Diffusion bonding of a titanium alloy to a stainless steel with an aluminium alloy interlayer

Diffusion bonding is the process of joining similar or dissimilar materials at the interface rather than edge joining using welding process or other metal joining processes. It is a solid state welding process carried out under the conditions of soaking temperature, bonding time and load on the surfaces to be joined. Generally this is carried out under vacuum or inert atmosphere to avoid interface reactions to actuate the corrosion. Diffusion bonding process done at different conditions of temperature, bonding time and load. These parameters are optimised for better bond strength, the bonded specimens are analysed for microstructural changes and bond strength since diffusion bonding is an atomic transfer between the surfaces. The micro hardness test is conducted to evaluate the bond strength at the interface. The strength of the bond can be improved by inserting another material at the interface as an interlayer. The base metal titanium with stainless steel is bonded with aluminium as an interlayer.


Introduction
The titanium alloy has high tensile strength, high corrosion resistance, can withstand moderately high temperature without creeping and therefore titanium alloy are used in aircraft. The use of titanium alloy has been increased in aircraft industries, so its proper utilization is required [1-3]. The joining of titanium alloy to the stainless steel has also found some of its implementation in satellite cooling system. The highly précised joints of titanium alloy/stainless steel having good bonding properties can be obtained by diffusion bonding process at different temperature, time and load.
Ti-Fe inter-metallic compounds are formed when stainless steel are joined directly to titanium alloy and it makes the joint brittle. Due to the large difference of linear expansion between Ti alloy and stainless steel, high internal stresses are formed and it also leads to bonding crack. Hence, indirect bonding with addition of interlayer metal is in use [4]. Aluminium alloys possess few erosion resistance properties and also have good plasticity, therefore the practicability of diffusion bonding of titanium alloy to stainless steel with an aluminium as an interlayer improves the bonding strength and is demonstrated in this paper [5][6][7][8]. The microstructural analysis at the interface shows rearrangement of the atoms with the diffusion process thereby increasing holding strength of the atoms at the boundary layer [9][10][11][12].

Diffusion bonding setup
The Diffusion bonding setup used for joining the two similar or dissimilar metals is shown in the figure 1. This setup mainly consisting of heating chamber with a carbon rods or plates for producing temperatures above 900℃ with a vacuum generation facility provided by diffusion pump and a vacuum pump. The required vacuum to avoid the reactions between the bonding surfaces can be produced at the order of 10 -6 supplying liquid nitrogen to the diffusion pump. Since the heating chamber should not over heated hence constant cooling using water cooling system. Overcooling may result in the production of vapors which indirectly affects the diffusion bonding process hence0 moderate cooling is required and done with a electronic controller. Precise loading of the bonding surfaces possible with a use of hydraulic loading [8].

LOAD CELL
The diffusion bonding process is a means of transferring atoms from one surface to another and breaking of aspirates on the surfaces of bonding or diffusing of clusters of atoms of the similar and dissimilar metals. Vacancy is referred to an unoccupied site in a lattice structure. Diffusion of atoms is a thermodynamic process where temperature and infusibility of the material are considerable parameters. The atomic transfer during the diffusion bonding process is a result of activation energy of the atoms achieved by heating the surface maintaining the temperature at less than or equal to 0.6 times the melting temperature of the base metal. The plastic flow of the material helps in the determination of the flow stress to be applied on the surface to avoid the buckling of the material or creep at elevated temperatures.

EXPERIMENTAL
Diffusion bonding experiment comprising of titanium alloy, stainless steel and aluminium alloy as an interlayer was conducted. The specimen of 1mm thickness with 10×10mm size is diffusion bonded under the various condition of bonding temperature, time and load. The master alloys to which the bonding is to be done were machined and so was the aluminium alloy to join them. The surface of the material used for bonding are very well polished with different emery paper of different grades starting with 200 to 3000 so that irregularities at the surface can be made smooth for better bonding. Suitable chemical agents (ethanol to remove grease or any dust) are applied for cleaning the surface to avoid reactions at the interface so that better strength can be achieved at the interface.
A vacuum pressure of 3×10 -3 Pa was applied during the experiment. A high-frequency induction equipment was used in the experiment and the heating rate of 2000℃/min was applied throughout the experiment. For tensile testing, small blocks were taken out and the testing was performed at nominal strain rate of 3×10 -4 s -1 . The microstructure at the interface of three materials can be seen by the use of SEM images.  The percentage of titanium is given approximately

Result of bonding temperature T on strength of diffusion bonding
The experiments are conducted under the various conditions of bonding parameters. The samples are made with a size of 10×10mm. These samples are kept under the furnace free from atmospheric inclusions by creating sufficient vacuum with a use of diffusion pump, vacuum pump with a water cooling system to prevent excessive heating and production of vapours which activates reaction at the interface of the bonding area. The experimental results are tabulated for different conditions of temperature, bonding time and pressure applied on the surfaces to be bonded. The bonding temperature T influences the yield strength which results in atomic diffusion of the base metals, this can be analysed with an empirical relations as explained in the introduction. With an experimental results it was found that the bonding strength will increases based on the type of the material to be joined, the conditions of the surface like aspirates, regularity in the surface cleanliness of the surface. Effect of bonding temperature T on the tensile strength of the joints (as shown in figure 3). At the temperature of 623K, there is a regular variation of bonding temperature with the tensile strength of the bonded joints. At this temperature, the joints shows tensile strength of only 122×10 6 N/m 2 .
The stainless steel shows a high yield strength and therefore the contact is very poor between the bonded surface. Simultaneously there is a decrease in atomic diffusivity due to low thermal excitation. The fracture of specimen takes place mostly on stainless steel/LF6 interface. As the bonding temperature increases from 623K to 723K, decrease in yield stress of master alloy and increase in atomic diffusivity can be seen. Hence, it results in high interfacial deformation and chemical bonding becomes easier. Due to this, there is a increase in tensile strength of the joints. As the bonding temperature approaches 723K, the tensile strength of the bonded joints reaches a maximum value of

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2nd International conference on Advances in Mechanical Engineering (ICAME 2018) IOP Publishing IOP Conf. Series: Materials Science and Engineering 402 (2018) 012124 doi:10.1088/1757-899X/402/1/012124 183×10 6 N/m 2 and at the stainless steel/LF6 interface specimen fracture takes place. It occurs mostly on interlayer metal LF6, and the fracture was ductile fracture. As the bonding temperature is increased to 873K, there is a drastic decrease in tensile strength of joints and it approaches a minimum value of 34×10 6 N/m 2 . The fracture that occur at stainless steel/LF6 interface was entirely brittle (as in figure). As T starts exceeding from 723K, the bond strength starts decreasing with increase in bonding temperature. As per result it was observed that there is an optimization of the bonding parameters with the values of bonding temperature of 623K, 60min bonding time and 1200kg as a load. The microstructural analysis and variation of strength are plotted using the bar graph shown in figure 3.

Dispersion of elements
The dispersion of the elements and rearrangement of the atoms at the interface of three materials shows that there is a drastic change in the roughness value due to the presence of gap between the surfaces. Hence it is needed to have smooth surface free from any foreign particle during the diffusion bonding in the vacuum atmosphere.

Result of average pressing speed on strength
The rate of deformation of the three materials aluminium, titanium and stainless steel was affected due to the average speed and its affects on strength can be seen in figure 3b. The highest value of strength (183×10 6 N/m 2 ) was obtained at the pressing speed of 70mm/min. It is observed that the bonding stress is low for low value of pressing speed V. As the oxide film is not removed well, lower value of friction force at the interface is obtained. It can be observed that the specific bonding stress has a very important role in formation of practical contact and atomic bonding process.

Microstructure analysis
The microstructure at the interface of three materials aluminium, titanium and stainless steel shows that the atomic transfer is very effective at the interface on the application of bonding temperature so that melting temperature of these materials is very high [3]. Therefore application of high temperature results in handling of bonding process. Therefore an effective cooling system should be provided for the furnace to avoid formation of reactions agents. (1) The strength of the bonding depend on the nature of the surface to be bonded, pair of materials to be bonded and the operating process parameters. The experimental parts covering the process parameter optimization with surface indicates that the optimized bonding temperature is 823K, one hour bonding time and a force of 1200kgs on a specimen sample size of 10×10mm with 1mm thickness of sheet.
(2) The micro hardness at the interface of the bonding shows that the hardness values varies from the contact zone to the farther zones.
(3) The maximum hardness value purely depends on the surface in contact, activation energy of the atoms, minimum voids and maximum contact surface.
(4) During the bonding process it is observed that the strength is better if sufficient vacuum is created during the bonding process to avoid the oxide layer formation and the bonding surface.