Effects of Surface Preparations of Base Metal on the Butt Joint Strength of Mild Steel Soldered by Pb-Sn Alloy (Report 3)

Effects of Roughnese on Soldered Joint

Abstract

Usually, the mechanical properties of soldered joint of mild steel are affected by various factores, for instance, substance of base metal, solder and flux, surface preparations such as finishing and roughness, joint clearance, presence of defects as prosity and unsoldered area or void. Among these factores, surface preparation especially roughness of base metal is very important for the tensill strength of soldered joint of mild steel. This paper presents some experimental evidences that roughness of base metal prepared by following process affects the tensile strength of mild steel (SS41) butt joint soldered by Pb-Sn alloy:
(1) Polishing by emery paperes (emery No. 60, 120, 240, 400, 600)
(2) Machining by plain milling cutter.
(3) Grinding by 604# grinding wheel.
(4) Grinding by 80# grinding wheel.
Above process were carried out with one direction on the soldering surface of the base metales. To obtain some experimental data, soldering operation of these specimens was carried out in the 0.3 Kw small electric soldering bath installed the heat-controll (±5°C) unit using ZnCl₂-NH₄Cl flux. These soldering temperatures were 200°C, 250°C, 300°C, 350°C, and joint clearance was adjusted by using the device of Photo. 3 for very short time. Tensile test of these specimens gave the results as follows:
(1) Among the these specimens polished by various emery paperes, specimens polished by emery No. 600 gave the highest tensile strength, its value being 20.87 Kg/mm² (t=0.18 mm). But specimens polished by emery No. 60 gave lower strength, its value being 13.74 Kg/mm² (t=0.22 mm) at the 250°C soldering temperature.
(2) These specimens soldered at 250°C gave high tensile strength and specimens soldered at 350°C gave lowest strength, also specimens soldered at other temperature gave lower strength than that of the specimens soldered at 250°C.
(3) Tensile strength of specimens ground by 80# grinding wheel was very high, its value being 22.05Kg/mm² (t=0.28 mm), and specimens ground by 60# grinding wheel was lower, its value being18.22 Kg/mm² (t=0.24 mm). Also, tensile strength of specimens machined by milling cutter showed17.51 Kg/mm² (t=0.35 mm).
Also, studies on spreadabilty of Pb-Sn alloy on the base metal with various roughness were conducted at the soldering temperature 250°C and 350°C. To obtain some experimental data, mild steel plates (SS41 500φ × 1.6/) polished by emery paper No. 60, 120, 240, 400, 600, and mild steel disc machined by milling and grind ing process were prepared. Also, Pb (61.9%)-Sn (38.1%) filler metal (wt= 1 g) were prepared. This experiment was carried out using a small resistance soldering bath (0.3 KW). As a result, spreadabilty of filler metal on the specimens polished by large grain size emery paper was better than that of one polished by small grain size emery paper, and there are a few difference between spreadabilty of filler metal on the specimens prepared by milling and grinding process.