Influence of Deep Etching Surface Treatment to The Tensile Strength and Microstructure of Brazing Aluminium - Mild Steel

Since hard metals are among the materials that liquid brazing alloys scarcely adhere to, they should not be brazed without first removing the surface layer after sintering. The electrolytic etching method, which gives the surfaces of materials that are hardly wettable very good energetic qualities, is given specific attention. Deep etching is done by using a high concentration of acid solution to roughen the surface. Both dry and wet settings are suitable for etching. An etching fluid is used for etching in a wet environment. Typically, the plate or metal is submerged in an etching solution, where the substance is chemically broken down. While in dry etching, the plasma’s gas phase is where the etching takes place. Due to the presence of plasma in this situation, the etching process that takes place combines chemical and physical processes. Plasma etching is another name for dry etching. Deep etching is an etching process to obtain a rougher surface. Wet deep etching requires a longer immersion time and even several immersions are required. The purpose of this study was to determine the effect of etching treatment in the brazing process of Aluminium-Mild steel for its microstructure and mechanical properties. The specimens in this study used aluminium 6000 series, mild steel, Alusol ER4043, and etching fluid. Mild steel surfaces are deep etched before brazing. Specimen manufacturing standard refers to ASTM D1002. A micro photo test was conducted to compare the surface conditions of aluminium and mild steel with and without etching treatment. The deep etching process is carried out in three variations of time, namely 20, 30, and 40 minutes. The results showed that the tensile strength increased with deep etching treatment on the surface.


Introduction
The brazing process is a welding technology that is widely used in industry for joining materials in the form of pipes, sheets, or plates, both similar and dissimilar metals [1].Brazing or soldering is conducted by using a metal filling or solder metal between the metal surfaces to be joined [2].The filler metal always has a lower melting point than the metal to be joined, which is 840°F (450°C) below the melting point of the parent metal [3].
Parameters that affect the strength of brazed joints are welding temperature, differences in parent material (interfacial), wettability, and etching (deep etching).Deep etching can be done by using a high concentration of acid solution to scrape the surface of the brazed area [4].The occurrence of fundamental physical-chemical processes on the surface of the base material is a prerequisite for getting a correctly constructed soldered joint that meets the prescribed operating requirements [5].It is crucial to first acquire the best wettability and spread-ability of the used solders in order to enable capillary phenomena and a tight filling of the soldering gap with liquid solder [6].As a result, the choice of solders and fluxes, two auxiliary materials for flame soldering, has a significant impact on the quality of a soldered joint [7].
The choice of appropriate auxiliary materials for joints made of dissimilar metals is significantly more difficult and frequently leads to numerous issues, even if it does not typically cause a bigger difficulty in the case of similar-metal junctions.This happens because there aren't any all-purpose solders and fluxes that can thoroughly wet the surfaces of different kinds of materials [8].One of the more difficult technical materials to solder is aluminum, making dissimilar-metal junctions is particularly difficult.This is determined by its following properties.Aluminum's low melting temperature (about 660 o C) makes it difficult to choose a solder.Aluminium surface is covered hermetically by a high-melting (2050 o C) and chemically resistant (11,163 kJ/mol O 2 ) layer of oxides Al2O3.It has low mechanical characteristics, particularly over 500 o C. Strong thermal conductivity (237 W/m K), significant volume contraction (7%), and high thermal expansion (26x10 -6 K -1 ) all contribute to the development of thermal stresses and deformations [9].
The surfaces of acid-resistant steel grade X5CrNi18-10, low-alloyed steel grade DC01, and wellwettable surfaces of aluminium grade EN AW-1050A are soldered using solders of type Zn-Al, however, the surfaces of copper grade Cu-ETP are not guaranteed to be suitably wettable [4].
Copper's insufficient wettability with zinc-based solders is not the sole issue.The interaction between Zn-solders and copper Cu-ETP and steel DC01 produced high reactive zones that made soldered junctions with aluminum fragile [5].The reactive zones may be composed of intermetallic phases of Al-Cu or Al-Fe from the system.Hard zones in dissimilar-metal soldered junctions with aluminum may be eliminated by using a sufficiently thick galvanic layer Zn-Ni that serves as a diffusion barrier [10].

Materials and Method
Zn-based solder with an Al concentration of no more than 15% was used to create junctions between dissimilar metals.Table 1 shows the chemical composition of the solder.Mild steel and aluminium series 6 with a thickness of 1.8 mm and 2 mm respectively are the materials to be combined.Both specimens were cut according to ASTM D1002 standard.The type of joining is the lap joint.The etching liquid used in this etching treatment process is a solution of nitric acid (HNO3) with a solution ratio of 1: 5 reacted with 95% alcohol (C2H5OH).The variation of time in the etching treatment process is 20, 30, and 40 minutes.Fig 1 shows the materials used in the research.There are three kinds of specimen testing, namely shear tensile test, micro test, and SEM (Scanning Electron Microscopy).Tensile tests were performed using the Universal Testing Machine for three specimens for each time variation of the etching treatment.Then the photo micro test was carried out aiming to compare the structure of the metal surface.SEM was performed on the specimen with the strongest tensile test results.

Results and Discussions
Figure 2a shows the results of the analysis of the stress and strain graphs of the shear test of three specimens.The researcher obtained the results of shear testing at the lap joint between aluminium and mild steel with a thickness of 2 mm using Alusol filler without etching treatment.At the connection of the first specimen, the shear stress was 7,703.5 MPa and the strain was 0.71%, while in the second specimen, the shear stress was 15,865 MPa and the strain was 1.74%.In the third specimen, the shear stress was 4,234 MPa and the strain was 0.30%.The result was agreed with the other researcher [11].Figure 2b shows the results of the average stress and strain tensile testing of lap brazed joints between aluminium and mild steel.The joint is treated with deep etching on the brazed surface.At the connection with the etching treatment, the average shear stress was 23,306 MPa for 20 minutes of etching treatment.Meanwhile, the connection with 30 minutes of etching treatment, the average shear stress was 19,708 MPa.The highest average shear stress was obtained at 27,924 MPa from the specimen with 40 minutes of deep etching.
Figure 3 shows the results of the shear tensile test fracture of the specimen with deep etching treatment.Figure 3a and 3b are specimens treated with etching for 20 and 30 minutes.Both specimens were fractured in the brazing or joint area.This shows that the joint strength is lower than the parent metal, namely series 6 aluminum.Fig 3c shows the fracture of the specimen with etching treatment for 40 minutes.The fracture occurs in the aluminum base metal.Based on the fracture results, the joint strength is higher than that of the 6 series aluminum.Figure 4 shows a cross-sectional photo micro of the joint area, aluminium, and mild steel.In the area between the connection with mild steel, a thin layer is seen along the brazed area.The thin layer is an intermetallic formed during the brazing process.The formation of the intermetallic layer is due to heating and reaction with Allusol which is mostly zinc and aluminium.To ensure the formation of an intermetallic layer, further testing needs to be carried out.
Figure 5a with 3000x magnification shows the mild steel surface eroded due to the etching treatment.Red lines are given to indicate uneven surfaces or open gaps due to etching treatment.The surface scraping serves to reduce the surface tension of the specimen to increase the wettability.Thus, during the brazing process, the filler becomes easy to fill in the gaps in the joint area.area.It can increase the strength of the connection.The defects detected in the image area are microvoids and cracks.The black boundary line shows the widening gaps due to the etching treatment so that the filler can fill the open gaps.Intermetallic regions are also formed due to heating during the brazing process at a certain temperature [12].
The connection section with mild steel is shown in Fig 6b .It can be seen that there is an element of mild steel diffused filler.From the picture, it can be seen that the filler crosses the surface line on the mild steel.The diffusion is indicated to occur due to the effect of a longer etching (deep etching) treatment, causing the gaps that open deeper into the mild steel surface to be filled by the filler during the brazing process [13].

Conclusion
The deep etching treatment can increase the shear strength of the brazed joints between aluminium plates and mild steel.The etching treatment affects the shear strength of brazed joints.Connections with etching treatment for 40 minutes obtained higher average shear stress than joints without etching treatment.The yields were 27,924 MPa and 9,267 MPa respectively.The strains obtained were 3.96% and 0.9%.The results of the photo micro comparison show the difference in the surface between the deep etching treatment and without etching.The etching treatment for 40 minutes showed that there were gaps formed, open pores, and a wavy surface.This can be seen in both aluminium and mild steel specimens.The eroded surface layer can make it easier for the filler to fill in the gaps and increase the wettability of the surface.The results of the Scanning Electron Microscopy test on the connection with the deep etching treatment show the presence of diffusion, and the entry of filler into the parent metal.

Figure 1 .
Figure 1. Materials used in the research.

Figure 2 .Figure 3 .
Figure 2. The results of the stress and strain, (a) without etching, (b) with etching treatment.
Figure4shows a cross-sectional photo micro of the joint area, aluminium, and mild steel.In the area between the connection with mild steel, a thin layer is seen along the brazed area.The thin layer is an intermetallic formed during the brazing process.The formation of the intermetallic layer is due to heating and reaction with Allusol which is mostly zinc and aluminium.To ensure the formation of an intermetallic layer, further testing needs to be carried out.Figure5awith 3000x magnification shows the mild steel surface eroded due to the etching treatment.Red lines are given to indicate uneven surfaces or open gaps due to etching treatment.The surface scraping serves to reduce the surface tension of the specimen to increase the wettability.Thus, during the brazing process, the filler becomes easy to fill in the gaps in the joint area.Fig5bshows the SEM results of the aluminum surface with the same treatment.The surface of aluminum erodes less than mild steel.However, there is still visible erosion caused by deep etching for 40 minutes.

Table 1 .
Chemical composition of zinc-based solder