【作者】 王晓林；

【导师】 杨士勤； 闫久春；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2007， 博士

【摘要】 聚醚醚酮(PEEK)具有高强度、高模量、耐冲击、耐高温、耐辐射以及尺寸稳定性好等一系列优点,在宇航、电子、信息、能源等高技术领域具有非常广阔的应用前景。聚醚醚酮作为热塑性材料还具有可焊接的特点,因此可以不采用机械连接和粘接等连接工艺,从而避免因应力集中、密封性降低、耐腐蚀性下降导致的接头使用性能变差的问题。目前,在各种塑料焊接方法中,超声波焊接方法具有成本低、自动化程度高、效率高、适用范围广等优点,因此其能否成功的应用于PEEK,对PEEK构件制造水平的提高具有非常重要的意义。超声波焊接PEEK存在的主要问题是接头强度偏低,这是由PEEK的高熔融温度和高结晶度导致的。因此,本文针对PEEK的这两个特性,围绕超声波焊接界面的产热-形变过程和焊接界面的成型过程展开研究。根据聚合物材料在循环振动载荷下的生热理论,建立了热力耦合的超声波焊接过程有限元模型。采用此模型对焊接振幅、焊接压力、导能筋的形状和尺寸等因素对超声波焊接过程的影响进行了研究。结果表明,焊接界面上的温度在焊接初期会以非常快的速度升高到一定值,之后便会保持在此定值附近。其内在原因是由于导能筋的变形,使焊接界面上的产热功率不断降低以及导热面积不断增加。焊接界面能够达到温度随着焊接振幅的增加而提高。因此在焊接PEEK时,只有焊接振幅超过一定值时,才能顺利完成焊接过程。焊接压力、导能筋的形状、导能筋的尺寸是通过影响导能筋的变形来影响导能筋焊接界面上的温度和温度分布的。焊接压力具有促进导能筋变形的作用,因此增大焊接压力会导致焊接界面温度的降低,以及下焊件表面温度的降低。矩形导能筋由于发生墩粗状的变形,使得导能筋同上下焊件接触面的边缘同时成为焊接界面上的最高温度区。半圆形导能筋由于产生滚动式变形,因此在焊接界面没有达到较高温度的条件下,导能筋同下焊件之间的接触面积也会有较大的增加,使得下焊件表面的温度始终处于非常低的状态。采用三角导能筋,在合适的工艺参数下,最高温度区同时分布在导能筋的尖端和下焊件表面,有利于有序熔化铺展过程的进行。对于三角形导能筋,顶角角度对焊接界面所能达到的温度未见显著影响。但是,30°顶角角度的导能筋由于刚度较小,仅在焊接压力的作用下导能筋就会发生较大的尖端膨胀变形,使得最高温度区远离焊接界面,而且容易发生尖端断裂现象。60°和90°顶角导能筋最高温度区同时包括焊接界面上导能筋尖端和下焊件表面,有利于熔融连接。120°顶角导能筋焊接时,导能筋的变形最小,但是与其接触的下焊件表面的高温区面积较小。通过控制导能筋下塌量,获得一系列导能筋铺展过程的中间结果方法,对导能筋的熔化铺展过程进行了研究。结果表明,PEEK的焊接界面上产生的大量密集孔洞缺陷是其超声波焊接接头性能下降的主要原因。焊接界面上的孔洞缺陷的尺寸和数量同焊接振幅成正比,同焊接压力成反比。增大导能筋的尺寸,导能筋向下板插入的深度增加,一定程度上减轻了熔融树脂从焊接界面上的飞溅,但是由于形成大尺寸孔洞缺陷,接头强度下降。增大三角形导能筋的顶角角度,也有利于减轻焊接界面上熔融树脂的损失,但是由于形成了连接面积小,接头强度降低。提出了采用预热+小焊接振幅来进行PEEK的超声波焊接的新工艺。结果表明,在预热温度260°C以上时,焊接界面上的成型过程发生变化。上下焊件之间先达到紧密接触,再发生熔融连接过程。在此过程中,焊接界面上的温度波动小,未见熔滴喷溅现象。采用28μm的焊接振幅,400N焊接压力、300°C的预热温度和2s的焊接时间,可获得~39MPa抗剪强度的PEEK超声波焊接接头。更多还原

【Abstract】 Polyetheretherketone (PEEK) is a kind of high performace thermoplastics, which has many merits, such as higher strength, higher stiffness, higher impact resistance, higher invironment resistance, higher radiation resistance and higher sevice temperature. As a result, PEEK finds many potential applicaons in high-tech industries. PEEK also can be welded, which avoids using mechanical and adhesive joining methods. These methods may bring some joint problems such as stress concentration, poor leakproofing performance and lower strength. Though many plastics welding methods can be used to bond PEEK, ultrasonic welding has most advantages. It has shortest welding time, lowest cost, cleanest joint and higher degree of automatization. So the success of appling ultrasonic welding to PEEK is most significant to improve its manufacture level. The main problem existing in ultrasonic welding of PEEK is lower joint strength, which is caused by higher melting point and crystallization degree. Therefore, the focus of the present dissertation is to investigate the effects of these two properties on the thermal and formation process of welding interface.Based on the viscoelastic heating theory, a mechanical-thermal coupled finite element model was developed. And using this model the effects of welding amplitude, welding pressure, energy director shape, energy director size and energy director tip angle were investigated. The results show that the temperature of welding interface initially rapidly rises to a certain value, and then willl level off. The reason behind is that the strain amplitude in the welding interface decreases with energy director deforming, which lead to the decline of heat generation power of welding interface. The temperature can be reached is positively related to welding amplitude. Only when the welding amplitude exceeds 30μm, welding process can proceed. Orther factors such as welding pressue, energy director shape, energy director size and energy director tip angle taking effects on temperature profile of welding interface is through affecting the process of energy director deforming。Welding pressue plays a role to promote the deforming of energy director, so increasing it leads to the decreasing of temperature of welding interface. Using rectangular energy director the highest temperature zone locates at the border of interface of energy director with upper part, which is harmful to welding process. Using semicircular energy director the part temperature keeps at room temperature, which is also harmful to welding process. Using triangular energy director, the highest temperature zone locates at the tip of energy director and the part temperature can rise to 180°C, which is benefit to welding process. For triangular energy director, the optimum tip angle is 90°and optimum cross-section area is 0.25mm2 as welding PEEK.The melting down process was investigated in the present dissertation through controlling the collapse distance of energy director. The results show that the porous microstructure of welding interface is the main reason for the lower joint strength during ultrasonic welding of PEEK. The size and quatity of pores is positively proportional to welding amplitude and is negatively proportional towelding pressure. But too high pressure leads to excessive plastic flow of interface resin which undermines the joint strength. Increasing enegy director size can increase the depth of curved welding interface, which decreases the splashing of liquid drop of PEEK and alleviates the porous defects. But using too large energy director weak bonding joint will formed, the actual joint strength declines. Increasing energy director tip also can slightly allevate porous defects, but the bonded area decreases at same time, which significantly lower the joint strength.An improved ultrasonic welding process for PEEK is developed. The results show that using this process the porous defects can be eliminated from welding interface. As using 28μm welding amplitude, 400N welding pressure and a preheat temperature of 300°C a porous free PEEK joint with a shear strength of ~39MPa can be obtained.更多还原