Special features in welding small-diameter tubes with tube grids in making heat-exchange apparatus

Abstract

abroad (in the USA, the Sulzer and Gardner Cryogenic Corporation companies; in West Germany, the Messer and Linde companies;and in Czechoslovakia), but so far only the welding of aluminum tube more than 10 mm in diameter has been mastered. We have investigated the possibility of connecting aluminum tubes of smaller diameter with massive grids. As constructional materials for the heat-exchanger prototypes we used tubes of AMts alloy having the dimensions 8  1, 6  1,and 6 x 0.5 ram; and also grids of AMts and AMg5 alloy plate. A part of the experiments was performed on tube grids from AMtsPS alloy plate, in accordance with Technical Specification 42-115-67. In the procedure for evaluating the reliability of the tube-tube grid connections, we took into account the requirements usually imposed on heat-exchange apparatus. The program of studying the heat-exchanger prototypes included the following: hydraulic testing for strength with a pressure of 9 atm; pneumatic strength-testing with a pressure of 6 arm; checking for vacuum tightness with a helium leak-tester; thermal cycling, which consisted in a 20-fold chilling of the tested samples to 77~ with subsequent warming to 300~ and repeated tests for vacuum tightness. Initially, to select the connection construction and operation of the techniques and welding regimes, we made up small prototypes hhving six welded in tubes, and the reliability of a unit of the selected construction was evaluated from the results of testing heat-exchanges having 40 to 60 tubes. We also compared the strength of various methods of connecting tubes into the grid by testing the specimens shown in Fig. 1. Welding was conducted with a microplasma arc. The cylindrical column form of this arc makes it possible to vary to a comparatively large extent the length of the arc gap without impairing the quality of the seams, which is particularly important in manual welding [1]. A semiconductor A 1281 M apparatus was used as the current source. Pure argon (All-Union State Standard 10157-73, grade A or B) served as the plasma-forming gas~ helium {Inter-republic State Specification 51-77,66)was used as the protective gas. With the objective of increasing the fusing power of the arc, we used nozzles of minimum diameter, whose limiting size was designated from the condition of stability of the plasma arc. At currents of 20-30 A, the nozzle diameter was 1.1 mm; at currents of 40-50 A, it was 1.3 ram. The generally accepted preparation of the aluminum tubes and grids for welding was performed by etching in alkali and brightening in nitric acid. The etching time of the tubes was strictly limited, since in chemical etching the walls of the pieces are thinned. As a result of the experiments performed, a construction of the connection of tubes 6  0.5 or 6  1 mm in size with the tube grids which is shown in Fig. 2 was recommended. Usually the thickness of the projection is selected as equal to the tube thickness. In the present case, for tubes 6  0.5 mm in size it was increased by (t2-(t3 ram. In this way a greater thickness of assembly was ensured. The minimum permissible interval between tubes, which was ascertained experimentally, was 10. 5-11 ram. At smaller intervals, "jump-over" of the arc to neighboring beads occurred, which led to a difficultly eliminated burn-through. Welding was effected by aiming the electrode exactly along the center of the joint. Formation of the seam took place as a result of joint fusion of the tube edges and of the annular projection. Special attention was paid to eliminating the crater in going over from one tube to another, at the moment of arc breakoff. It is known that pore formation takes place basically exactly at this time. When account was taken of this fact, pores either did not form at all, or the number of them was slight.