Reminder 8th International Congress Aluminium Brazing

The 8th International Congress Aluminium Brazing will start on
June 3rd in Dusseldorf, Germany. The NOCOLOK® Team is taking part and we cordially invite you to attend this 3-day-event, where expert authors will present papers about new developments and processes in and around brazing technology.

You can find more information, registration and hotel reservation on the website. Finally, the programme is available. The programme of the 8th International Congress Aluminium Brazing as well as the registration forms can be downloaded as a PDF file here.

New NOCOLOK® Plant in China will Start Soon

"We are right on schedule," says Werner Schmitt, Global Sales & Marketing Manager NOCOLOK®. "The work on the equipment will be completed by the end of May." Commissioning is scheduled for the beginning of June.

The plant will then be started up and thoroughly tested. The first batch of NOCOLOK® is scheduled to leave the factory at the beginning of July.
"The cooperation with our Chinese partner is going ahead at full steam," emphasised Werner Schmitt.

Lansol Fluorchemicals is a joint venture between Solvay and Sinochem Lantian, a Chinese chemical company. The new facility in Quzhou (Zhejiang province) will offer approx. 25 new jobs and will begin by producing around 3,000 tonnes of NOCOLOK® per annum. The production capacity can also be adapted very quickly if demand increases.

With the new production facility, Solvay aims to serve the fast-growing automotive and stationary HVAC+R markets in Asia, and specifically in China.

Brazing of Aluminium Alloys with Higher Magnesium Content using Non-Corrosive Fluxes – Part 1 & 2

Technical Information by Leszek Orman, Hans-Walter Swidersky and Daniel Lauzon

Abstract

For just as long as aluminium has been used for brazing heat exchangers, there has been a trend to down-gauging components for weight savings. The most common alloying element to achieve higher strength alloys for the purpose of down-gauging is magnesium. While magnesium additions are helpful in achieving stronger alloys, the consequence is a decrease in brazeability. This article discusses the mechanism of brazing deterioration with the addition of magnesium and proposes the use of caesium compounds as a way of combating these effects.

We split the article in five parts:

1. Introduction
2. Effects of Mg on the Brazing Process
3. Mechanism of Magnesium Interaction with the Brazing Process
4. Caesium Fluoroaluminates
5. NOCOLOK® Cs Flux

Introduction

Aluminium brazing using non-corrosive fluxes is the leading process for manufacturing automotive heat exchangers. Recently, this process has become more wide spread in the stationary Heating, Ventilation, Air-Conditioning and Refrigeration (HVAC&R) industry, both for domestic and commercial applications. The standard brazing process involves joining of components with a brazing alloy, typically an aluminium-silicon filler alloy. Al-Si brazing alloys have melting ranges from 577°C to 610°C, which is appreciably lower than the melting point range of the base aluminium alloys used for heat exchangers (630°C – 660°C). Fluoride-based non-corrosive fluxes of the system KF-AlF₃ are used to displace the surface oxide film during the brazing process. A commonly used non-corrosive flux of the general formula K_1-3AlF_4-6 is known under the trademark name NOCOLOK® Flux with a melting range between 565°C and 572°C. The flux works by melting and disrupting the oxide film on aluminium, protecting the surfaces from re-oxidizing during brazing thus allowing the Al-Si brazing alloy to flow freely.

A consistent and on-going trend across all heat exchanger manufacturing sectors is towards lighter weight, accomplished by down-gauging of components. Also corrosion resistance is a key factor – particularly when there is no additional post brazing coating or treatment. These often contradictory trends call for aluminium alloys having higher and higher post brazed strength. While alloys from the 7xxx (alloyed with Zn) and 2xxx (alloyed with Cu) series can be precipitation hardened to the highest strengths of any aluminium alloys, their corrosion resistance without any additional coating is low and their solidus temperatures are below the melting range of currently used flux and filler metal combinations, and therefore they are not suitable for heat exchanger manufacturing by brazing.

The most common alloys used for aluminium brazing are from the 3xxx series (alloyed with Mn). After being subjected to the high temperature during the brazing cycle, these alloys have relatively low post-braze mechanical strength. Higher strength is offered by alloys from the 5xxx series (alloyed with 2 to 5 wt% Mg) where post brazed strengthening is achieved by solid solution hardening or by the 6xxx series (alloyed with Mg and Si) which can be precipitation hardened. A more comprehensive survey of mechanical properties of brazeable aluminium alloys is presented in [1]. It is worth observing that the brazing cycle itself could be considered as a thermal treatment for obtaining the precipitation hardening effect providing the cooling rate from the brazing temperature is sufficiently fast [2]. An example of such an alloy designated for specific use for aluminium brazed heat exchangers is described in detail in [3].

As well as increasing post-braze mechanical strength, the addition of Mg to certain alloys allows for improved machinability. Machining is necessary for heat exchanger components such as connecting blocks and threaded fittings.

There is however a certain limitation with the above mentioned alloys. They all contain magnesium. During the brazing cycle Mg negatively influences the process of oxide removal and it is generally accepted that Mg levels only up to 0.3% can be safely brazed with the standard brazing flux. This negative influence can be mitigated with the use of caesium containing compounds. The mechanism of Mg interference with the brazing process and the positive role of Cs additions to the flux in combating the effects of Mg are the subjects of the current paper.

Effects of Mg on the Brazing Process

To illustrate the effects of Mg on the brazing process, Bolingbroke et al [4] chose the angle-on-coupon method. In this technique, an aluminium angle is laid on top of a cladded aluminium coupon where the legs of the angle are raised using stainless steel wire (see Fig. 1). Brazeability is thus measured as a function of the length of the fillet formed. In this set of experiments, the coupon base alloy is 3003 with Mg additions ranging from 0.1 to 0.58 w%. Only the coupon was fluxed at pre-defined loads ranging from 2 to 10 g/m². The results of the Mg content on brazeability are shown in Fig. 2.

Fig. 1: Experimental set up for brazeability measurement [4].

Fig. 2: Brazeability as a function of magnesium content [4].

Fig. 2 shows that increasing the flux load can reduce the negative influence of magnesium.

The solid state diffusion is time-temperature dependent and becomes rapid above 425°C. Thus brazing at higher heating rates should reduce the negative influence of Mg. The influence of heating rate on brazeability is shown in Fig. 3.

Fig. 3: Brazeability of 3003 alloy + 0.31 wt% Mg as a function of heating rate and flux load [4].

The influence of heating rates when kept within the values attainable for the CAB process is rather weak. Increasing the flux load is more effective in combating the negative influence of Mg for CAB processes.

In flame or induction brazing, where the heating rates are about two orders of magnitude higher than in the CAB process, alloys with Mg concentration even as high as 2% can be successfully brazed.

It should be noted that when one speaks of the brazing tolerance to Mg, it is always the total sum of the Mg concentrations in both components:

[Mg] _{component 1} + [Mg] _{component 2} = [Mg] _total   (1)

The effect of magnesium content on the appearance of the brazed joint is shown in Fig. 4.

Fig. 4: Effect of Mg content on appearance of brazed joint [4].

At 0.1 wt% in the base coupon, the fillet is large and joining is complete. At 0.4 wt% Mg in the base coupon, the fillet volume is smaller.

References:

1. S. W. Haller, “A new Generation of Heat Exchanger Materials and Products”, 6th International Congress “Aluminum Brazing” Düsseldorf, Germany 2010
2. R. Woods, “CAB Brazing Metallurgy”, 12th Annual International Invitational Aluminum Brazing Seminar, AFC Holcroft, NOVI, Michigan U.S.A. 2007
3. T. Stenqvist, K. Lewin, R. Woods “A New Heat-treatable Fin Alloy for Use with Cs-bearing CAB flux” 7th Annual International Invitational Aluminum Brazing Seminar, AFC Holcroft, NOVI, Michigan U.S.A. 2002
4. R. K. Bolingbroke, A. Gray, D. Lauzon, “Optimisation of NOCOLOK® Brazing Conditions for Higher Strength Brazing Sheet”, SAE Technical Paper 971861, 1997
5. M. Yamaguchi, H. Kawase and H. Koyama, ‘‘Brazeability of Al-Mg Alloys in Non Corrosive Flux Brazing’’, Furukawa review, No. 12, p. 139 – 144 (1993).
6. A. Gray, A. Afseth, 2nd International Congress Aluminium Brazing, Düsseldorf, 2002
7. H. Johansson, T. Stenqvist, H. Swidersky “Controlled Atmosphere Brazing of Heat Treatable Alloys with Cs Flux” VTMS6, Conference Proceedings, 2002
8. U. Seseke-Koyro ‘‘New Developments in Non-corrosive Fluxes for Innovative Brazing’’, First International Congress Aluminium Brazing, Düsseldorf, Germany, 2000
9. K. Suzuki, F. Miura, F. Shimizu; United States Patent; Patent Number: 4,689,092; Date of Patent: Aug. 25, 1987
10. L. Orman, “Basic Metallurgy for Aluminum Brazing”, Materials for EABS & Solvay Fluor GmbH 11th Technical Training Seminar – The Theory and Practice of the Furnace and Flame Brazing of Aluminium, Hannover, 2012
11. K. Suzuki, F. Miura, F. Shimizu; United States Patent; Patent Number: 4,670,067; Date of Patent: Jun. 2, 1987
12. J. Garcia, C. Massoulier, and P. Faille, “Brazeability of Aluminum Alloys Containing Magnesium by CAB Process Using Cesium Flux,” SAE Technical Paper 2001-01-1763, 2001