Heat is atoms or molecules in motion. In gases, they are flying between occasional collisions with each other. In solids, by contrast, they vibrate about their mean positions, the higher the temperature, the greater the amplitude of vibrations. From this perception emerges all our understanding of thermal properties of solids: their heat capacity, expansion coefficient, conductivity, even melting. Heat affects mechanical and physical properties too. As temperature rises, materials expand, the elastic modulus decreases, the strength falls and the material starts to creep, deforming slowly with time at a rate that increases as the melting point is approached until, on melting, the solid loses all stiffness and strength. As brazing is a significant heat treatment processs, its effects must be accurately identified and regulated. The depth of Sn diffusion has dependence of brazing time, temperature and diffusion coefficients. The penetration is controlled by the degree of hardening of the core material, melting point and the duration of the brazing cycle. After wetting all the free surfaces of the core material, the molten metal preferably moves towards the points of contact between the parts to be assembled. The liquid then enters the core material via its seal network of grains. The diffusion of the silicon from the liquid towards the core leads to a local decrease in the point of melting of the base alloy and thus its dissolution. Under carefully monitored braze conditions, a metallurgical bond can be formed between the parent material and the molten filler.

Brazing has many advantages when compared to other metal-joining processes. Given that brazing does not melt the base metal of the joint, it allows for more precise control of tolerances and provides a clean joint with no need for additional finishing. The meniscus (crescent shaped) formed by the filler metal in the brazed joint is ideally shaped for reducing stress concentrations and improving fatigue properties. The solidus is the highest temperature at which the metal is completely solid; the temperature at which melting starts. The liquidus is the lowest temperature at which the metal is completely liquid; the temperature at which solidification starts. Due to rapid diffusion kinetics at high brazing temperatures, the metallic bond is characterized by an interexchange of atoms between the filler and parent metal.

Oxides and other nonmetallic species are usually present on material surfaces that have been exposed to ambient atmospheres. These impurities can interfere with, or even inhibit, wetting and alloying. Aluminum forms a natural refractory oxide, that is remarkably stable, mechanically durable, and with a hardness that is inferior only to that of diamond. Moreover, the rate of oxidation roughly doubles with each 25°C rise in temperature. Thus, stable oxides can become progressively thicker and tenacious. Oxide films often endow beneficial attributes to metals, such as corrosion resistance, but their presence on braze joint surfaces present more than a nonmetallic barrier to wetting and spreading. Oxides are generally poor thermal conductors, compared with metals, and may impede heat transfer, thereby exacerbating temperature gradients and delaying fusion to the parent metal. While molten, flux forms a thermal blanket around the joint that helps to spread the heat evenly during the heating cycle. It reduces the surface tension between the joint surfaces, thereby enhancing wetting behavior. Reduction will occur when the free energy curve for metal-oxide formation lies above the oxygen partial pressure curve, at the temperature of interest. The effectiveness of using the process temperature and oxygen partial pressure to control the oxide reduction is limited further by the presence of absorbed water vapor. The desorption of water vapor increases the oxygen partial pressure in the chamber, and this has a deleterious effect on the oxide removal process. At any given temperature, the equilibrium partial pressure of oxygen in the metal oxide becomes less significant; however, the bond between the oxide and the parent metal is more substantial with respect to oxide stability. Thus, the tendency of the oxide to decompose will be greater with a reduction of atmospheric oxygen and an increase in temperature. Chemical thermodynamics can be used to determine the propensity for a metal to spontaneously oxidize, or conversely, for an oxide to disassociate. One may quantify the strength of a metal-to-oxygen chemical bond by measuring the change in the Gibbs free energy. This formation energy is sometimes referred to reciprocally as the dissociation potential of the oxide. Ellingham diagrams can be used to determine the thermodynamic stability of the surface oxides, thus predicting the equilibrium temperature between the metal oxide, reduced metal and oxygen. In response to a correctly designed braze profile, the molten filler metal and the parent metal, will result in joint formation via wetting and capillary reaction; this includes the diffusion of Si into the core alloy. Here, it is important to control the magnitude of the diffusion, in order to influence the erosion of the parent material and intermetallic phase formation.

Stress causes strain. If you are human, the ability to cope with stress without undue strain is called resilience. If you are a material, it is called elastic modulus. Stress is something that is applied to a material by loading it. Strain—a change of shape—is its response; it depends on the magnitude of the stress and the way it is applied—the mode of loading. Stiffness is the resistance to change of shape that is elastic, meaning that the material returns to its original shape when the stress is removed. Strength is its resistance to permanent distortion or total failure. Stress and strain are not material properties; they describe a stimulus and a response. Stiffness (measured by the elastic modulus E, defined in a moment) and strength (measured by the elastic limit σy or tensile strength σts) are material properties. Materials are better at supporting static loads than loads that fluctuate. The long-term cyclic load a material can tolerate is barely one-third of its tensile strength (fatigue resistance). Repetition is tiring, the cause of many human mistakes and accidents. Materials, too, grow tired if repeatedly stressed, with failure as a consequence. Even when the amplitude of the cycles is very small, some energy dissipation or damping occurs. Larger amplitudes cause the slow accumulation of damage, a little on each cycle, until a critical level is reached at which a crack forms. Continued cycling causes the crack to grow until the component suddenly fails. The cyclic stress hardens the material and causes damage in the form of dislocation tangles to accumulate, from which a crack nucleates and grows until it reaches the critical size for fracture. Fatigue in materials is the result of cumulative damage processes that are usually induced by repeated loading cycles. Since the energy dissipation associated with damage is irreversible, and the loading cycles are accompanied by the evolution of heat, the corresponding relationship stress and strain is not single-valued, but rather exhibits a memory-dependence, or hysteresis.

In real operational conditions there’s of course also the variating amplitude. This problem can be addressed approximately with Miner’s rule of cumulative damage. Coined by M.A. Miner in 1945, Miners Rule is one of the most commonly used cumulative damage equations for failures caused by fatigue. Miner's rule assumes that the damage done by each stress repetition at a given stress level is equal, meaning the first stress cycle at a uniform stress level is as damaging as the last. Miner's rule operates on the hypothesis that the portion of useful fatigue life used up by a number of repeated stress cycles at a particular stress is proportional to the total number of cycles in the fatigue life, if that were the only stress level applied to the part. For example, if a part is stressed for 5,000 cycles at a stress level which would cause failure in 100,000 cycles, 5 percent of the fatigue life would be expended. Repeated stress at another stress level would consume another similarly calculated portion of the fatigue life. When 100 percent of the fatigue life is expended in this manner, the part could be expected to fail. The order in which each of these individual stress cycles is applied is not considered significant in Miner's analysis. Understanding and controlling the impact of both the internal and external loads and local stress concentrations affecting the product is the key to reliable long-life operation. Correct choice of alloys and brazing process is of an essence.

The processing from ore to metal binds energy to the metal. Thus, the metal contains more internal energy than the corresponding ore. Corrosion is a process in which this bound internal energy is trying to get released back to its lower, natural level. Simply, the metal is trying to return to its original state in nature. Thermodynamic systems proceed towards thermodynamic equilibrium – the state of maximum entropy (minimal free energy). Corrosion cannot be totally stopped but there are many ways to lower and control the rate. That area of expertise is called corrosion engineering. The general trend is an engine room built more compact, and the performance requirements for car engines increasing, resulting in elevated operating temperatures and pressures. Corrosion reactions are accelerating with increasing temperature. The continuing pressure for weight and cost reduction has resulted in strong down-gauging for the various automotive heat exchanger materials. Downgauging, on the other hand, requires an improvement of the corrosion resistance of both tube and fin alloys. The corrosion resistance of aluminum is based on an oxide film that is bonded strongly to its surface and, if damaged, it re-forms immediately in most environments. On a freshly abraded surface the oxide film is only 1 nm thick. If the destructive forces are too strong, the oxide will be hydrated faster than it is formed. Normally, relatively thick (20 to 200 nm) natural oxide films are formed on aluminum. There is no specific form of corrosion for aluminum. General corrosion, pitting, stress, intergranular and exfoliation corrosion etc. can appear when conditions are favorable for them.

Corrosion of aluminum in the passive range is localized, characterized by random formation of pits. For aluminum, pitting corrosion is mostly produced by halide ions, of which chloride (Cl-) is the most frequently encountered in service. Wrought alloys of the 3xxx series (Al-Mn and Al-Mn-Mg) have very high resistance to corrosion. Manganese is present in the aluminum solid solution, in submicroscopic particles of precipitate and in larger particles of Al6(Mn,Fe) or Al12(Mn,Fe)3Si phases, both of which have solution potentials almost the same as that of the solid solution matrix.

Then there’s so called Brown Band Phenomena (BBP). Al-Mn solid solution of the Brown Band Phenomena (BBP) has its reduced manganese concentration thus decreasing the corrosion potential of this zone, which favors the lateralization of the corrosion in the BBP (another form of sacrificial corrosion protection). Additional protection of the external surface of the tube can be achieved by using fin alloys that preferentially corrode in contact with the tube alloy. For this reason, the fin alloy must be less noble than the tube alloy. This helps in suppressing pitting of tubes. Pitting can occur in the defects (scratches etc.) of the aluminum tube surface, as the large surface oxide area is more noble than the base material inside the defect. The nobility and area differences create an electrochemical potential difference, where the less noble area will act as an anode in a corrosion cell and will be suffering from pitting corrosion. The nobility concept (galvanic differences) in aluminum core is opposite to the nobility concept of copper/brass radiator core, where tubes and solder protect catholically fins. When utilizing nobility differences for corrosion prevention, the corrosion is directed to the less important component. This type of protection, where one metal protects another one, is called cathodic protection. It works only when the more and less noble metals are in physical contact with each other and they are covered by a conductive electrolyte, usually salt containing water or water film. This way the electrical circuit becomes closed. The nobility difference of the metals and the volume and conductivity of the electrolyte determine the protection distance. In a dense radiator core, it often works between fins and tubes, as they are close enough to each other. The preferred option for achieving a fin that protects the tube is the addition of zinc, typically 1.0 to 2.5 wt. % Zn will reduce the nobility of the fin material enough. The low melting point Al-Si alloys which are used to clad the tubes for joining purpose are after brazing already slightly less noble to most of the Al-Mn based tube alloys.

Technology development and aluminum alloy development continues as new applications and requirements emerge (i.e., BEV and FCEV thermal management systems), as well as the continuous focus on topics like lightweighting the transportation, energy-efficient and sustainable manufacturing processes, and enhancing material properties for various industries. Innovation potential certainly involves developing new high-strength alloys, corrosion-resistant variants, with increasing focus on recycling and circular economy initiatives. Developing ways to use materials more efficiently is a prerequisite for a sustainable future.

Heat exchanger manufacturers should explore the development opportunities and match the material to design requirements. The process is one of narrowing the materials search space by screening out materials that cannot meet the design requirements, ranking those that remain for identifying the most promising choice. The material decisions do not end with the establishment of production. Products fail in service, and failures contain information. This information is then feed back into D/PFMEA.

On the brazing front, there’s also developments such as integrating so-called medium-vacuum purging to the CAB. This reduces nitrogen consumption significantly due to the need of single backfilling instead of several exchanges in the purging chambers. Despite the reduced N₂ consumption, the brazing atmosphere purity is higher across the whole heating, brazing and initial cooling process, usually on the level of 10-20 ppm of oxygen. This provides the highest quality of brazing atmosphere for forming the joints and excellent look of the cores after brazing.**