Weld gap for vacuum brazing

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The weld gap of vacuum brazing refers to the assembly gap between the surfaces of the weldment during brazing. In the process of vacuum brazing, the brazing gap affects the capillary filling process of the solder, and also affects the degree of interaction between the solder and the substrate, as well as the process of plastic flow when the substrate is stressed to the brazing suture layer. Greatly affects the quality and performance of brazed joints.

On the one hand, if the gap is too large, the capillary action will be weakened, and it will be difficult for the solder to fill the gap, and the alloying effect between the solder and the substrate will be reduced (or a hard and brittle intermetallic compound phase will be produced), resulting in a decrease in the mechanical energy of the joint.

On the other hand, too small gap will affect the solder filling, and for eutectic or single-element solder, it may make it difficult to form joints with high solder penetration.

In the actual vacuum brazing process, maintaining a small brazing gap is conducive to the distribution of the solder, and it is also beneficial to reduce the generation of voids and shrinkage holes when the solder solidifies, as well as the alloying of the solder and the substrate. One point is that in the narrow gap, the plastic deformation of the solder is limited by the surrounding substrate when heated, and complex body stress is formed in the joint, which will increase the strength of the joint. Conversely, with the increase of the brazed seam gap, the strength of the weldment joint shows a downward trend.

Therefore, in general, the weld gap of vacuum brazing is required to be as small as possible without hindering the filling of the solder.

There are many ways to damage shearer picks, among which desoldering and alloy head fall off are the main forms of pick failure and damage. The cause of the damage is not only related to the material and production process of the pick, but also to the gap of the vacuum brazing weld of the pick. Therefore, by analyzing the influence of vacuum brazing seam gap on pick strength, this paper proposes a calculation formula for reasonably selecting pick brazing seam.

1 vacuum brazing test of pick

The pick body is made of high-quality alloy structural steel 35CrMnSiA, 42CrMo, 40CrNiMo, 30CrMnSiA; the brazing material is 105″ copper-based solder; the brazing flux is prepared with borax and boric acid. Vacuum brazing and vacuum quenching are carried out simultaneously in induction heating furnace.

2 Relationship between gap and strength of vacuum brazing seam

The brazing gap is the distance between the brazing surface of the cemented carbide and the brazing surface of the pick hole. The brazing seam is formed by the brazing filler metal, tooth body and cemented carbide, which are heated and melted, filled in the gap, cooled and crystallized.

The reasonable selection of the brazing seam gap is the key link to obtain a firm brazing seam and an important prerequisite for obtaining high-quality picks.

The correct selection of the brazing seam gap will affect the compactness, air tightness and strength of the brazing seam to a great extent. Filling the brazing seam gap densely is a necessary condition to obtain the strongest brazing seam. When the brazing metal and brazing material are fixed, the density and strength of the brazing seam depend on the size of the gap. When the gap value is too small, due to the difficulty of filling the solder, the gas flux residue in the gap is difficult to discharge, resulting in defects such as incomplete penetration, pores or slag inclusions. When the gap value is too large, the capillary action is weakened, so that the brazing material cannot fill the gap, the alloying effect disappears, the brazing seam crystallizes to form a columnar casting structure, and the supporting effect of the base metal is weakened, which will lead to a decrease in the strength of the brazing seam.

3 Selection of brazing seam gap

Maximum braze seam densities can only be achieved under certain clearance conditions. Therefore, in order to obtain the most suitable brazing material layer thickness, the brazing gap must maintain a certain size, which is called the minimum safe brazing seam gap, and the gap can be calculated by the following formula:

In the formula

b——Minimum safe brazing gap

aC – coefficient of linear expansion of steel

aT——the coefficient of linear expansion of cemented carbide

Tn – brazing solder solidus

T0 – room temperature

L – the length of cemented carbide on the radius of the tooth body

h——height of cemented carbide

λ——Maximum allowable elongation of brazing solder

The above formula shows that the minimum safe brazing seam B is proportional to the linear expansion coefficient of the gear steel and cemented carbide, the brazing temperature and the brazing area, and is inversely proportional to the elongation of the brazing material. When designing the brazing gap, in addition to the condition that it should not be smaller than the minimum safe brazing gap, it should also be within the optimal brazing strength range.

The relationship between the rising height of the solder and the gap of the brazing seam is as follows:

In the formula δ——the thickness of the brazing seam

λ——The allowable deformation of the brazing seam

ac – coefficient of linear expansion of steel

ar – coefficient of linear expansion of cemented carbide

L——The length of the brazing seam

T – brazing temperature

T0 – room temperature

The above formula shows that the thickness δ of the brazing seam is proportional to the length of the brazing seam and the brazing temperature. The longer the brazing seam, the higher the brazing temperature, and the greater the δ required to ensure that the brazing seam does not crack.

The allowable brazing thickness δ value is 1% of the brazing seam length. If the δ exceeds a certain range, the capillary action will be greatly weakened, which will inevitably affect the filling ability of the brazing material, and defects such as unfilled holes often appear. , thereby reducing the strength of the brazed seam.

4 Conclusion

The above test and analysis show that the brazing gap is related to the brazing material and flux, and affects the vacuum brazing strength. Through calculation, the recommended selection of pick material, brazing process and brazing gap are as follows:

(1) The material of the pick body is 35CrMnSiA, 3oCrMnSiA, 42CrMo, 40CrNiMo, etc.; the brazing material is 105″ of Cu-Zn-Mn, the pot-based solder; the hard alloy is YG series; the brazing flux is prepared with borax and boric acid .

(2) The best choice for the brazing gap is 0.10~0.30 mm.

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