Transient liquid phase bonding, also known as diffusion brazing, is a metal joining process that produces a high-quality bond through the isothermal solidification of a melting point depressant-rich interlayer. The advantage of using diffusion brazing over conventional processes is that the microstructural properties in the fusion zone are very similar to those of the base metal. The process is broken down into four distinct stages; heating, dissolution, isothermal solidification, and homogenization. The rate-controlling stage of the process is the isothermal solidification stage which may take hours or even days.

Recently, VAC AERO teamed up with researchers from the Mechanical Engineering Department of the University of Waterloo (Waterloo, Ontario) to develop a practical method for diffusion brazing of Inconel 738. This alloy is used extensively in turbine engine hot section components. Brazing techniques are often used to restore these components when they have been damaged in service, but conventional vacuum brazing produces a repair with limited durability.

VAC AERO’s latest advancement in vacuum furnace controls is a hybrid system based on the Honeywell HC900 controller to regulate mechanical and thermal functions integrated with the Honeywell Experion HS software to provide Supervisory Control and Data Acquisition (SCADA) using a large color touch screen LCD for operator interface. The operator interface is provided through a large, color touchscreen. Using this system, process information is accessible by operators at the furnace and by process engineers via the network. This provides comprehensive control and monitoring capabilities for higher productivity, reduced costs, and increased quality.

VAC AERO’s Furnace Control System is Versatile and Operator-Friendly

VAC AERO’s programmable, logic based control system is comprised of proven hardware components, suitably hardened for an “industrial shop” environment and optimized to cover all normal operating and alarm conditions. Process information is accessible by operators and across a company’s network for process engineers, allowing control and monitoring for higher productivity, reduced costs, and increased quality.

The new system has several features intended to improve security and reduce dependence on human operators. For example, it can be configured to send an e-mail message or call a pager in the event of an alarm condition. It has customizable operator sign on and sign-off functions with multiple levels of access security. It also has extensive reporting capabilities and can generate reports in flexible formats including mathematical and statistical functions.

VAC AERO control systems have been operating reliably through thousands of service hours. All control system innovations are fully tested and refined in our own heat treating facilities before being offered to our customers.

For more information on our HC 900 Control System please Click Here

High strength steels based on the 43XX series are commonly used for aircraft landing gear components because of their desirable mechanical properties. To achieve optimum mechanical properties, these alloys are usually heat treated by quenching in oil from the hardening temperature, then tempered. This produces a microstructure with high strength and excellent toughness, ideal for applications involving heavy loading and cyclic fatigue. During hardening, internal stresses created from the thermal shock of direct oil quenching almost always cause some degree of distortion in the heat treated component. A final machining allowance is often left on critical dimensional features to compensate for distortion. However, due to the difficulty of machining these alloys in the hardened condition, machining allowances must be minimized.

Many heat treaters have developed hot straightening techniques to reduce quench distortion. However, these techniques are very labour intensive and often involve expensive tooling. In the current environment of cost and lead-time reduction, reducing distortion during thermal processing has become a primary objective for heat treaters of landing gear components. Interrupted quenching techniques such as ausbay quenching offer good potential for reducing quench distortion. Shown in Figure 1 is an isothermal transformation (I-T) diagram for 300M (4340M) alloy, the most commonly used steel for ultra-high strength aircraft applications. The I-T diagram illustrates the microstructural transformations the steel undergoes over time at constant temperature. Superimposed on this diagram is a curve depicting the cooling rate required for successful ausbay quenching. BY JEFF PRITCHARD