Wetting of Al Alloys for Hot Dipping Coating Process

*Qiaoli Lin, Ran Sui and Weiyuan Yu* 

### **Abstract**

 Wetting phenomenon, as a basic physical process, also relates to the many material processes, such as coating process, brazing or soldering process, casting, preparation of MMCs, etc. In this chapter, the method of wetting characterization at the high temperatures was presented (especially for the metallurgical melts), also the wetting behavior and mechanism of Al alloys (4043 and 6061 alloys) on the different metallic substrates at isothermal dwelling process as well as the characteristics and formation mechanism of precursor film were discussed. Thermodynamics of segregation of solute element were also discussed, which can be predicted by the thermodynamic model. We believe the content of this chapter would be a guidance for hot dipping process based on the wetting theories.

**Keywords:** wettability, aluminizing, brazing, interfacial reaction, adsorption, precursor film

#### **1. Introduction**

At high temperatures, wetting of a solid (metal or ceramic) by the molten metallurgical melts is of great technological importance in a variety of metallurgical processes, e.g, hot dipping coating process, brazing process, casting process and sintering process. Each process has the different optimal wetting condition. For the hot dipping coating process, the perfect wettability of base materials by liquid metal is demanded. Two key issues for wetting at high temperatures include the spreading dynamics (wetting behavior) and the final wettability (the degree of wettability). For the former issue, it determined the technological parameters in process; for the latter issue, it would be one of the critical evaluation bases for whether the process can be carried out or not.

 The description of wettability since from 1805 in Young's work [1], has been well developed, as following,

$$\mathbf{\dot{\cos}}\Theta \; = \begin{array}{c} \frac{\sigma\_{\gg\nu} - \sigma\_{sl}}{\sigma\_{lv}} \end{array} \tag{1}$$

 where θ is contact angle for a liquid equilibrium with ideal solid at the point of triple line, σ*lv*, σ*sv* and σ*sl* are the tension between liquid-vapor, solid-vapor and solid-liquid interfaces. The value of θ represents the degree of wettability, i.e., the better wettability and the smaller θ, also cosθ stands for the energy in a dimensionless form. The methods for obtaining θ are numerous, but for the

wetting system of metallurgical melts at high temperatures, need to consider many other factors, not just the high temperatures, metallurgical process, atmosphere, interfacial reaction, etc. Further, the characterization of wetting should pay attention to two points, one is the wetting under the quasi-ideal condition, another is the wetting in a real process which including the consideration of many other factors, such as nonequilibrium temperature field, heat input and output, physic-chemistry effect of flux on interface, etc. Due to the complexity, the method for the characterization of the wetting under the quasi-ideal condition would be elaborated in Section 2.

 The coating process by using a hot dipping method, although a traditional technology with relatively high energy consumption, is also a reliable technology with high efficiency. The quality of coating directly depends on the wetting of base metal by the coating metal. The performance of coating as well as the technological parameters in the coating process is also affected by the trace addition in the alloys which may act on the solid/liquid interface and(or) the liquid/vapor interface. In this chapter, based on the effect of trace elements on the wetting behaviors and the interfacial structures, the wetting mechanism of base metals (steel, Ti6Al4V (TC4) alloy, pure Ti (TA2)) by Al alloys (4043 alloy and 6061 alloy) as well as the formation of precursor film in these systems would be presented in Section 3.

This chapter presents an overview of wetting parameters at high temperatures, wetting mechanism in the isothermal spreading, the effect of trace elements on the formation of interface and the possibility of designed interfacial structures.
