F. Harati, M.a. Jabbareh, S.m. Mousavizadeh,
Volume 8, Issue 2 (1-2023)
Abstract
The present research aims to study the liquation and re-solidification of liquid during friction stir spot welding of AZ91 alloy. Although friction stir spot welding is a solid-state process, the presence of Mg17Al12 intermetallic compounds results in liquation during the welding process. In this study, friction stir spot welding was performed with a tool rotational speed of 2500 rev/min and a tool dwell time of 5 seconds. The microstructural assessment was carried out by optical and scanning electron microscopes. The results showed that initiation of liquation from the inner and outer edge of the eutectic precipitates occurred based on the melting of residual eutectic. Moving toward stirred zone, a liquid film formed along the grain boundaries. The liquid re-solidified as a composite structure of α-Mg/, which α-Mg phase dispersed in γ-Mg17Al12 matrix. Also, the results showed that eutectic morphology resulting from re-solidification is related to the cooling rate. Eutectic morphology changed from granular to fibrous by increasing the cooling rate. Also, the liquid film along the grain boundaries re-solidified as a divorced eutectic.
Engineer Amri Hossein Jafarzade, Engineer Mohammad Saeed Shahriari, Ph.d Ruhollah Ashiri,
Volume 9, Issue 2 (8-2025)
Abstract
Repair welding of nickel-based superalloy Inconel 939, which was under working conditions of 100,000 hours, was performed by gas tungsten arc welding using Inconel 617 filler metal. The main objective of this study is to investigate and analyze the challenges during welding such as irregular distribution of primary MC carbides and crack formation in the heat-affected zone, and also to investigate the effect of post-welding heat treatment cycle on the microstructure and hardness of different weld zones. During welding, a crack of 91 micrometers length was observed in the heat affected zone, which due to the presence of a liquation film and accumulation of carbides around the crack, the crack was categorized as a liquation crack. Then, due to post-welding heat treatment, improvement of microstructural characteristics and hardness of the weld zone, partial melted zone, and heat-affected zone was observed, which resulted in homogenization of the hardness profile of the weld. In other words, in post-weld heat treatment, the improvement and uniformity of carbides allow for a better response to hardness properties in different welded areas. It was observed that post-welding heat treatment caused the crack formed during welding to grow and spread to reach a length of 386 micrometers, which was classified as a strain-aging crack due to its formation and growth during post-welding heat treatment.
