The influence of construction factors on the weldability of AZ91E alloy

Abstract

Magnesium alloys, together with aluminum and titanium alloys, are a member of the group of light alloys that are of the greatest practical importance in construction applications. Magnesium alloys are currently used in mass sand-mould castings, high-pressure castings, and precision castings. Magnesium alloy casting defects (misrun, micro-shrinkage and cracks) are frequently found, particularly in the case of mass castings. These defects can be repaired with surfacing and welding methods that are common practice in foundries. The purpose of this work was to assess the impact of construction factors on the weldability of AZ91E alloy. Various tests were conducted: transvarestraint tests to determine the range of high-temperature brittleness, critical speed of strain and critical temperature intensity of weld strain under the conditions of forced strain; the Fisco test, which simulates welding under the conditions of permanent stiffening of the weld; and the Houldcroft test, which simulates variable strain distribution found in the weld. One concluded that the strain of castings is characteristic of the process of pad welding and welding. Assessment of susceptibility of AZ91E alloy to cracking under the conditions of forced strain (transvarestraint test) allows to determine the width of the range of high-temperature brittleness HTBR, critical strain speed of the weld CSS and critical temperature strain intensity CST. These parameters are the criteria of hot cracking of welds from AZ91E alloy, therefore, they are indicators of the assessment of the alloy's weldability. Castings from AZ91 E alloys with constant rigidity should be classified as easily weldable. On the other hand, variable rigidity of the casting, resulting from e.g. diverse thickness of the walls, causes significant increase of the alloy's susceptibility to hot cracking. Thermal treatment of gravity castings affects weldability of AZ91E alloy. Gravity castings from AZ91E alloy should therefore be pad welded or welded by TIG method, filler metal with chemical composition similar to the basic material after supersaturation with linear energy of the arc in the range from 3.0 kJ/cm to 4.0 kj/cm.