Comparisons between Assembly Technologies by Welding and Brazing of Aluminium Alloys from the Sight of Assembly Quality

Aluminum and its alloys are finding use in more and more industrial fields. Since the early 1970s, when many companies in the automotive, pharmaceutical and aeronautical industries replaced the classic components of iron alloys with those of aluminum alloys, they faced various difficulties in the non-removable assembly of designed components. The first assembly technologies used were welding, followed in the 1980s by brazing. Due to its economic advantages, in many cases brazing technologies are preferred by manufacturers. The present paper aims to make a comparative study, based on experimental data, between the non-removable assembly technology by welding and brazing for aluminum alloys of group 6000. Due to the high costs of materials and labor, it is preferable to use relevant non-destructive control methods that can indicate any discontinuities that may occur in welded or brazed structures. These defects can provide valuable information and may indicate either non-compliance with the required technological conditions or the wrong choice of assembly method. Non-destructive testing methods that can highlight interior defects, such as radiographic control with penetrating radiation and ultrasound will be used, as well as less expensive equipment and labor methods that may highlight surface defects or which communicates with the surface such as the method of control with penetrating liquids. Thus, the joint areas of the base and filler materials will be analyzed, as well as the areas in the immediate vicinity that have been thermally influenced. The experimental results will be able to lead to pertinent conclusions in the way of choosing the non-removable assembly technology, results that can be easily generalized in the industrial fields. Thus, each company will be able to choose according to the conditions imposed by the product specifications a certain technology that will satisfy its requirements.


Introduction
Due to its very good mechanical properties, aluminum alloys have revolutionized many technological fields.
Although it is lighter than other metallic materials, it has a very good mechanical power, which is why it is often used in fields such as aeronautical engineering, machine building and due to its resistance to many forms of corrosion that do not require high maintenance costs in the pharmaceutical and food industry.
Another advantage of using aluminum alloys is the relatively easy recycling and reshaping of this material.It is known that the energy required for recycling is only 5% of the total consumed for its initial production.It is therefore not surprising that 85% of aluminum used in construction at European level comes from recycling [1], as can be seen in Figure 1 [3].

Figure 1. World production of aluminium.[3]
We can say that aluminum is an "environmentally friendly" material that can be recycled an unlimited number of times, while maintaining its characteristics, without degrading over time.[2]
The chemical element that generates the biggest problems is Silicon, which if it exceeds 0.6% makes it difficult to apply the brazing technology.
The filler material must be chosen so that it can be applied by both the welding process and the brazing process.This choice will be made because it is desired to eliminate any discontinuities from the formed assemblies introduced by the deposition material.
For brazing, avoid materials with a Silver content that have a very good brazability, as well as tubular rods with a built-in flux content, because their melting temperatures are too low.[4] Precision rod-type additives shall also not be used Durafix [5] and A103.
Thus, an AlSi5 or SAl4043 rod type filler material will be chosen.It is used for welding aluminum and aluminum alloys with a maximum silicon content of 7% in forged and cast.Recommended for

Technologies used
The WIG welding process will be used for the welding operation because the assembly quality is very good using the parameters presented in the table 2: The brazing process with oxy-acetylene flame is used for the brazing operation, with the characteristics presented in the table 3.One of the most important operations for both welding and brazing aluminum alloys is surface preparation.In the experimental model that will be presented below, a simple preparation was chosen which consists of the following steps: -Washing with warm water and detergents that do not contain more than 5% soda; -Natural drying; -Mechanical cleaning with stainless steel wire brush; -Degreasing with acetone of the surfaces that will come into contact.These operations must be performed no later than 30 minutes before assembly.

Collectoin of Experimental Data
The WIG welding and brazing operations with filler material resulted in the sample sets shown in Figures 2 and 3.The non-destructive testing methods that have been analyzed are:

Examination method with penetrating liquids
This method highlights discontinuities in the surface of the material or that communicates with the surface.In the case of samples obtained by welding, this method was performed before cutting, and for brazed samples after the cutting operation.Following the control technology described in detail in the paper results the following data sets presented in Figures 4 and 5

Ultrasound examination method
This method was used to check the area of thermal influence of the base material.There is a possibility that in case of overheating, especially after the WIG welding process, there will be major changes in the structure of the base material.
Also by this method, changes in the thickness of the base material in the immediate vicinity of the joint cord can be highlighted.Several points have been established for performing the examination shown in Figures 6 and 7.

Ultrasonic immersion examination method
Due to the thickness of the base material which is very small (2mm), and the irregularity of the surfaces resulting from the joining processes, this method of examination is especially recommended for checking the weld bead by the WIG process.In this case the entire weld bead was scanned by the "C-scann" mode and the areas where the indications were inconclusive were checked by the "Ascann" mode.

Interpretation of results
Analyzing the samples examined by the method with penetrating liquids described in Chapter 4.1, we notice that we have no indications of discontinuities in the case of welding by the WIG process, and in the case of brazing in a single sample there is an indication of high color intensity which confirms a defect.In other brazed samples, color stains may be the result of poor cleaning of excess penetrating liquid.
In the case of the ultrasonic examination method, it can be seen from Figure 6 that the base material did not change its structure significantly, and no oscillations of defect appear on the oscillograms and neither did the end echo significantly decrease its amplitude.
Performing a comparative analysis shown in Figure 12, a thinning of the base material in the thermally influenced area.After measuring the thickness of the base material in areas further away from the weld cord, it was found that its thickness is 2.13 mm, while in the immediate vicinity of the weld the thickness has values of 1.94 mm, 1.97 mm and 2 mm.
For brazed samples the use of this method was difficult due to the small thickness of the materials.In many cases the echo was distorted as seen in the first image in Figure 7 as it reached the edge of the material.However, no discontinuities in the base or filler material were observed after the examination.
From the analysis performed by the immersion ultrasound method, many discontinuities can be observed in the welding bead: some minor ones as seen in figure 10 e) and f), but also major defects as in figure 10 d).Many areas are flawless and this is also highlighted in the first image.
In the case of brazed samples, few minor defects were identified.The end echo decreased its intensity very little as can be seen in Figure 11 a) and c).

Conclusions
The comparative study was performed between the WIG welding process and the oxy-acetylene flame brazing process using the same filler material and the same base material.The experimental difficulty lies in the melting temperature of the filler material which is close to the melting temperature of the base material.
In the case of both assembly technologies, both WIG welding and brazing do not produce surface defects or those that communicate with the surface, defects that can be highlighted by the method of non-destructive examination with penetrating liquids.
Using the ultrasonic examination method, the change in the thickness of the base material in the immediate vicinity of the WIG weld bead was determined, which can have a very large influence by reducing the quality of the assembly.This thinning is mainly due to the way the molten bath is formed.A detailed study can provide additional information on the correlation of thickness and mechanical properties.
In the case of WIG welding, minor defects appear, following the analysis by the ultrasonic immersion method.The main cause of their occurrence may be the uniform misalignment between the base material and the filler material.The major defects found are of the type of oxide inclusions, which in the case of aluminum have higher melting temperatures.
Few minor defects were identified for the brazing operation, which proves that the surface preparation technology, the choice of flow and the choice of brazing technology were well chosen and led to the achievement of flawless joints.

Figure 12 .
Figure 12.Change in the thickness of the base material in welded samples.

Table 2 .
WIG welding regime parameters