1. Wetting and Spreading of Filler Material

During brazing, only when the molten filler material effectively wets the surface of the base material can it fill the joint. The ability of the filler to wet the base material can be measured by the contact angle when the filler (liquid phase) contacts the base material (solid phase). The main factors affecting the wetting of the base material by the filler include:

1. Composition of Filler and Base Material

If there is no physical or chemical interaction between the filler and base material in both solid and liquid states, their wetting action will be poor, such as lead and iron. If the filler and base material can dissolve into each other or form compounds, it is considered that the filler can better wet the base material, for example, silver on copper.

2. Brazing Temperature

An increase in brazing heating temperature can improve the wetting of the base material by the filler due to a decrease in surface tension of the filler, but the brazing temperature should not be too high; otherwise, it may cause loss of filler, grain growth, and other defects.

3. Oxides on Base Material Surface

If there are oxides on the surface of the base metal, the liquid filler often tends to coalesce into spherical shapes and does not wet the base material. Therefore, it is essential to thoroughly remove oxides before brazing to ensure good wetting.

4. Surface Roughness of Base Material

When the interaction between the filler and base material is weak, rough grooves on the surface of the base material can play a special capillary role, improving the wetting and spreading of the filler on the base material.

5. Flux

Using flux during brazing can remove oxides from the surfaces of both filler and base material, improving wetting.

2. Capillary Flow of Filler Material

During brazing, liquid filler must flow along gaps to fill joints; since these gaps are very small, similar to capillary tubes, this is referred to as capillary flow. The capacity for capillary flow determines whether the filler can fill joint gaps.

Many factors affect the capillary flow of liquid filler, mainly including its wetting ability and joint gap size. For instance, if a filler has good wettability with a small gap in joints, good flow and filling performance can be achieved.

3. Interaction Between Filler and Base Material

During capillary filling processes, liquid fillers undergo physical and chemical interactions with base materials; these interactions significantly affect joint performance and can be divided into two types:

1. Dissolution from Base Material into Filler

During brazing, there is generally a dissolution process from base material into liquid filler, which can alloy the components of the filler and enhance joint strength. However, excessive dissolution from the base material may raise the melting point and viscosity of liquid filler, reducing its fluidity and often leading to incomplete filling of joint gaps while potentially causing defects like depressions on the surface due to excessive dissolution.

2. Diffusion of Filler Components into Base Material

During brazing, diffusion of filler components into base materials also occurs in two ways: one involves diffusion into entire grains within adjacent areas near joints forming solid solution layers without adverse effects on joints; another involves diffusion at grain boundaries which often makes them brittle, especially noticeable in thin parts during brazing.

To ensure a strong connection at brazed joints and enhance adhesion of fillers, flux should be used during brazing. Its role is to remove oxides from both filler and base material surfaces, protecting welded parts and liquid fillers from oxidation during brazing while improving wettability.

Commonly used fillers generally fall into two categories: one is hard fillers with melting points above 450°C; commonly used hard fillers include copper-based, silver-based, aluminum-based, nickel-based alloys. Common fluxes include borax, boric acid, chlorides, fluorides etc. Heating sources for hard brazing include torch flames, resistance heating, induction heating, salt bath heating, and furnace heating. Hard brazed joints have higher strength suitable for parts under significant stress or high working temperatures like carbide tools or bicycle frames; this type is usually referred to as hard brazing. The other category is soft fillers with melting points below 450°C; widely used soft fillers are tin-based alloys with most suitable welding temperatures between 200-400°C; fluxes include rosin or rosin alcohol solutions or zinc chloride solutions with heating methods commonly using soldering irons. Soft brazed joints have lower strength suitable for parts under less stress or lower working temperatures like containers or instrument components; this type is usually referred to as soft brazing.

Filler materials are metals that form brazed joints; their quality largely depends on fillers. Fillers should have appropriate melting points, good wettability and filling capabilities that allow diffusion with base materials while also possessing certain mechanical properties and physical-chemical properties to meet performance requirements for joints.