焊接是两个或两个以上的部件连接在一起，以确保它们完全结合在一起的过程。焊接的目的是确保两种不同或相似的材料连接在一起。这些整体结构的建造通常要经过强度测试和耐久性评估。这些因素对于提高航空航天行业的联合优势非常关键(Li et al.， 2008)。传统焊接方法中的问题是LFW等较新的焊接方法产生的原因。焊接通常是通过加热和加压来实现的。焊接形式通常会根据所使用的基材的状态而有所不同。液态焊接又称熔焊和固态焊接，是根据焊接状态特点而产生的焊接形式。
熔焊通常是将基材加热熔化，然后再用于准备连接或任何其他动作。在熔焊过程中，主要的特点之一是没有填充材料。通常会添加填充材料来增加关节的强度或用于其他目的。在熔焊过程中，这种填料是不存在的，焊接操作称为自生焊缝。熔态焊接的几种形式有氧燃料气焊、电弧焊和电阻焊。氧燃料焊接通常是作为切割和分离金属板到零件的单独组合进行的。这里使用的混合物是氧和乙炔，与通常的熔焊状态不同，填充剂可以作为例外(Li et al.， 2008)。这里使用的填料通常是用来模拟焊接过程的填料。火焰是由填料产生的，这使得它是一个非常经济和多才多艺的过程使用。它是利用低数量的生产或用于修理作业。另一方面，电弧焊形式的熔焊被认为是一种利用电弧的熔焊。电弧过程增加了焊缝的强度。这是一种良好的联合技术，在汽车机械厂。填充金属在这里再次使用，但只是因为它增加了正在制作的接头的强度。用电极产生电弧，熔化的焊缝就会凝固。电阻焊接不像电弧焊那么常用，它是一种依靠热压技术来实现焊接的焊接方法。这里使用电阻来控制界面上的电流流动，因此这里不需要使用填料(Schubert et al .， 2001)。
固态焊接不同于熔焊过程，焊接操作是在界面层进行连接或熔合。在这里，两个或两个以上的表面被带入液体或熔融状态，然后关节被创建(Vairis & Frost, 1998)。通常，固态焊接中比较流行的焊接种类是扩散焊，其次是摩擦焊和超声波焊。扩散焊接是在高温高压条件下进行聚结的过程。在扩散焊接的情况下，通常建立在表面的变形是最小的，因此在航空航天应用的背景下，它发现了很多用途(Zink, 2001)。通常不同的材料需要焊接在航空航天工业的情况下，通常是用这种焊接(Abel et al, 2015;Tavares等，2013)。这些都是航空航天工业中使用的传统方法，它们本身存在一些问题和缺陷，这是寻求较新的焊接技术和对其进行分析和评估的原因之一。摩擦焊接属于固态焊接。摩擦焊接是通过表面的摩擦来获得焊接所需的热量的过程。工作部件按常规尺寸组装在一起，通常焊缝的形式是，与熔焊相比，所遇到的缺陷较小(Xin et al .， 2016;布鲁克斯,2015)。超声波焊接也是固态焊接过程的一部分。在超声焊接过程中，零件在压力作用下结合在一起，通过高频振动能实现聚结。焊接通常使用电极或一些超声能量头，这些电极或能量头夹在振荡器下，与焊缝表面平行工作(Sinke, et al .， 2010)。
Welding is the process where two or more pieces are joined in order to ensure that they are completely coalesced. The purpose of welding is to ensure that two dissimilar or similar materials are joined. The creation of these monolithic structures is usually tested for strength and assessed for durability as well. These elements are quite critical for increasing join strengths in the case of the aerospace industry (Li et al., 2008). The issues in conventional welding methods are the reasoning behind why newer welding methods such as LFW came into existence. Welding is usually achieved by means of heat and pressure. The forms of welding will usually be different based on the state of base material that is used. Liquid state welding which is called as fusion welding and solid state welding are some of the types based on welding state characteristics.
2.2. Fusion Welding
Fusion welding is where the base material is usually heated to melt before it is used for preparation of joint or any other action. In the case of the fusion welding process, one of the main characteristics is the absence of the filler material. Usually a filler material would be added to increase the strength of the joint or for other purposes. In the case of the fusion welding process such filler is absent and the weld operation is termed as an autogenously weld. Some of the forms of fusion state welding are oxy-fuel gas welding, arc welding and resistance welding. Oxy fuel welding is usually done as a separate mix of cutting and separation of metal plates to parts. Here the mix used is oxygen and acetylene and unlike the usual state of fusion welding a filler can be used as an exception (Li et al., 2008). The filler which is used here is usually one that is seen to imitate the weld process. The flame is produced by the filler and this makes it a very economical and versatile process to use. It is made use of low quantity production or for the use of repair jobs. On the other hand, the fusion welding in the form of arc welding is seen to be one that makes use of the electric arc. The electric arc process adds to the strength of the weld joint. This was a favourable joint technique in auto mechanic works. Filler metal is once again used here but only as it adds to the strength of the joint that is being made. With an electrode of creating an arc, the molten weld will solidify. The Resistance welding is not as commonly used as the arc welding and is one that relies on heat and pressure techniques to achieve the weld. Here an electrical resistance is made use of in order to control current flow at interface and hence it is not required to make use of a filler here (Schubert et al, 2001).
2.3. Solid-state welding
Solid state welding is different from the fusion welding process in that the welding operation will take place as a joining or fusion happening at the interface level. Here two or more surfaces are brought into a liquid or molten state and then the joint is created (Vairis & Frost, 1998). Usually, the welding category that is more popular in the case of solid state welding is the diffusion welding, and secondly friction welding and ultrasonic welding is done. Diffusion welding is a process in which the coalescence is usually achieved at elevated temperature with high pressures. In the case of diffusion welding it is often established that deformation at surface level is minimal and hence in the context of aerospace applications it finds much use (Zink, 2001). Often dissimilar materials that need to be welded in the case of the aerospace industry is usually done with this kind of weld (Abel et al, 2015; Tavares, et al, 2013). These are conventional methods used in the aerospace industry that lend itself some issue and defects, which is one of the reason for the search of newer welding techniques and analysis and assessments of the same. Friction welding comes under solid state welding. Friction welding is the process in which the heat obtained for the welding is usually achieved by means of the rubbing of surfaces. Work parts are held together in a conventional scale and usually the form of weld is such that the defects encountered are seen to be lesser compared to the fusion welds (Xin et al, 2016; Brookes, 2015). The ultrasonic weld is also a part of the solid state weld process. In the ultrasonic weld process, the coalescence is achieved by means of high frequency vibratory energy where parts are held together under pressure. The welding usually occurs with the use of an electrode or some ultrasonic energy tip which is clamped under an oscillator to work in parallel with the weld surface (Sinke, et al, 2010).