Choosing the Right Parameters for Diffusion Welding

Correctly selecting diffusion welding parameters is an important guarantee for obtaining high-quality joints. The main welding parameters for diffusion welding include welding temperature, welding pressure, holding time, and atmospheric conditions. These factors influence and restrict each other, and should be considered comprehensively when choosing welding parameters.

Welding Temperature

Welding temperature is the most important parameter in diffusion welding. The higher the welding temperature, the larger the diffusion coefficient, the better the plastic deformation ability of the metal, the less pressure required for the welding surfaces to achieve close contact, and the higher the strength of the joint obtained.

However, increasing the heating temperature is limited by the metallurgical, physical, and chemical characteristics of the welded material, such as recrystallization, low-melting eutectics, and the formation of intermetallic compounds.

In addition, increasing the heating temperature can cause softening of the base metal, which will directly or indirectly affect the diffusion welding process and the quality of the joint. Therefore, when the temperature exceeds a certain limit, increasing the heating temperature not only fails to improve the quality of the diffusion weld joint, but may even decrease it.

The welding temperature for different material combinations should be determined by testing based on specific conditions. The selection of welding temperature needs to consider factors such as the composition of the base material, surface state, intermediate layer material, and phase changes.

From a large number of experimental results, due to the limitations of the physical properties of the base material, surface state of the workpieces, and equipment, for many metals and alloys, the diffusion welding temperature is generally 0.6~0.8Tm (where Tm is the melting point of the base material, and in heterogeneous materials welding, Tm is the melting point of the lower melting side of the base material).

The most suitable temperature is generally close to 0.7 Tm. For transient liquid phase diffusion welding, the heating temperature is slightly higher than the melting point of the intermediate layer material or the eutectic reaction temperature.

After the liquid fills the gap, the isothermal solidification and homogenization diffusion temperature can be appropriately reduced. Table 3-4 lists the relationship between the diffusion welding temperature and the melting temperature of some metal materials.

Table 3-4 Relationship Between the Diffusion Welding Temperature and Melting Temperature of Metal Materials

Metal MaterialsDiffusion Welding Temperature T/℃Melting Temperature Tm/℃T/Tm
Silver (Ag)3259600. 34
Copper (Cu)34510830. 32
70-30 Brass4209160. 46
Titanium (Ti)71018150.39
20 Steel60515100. 40
45 Steel800, 11001490, 14900.54,0.74
Beryllium (Be)95012800. 74
Beryllium Copper with 2% Mass Fraction80010710. 75
Cr20-Ni10 Stainless Steel1000                               
1200
1454                       
1454
0. 68       
0. 83
Niobium (Nb)115024150.48
Tantalum (Ta)131529960. 44
Molybdenum (Mo)126026250. 48

During solid-phase diffusion welding, inter-element diffusion will trigger chemical reactions. The higher the temperature, the more intense the reactions, and the more varied the reaction phases that are produced. Simultaneously, under the same conditions, the thickness of the reaction layer increases with temperature.

Figure 3-7 shows the relationship between the thickness of the SiC/Ti reaction layer and time and temperature. As can be seen from the figure, when the welding time is the same, increasing the temperature can significantly increase the thickness of the joint reaction layer.

Figure 3-7: Relationship between the thickness of the SiC/Ti reaction layer, time, and temperature.

Under certain conditions, there is an optimal match between the welding temperature and the strength of the joint. For example, when diffusion welding tin bronze with titanium, if the temperature is below 800℃, even under high pressure, the joint strength remains very low.

This is primarily due to the low temperature, resulting in fewer atoms in an activated state at the interface, unable to form a good bonding interface. If the welding temperature is within the range of 800-820℃, the joint strength increases with the rise in temperature, as shown in Figure 3-8.

The maximum strength value (165MPa) is achieved at 820℃, but further increase in the welding temperature results in a gradual decrease in the joint strength. Fracture analysis reveals a brittle intermetallic compound at the bonding interface. This brittle layer thickens with temperature increase, thereby reducing the joint strength.

In conclusion, the choice of diffusion welding temperature should ensure the best welding quality in a short period of time, achieving full metallurgical bonding. Without causing adverse metallurgical reactions in the joint and parent material, a higher welding temperature should be used as much as possible.

Figure 3-8: Impact of connection temperature on the joint strength of tin bronze/titanium.

Welding Pressure

The primary function of applying pressure during diffusion welding is to induce plastic deformation on the surface of the workpieces and achieve a tight contact state. This activates the atoms in the interface area, accelerates diffusion, and bridges and eliminates interface voids, preventing the formation of diffusion holes.

The larger the pressure and the higher the temperature, the larger the area of tight contact. However, no matter how great the pressure is, in the first stage of diffusion welding, the welding surface cannot achieve 100% tight contact. There is always a small part of the local area that evolves into interface voids.

Therefore, during the pressurization deformation stage, efforts should be made to make the vast majority of the welding surface achieve a tight contact state. Increasing pressure can promote local plastic deformation. Under other constant parameters, a higher pressure is needed to form a joint with higher strength. The impact of welding pressure on joint strength is shown in Figure 3-9.

Figure 3-9: Relationship between diffusion welding joint strength and pressure.

However, too much pressure will cause deformation of the workpiece. At the same time, high pressure requires expensive equipment and more precise control, as well as the use of complex workpiece clamping methods. Therefore, considering the economic and processing aspects, it is beneficial to choose a lower pressure.

When welding similar materials, the pressure mainly acts in the first stage of diffusion welding, making the workpiece surface tightly contact. The effect of pressure on diffusion welding in the second and third stages is relatively small. In solid phase diffusion welding, the pressure can be reduced or completely withdrawn in the later stage to minimize workpiece deformation.

Currently, the range of welding pressure selection for diffusion welding is wide, with a minimum of only 0.04MPa (instantaneous liquid phase diffusion welding) and a maximum of 350MPa (thermal isostatic diffusion welding). In general, the pressure is between 10 and 30MPa. For dissimilar metal diffusion welding, using higher pressure has a good effect in reducing or preventing diffusion holes.

The pressure used for dissimilar materials diffusion welding is usually between 0.5 and 50MPa. Under normal diffusion welding temperatures, to limit workpiece deformation, the pressure should be reduced as much as possible. The value can be selected within the range given in Table 3-5.

Table 3-5: Commonly used pressures for similar metal diffusion welding.

MaterialsCarbon SteelStainless SteelAluminum AlloyTitanium Alloy
Conventional Diffusion Pressure/MPa5 ~10 7 ~12 3 ~7 
Thermal Isostatic Diffusion Pressure/MPa1007550

Diffusion Welding Time

Diffusion welding time, also known as soaking time, refers to the duration the workpieces are maintained under welding temperature. The necessary soaking time for diffusion welding is closely related to temperature, pressure, thickness of the intermediate diffusion layer, joint composition, and the requirement for uniform structure.

It is also influenced by the condition of the material surface and the intermediate layer material. Studies suggest that the average atomic diffusion distance or the depth of the diffusion layer is directly proportional to the square root of the soaking time. The higher the degree of uniformity required for the joint composition, the faster the soaking time will increase exponentially.

The relationship between the strength of the diffusion-welded joint and the soaking time is illustrated in Figure 3-10.

Figure 3-10: Relationship Between the Strength of Diffusion-Welded Joints and Welding Time

When the temperature is high or the pressure is large, the soaking time can be reduced. Under certain temperature and pressure conditions, the initial joint strength increases with time, but when the joint strength increases to a certain value, i.e., upon reaching the critical soaking time, the joint strength no longer continues to increase with time.

Conversely, if the high temperature and high pressure duration are too long, it could cause the parent material’s grains to grow larger, leading to joint embrittlement. Therefore, the soaking time should not be overly long, especially when welding dissimilar metals to form brittle intermetallic compounds or diffusion voids. One should avoid exceeding the critical soaking time.

In actual diffusion welding processes, the soaking time can range from a few minutes to several hours, or even up to dozens of hours. However, from the perspective of improving productivity, the shorter the soaking time, the better, provided that the joint strength is guaranteed. To shorten the soaking time, one must correspondingly increase the temperature and pressure.

For those joints that do not require uniform composition and structure, the soaking time generally only needs to be between 10 to 30 minutes.

Ambient Atmosphere

Diffusion welding is generally carried out in a vacuum or protective atmosphere. The degree of vacuum, the purity, flow rate, and pressure of the shielding gas all affect the quality of the diffusion-welded joint.

The higher the vacuum, the stronger the purification effect, and the better the welding results. However, a high vacuum can increase production costs, so the commonly used vacuum degree is between (1~20) x 10-3 Pa.

The common shielding gas used in diffusion welding is argon. Some materials can also use high-purity nitrogen, hydrogen, or helium. Pure hydrogen atmosphere can reduce the tendency to form oxides and thin many metals’ surface oxide layers at high temperatures. However, when using these gases, their purity must be very high to prevent recontamination.

Hydrogen can form disadvantageous hydrides with zirconium, titanium, niobium, and tantalum, so care should be taken to avoid this. The following factors should also be considered when selecting welding parameters.

(1) Allotropic transformations have a significant impact on the diffusion rate

Common alloy steels, titanium, zirconium, and cobalt all undergo allotropic transformations. At the same temperature, the self-diffusion rate of Fe is about 1000 times faster in the body-centered cubic lattice α-Fe than in the face-centered cubic lattice γ-Fe. Clearly, choosing to perform diffusion welding in the body-centered cubic lattice state can significantly shorten the welding time.

(2) Diffusion welding is easier when the parent material can exhibit superplasticity

The plasticity of metals during allotropic transformations is very high, so phase change superplasticity can occur when the welding temperature fluctuates near the phase transition temperature. Utilizing phase change superplasticity can also greatly promote the diffusion welding process. In addition to phase change superplasticity, fine grains also benefit the diffusion process.

For example, when the grains of the TC4 alloy (nominal chemical composition Ti-6Al-4V) are sufficiently fine, they can exhibit superplasticity, which is very beneficial to diffusion welding.

(3) Another way to increase the diffusion rate is through alloying, i.e., adding elements with high diffusion coefficients to the alloy system of the intermediate layer

In addition to accelerating the diffusion rate, elements with high diffusion coefficients usually have a certain solubility in the parent material. Although they do not form stable compounds with the parent material, they can lower the local melting point of the metal.

Therefore, it is necessary to control the problem of melting point reduction caused by alloying, otherwise, local liquefaction may occur at the joint interface.

(4) When welding dissimilar materials, diffusion voids sometimes form at the interface, and brittle intermetallic compounds may also form, reducing the mechanical properties of the joint

When performing diffusion welding of dissimilar materials with different linear expansion coefficients at high temperatures, large residual stresses can occur upon cooling due to the constraint at the interface.

The larger the component size, the more complex the shape, and the higher the welding temperature, the greater the linear expansion difference and the greater the residual stress, which can even immediately cause cracks near the interface.

Therefore, when designing diffusion-welded joints, efforts should be made to reduce residual stress caused by linear expansion differences, especially avoiding the use of hard and brittle materials to bear tensile stress. To solve such problems, the process can lower the welding temperature or insert an appropriate intermediate layer to absorb stress and reduce linear expansion differences.

Table 3-6 lists commonly used intermediate layer materials and welding parameters during solid phase diffusion welding.

Table 3-6: Commonly Used Intermediate Layer Materials and Welding Parameters During Solid Phase Diffusion Welding

Parent Material for WeldingIntermediate Layer MaterialsWelding Parameters
Pressure
/MPa
Temperature
/℃
Time
/min
Shielding Gas
AV/AISi7 ~155801Vacuum
Be/Be70815~900240Inert Gas
Ag7070510Vacuum
Mo/Mo701260~1430180Inert Gas
Ti70930120Argon
Ti8587010Vacuum
Ta/Ta701315~1430180Inert Gas
Ti7087010Vacuum
Ta-10W/Ta-10WTa70~14014300.3Argon
Cu-20Ni/SteelNi3060010Vacuum
AV/Ti1600~6501.8Vacuum
Ag1550~6001.8Vacuum
AV/SteelTi0.4610~63530Vacuum

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