Understanding Melting Points When Choosing the Right Silver Brazing Alloy
When it comes to choosing the right silver brazing alloy for your application, understanding melting points is crucial. The melting point of the alloy plays a significant role in determining how suitable it will be for your specific needs. In this blog post, we delve into the world of melting points and provide you with practical insights to help you make an informed decision.
The Solidus and Liquidus Temperatures
Most silver alloys used for brazing have more than one melting point. Let’s take the example of alloy 455, which melts over a range of 630–660°C. Within this range, there are two essential temperatures to consider:
Solidus Temperature
The lower temperature (630°C in the case of 455) is known as the solidus, and it represents the highest temperature at which the alloy remains partially solid.
Liquidus Temperature
The higher temperature (660°C in the case of 455) is known as the liquidus, and it represents the lowest temperature at which the alloy becomes fully molten.
The difference between these two temperatures is known as the melting range.
Working Temperature
Generally, brazed joints are produced at a temperature below the liquidus because the alloy becomes fluid enough to flow into the joint before it is fully molten. This is referred to as the working temperature.
Eutectic Alloys
Some brazing and soldering alloys have only a single melting point. These are known as eutectic alloys. For example, 2207 melts at 221°C. Eutectic alloys are exceptionally fluid and should only be used in situations with very tight joint gaps.
Considerations When Choosing Melting Temperature
Choosing the right melting temperature is critical for the success of your brazing or soldering project. Here are some key considerations:
1. Impact on Parent Metals
It’s essential to select an alloy with a melting point significantly below that of the parts being joined. Working at too high a temperature can affect the parent metals through annealing or discolouration, or in extreme circumstances, melting the parent metals.
2. Heating Equipment Capability
Your heating equipment must be capable of heating the parent metals above the melting range of the alloy you’re using, preferably quickly. If you’re looking for suitable heating equipment, check out our Sievert Heating Range available at CuP Alloys.
3. Joint Gap Control
Alloys with a narrow melting range, such as alloy 455 with a range of 30°C, should only be used in situations with close and well-controlled joint gaps. Using these alloys in wider gaps might require excessive amounts of alloy, resulting in a costly joint. If the joint gap is wide and/or cannot be controlled, consider a wider melting range alloy such as 438 (650–720°C).
4. Large Components and Heating Speed
For large components, or situations where your heating equipment may not heat the components quickly enough, use an alloy with as low a melting point as possible. During the heating cycle certain elements of the brazing alloy (for example silver, tin, and zinc) will begin to melt first and flow into the gap. This can leave the higher melting elements (for example copper) remaining in a solid state, reducing further capillary flow and potentially producing a weak joint.
This phenomenon is called liquation. To reduce the risk of liquation, opt for alloys with melting ranges of less than 70°C.
FAQs
What is the significance of the melting range in silver brazing alloys?
The melting range, defined by the solidus and liquidus temperatures, determines when the alloy begins to melt and when it becomes fully molten. Understanding this helps you choose the right alloy for your joint design, heating method, and application.
Can I use an alloy with a melting point close to the parent metals?
It’s not advisable. Using an alloy with a melting point too close to that of the parent metals increases the risk of overheating, which can distort, anneal, or even melt the parent metals.
What are eutectic alloys, and when should I use them?
Eutectic alloys melt at a single temperature and are very fluid. They are best used with very tight, well-controlled joint gaps where fast, complete flow is required.
How do I know if my heating equipment is suitable for my brazing project?
Your heat source must be able to bring the parent metals above the alloy’s melting range (and do so efficiently). If you’re unsure, contact us and we’ll help you choose the right setup.
What should I consider when dealing with joint gaps?
For narrow, controlled gaps, alloys with narrow melting ranges can work well. For wider or inconsistent gaps, a wider melting range alloy can be more forgiving and help avoid excessive alloy use.
How does melting temperature affect capillary flow in brazing?
If heating is too slow or uneven, some higher-melting constituents may not fully participate in the flow, which can reduce capillary action and weaken the joint. Choosing alloys with melting ranges under 70°C can help reduce the risk of liquation and improve joint integrity.
Are there other factors besides melting points to consider when selecting a brazing alloy?
Yes. Metal compatibility, corrosion resistance, service temperature, joint design, and the environment the joint will operate in should all influence your alloy choice.
How can CuP Alloys assist with my brazing and soldering needs?
CuP Alloys supplies a comprehensive range of high-quality brazing products, along with technical support and efficient delivery. If you need help selecting an alloy, see our best brazing practice guidance or contact our team.
Contact Us for Further Information
Choosing the right silver brazing alloy can be a complex decision, but understanding melting points is a fundamental step. If you have more questions about melting points or any other aspect of brazing and soldering, please don’t hesitate to contact us. We’re here to provide you with the information and expertise you need for successful brazing projects.