cause of the refining solidification structure. Therefore, if the resistivity data accumulated fully, we can use |Δr| to forecast the melt solidification organization. This is very important to the ultrasonic pretreatment melt solidifying structure.更多还原