Liquid Metal Embrittlement Formation in Galvanized 3rd Generation Steel After Resistance Spot Welding

Abstract

3rd generation steels have been of great importance in the applications of the automotive industry in recent years within the scope of weight reduction efforts. The weldability of 3rd Generation steels with special production methods, along with their forming problems, is being studied intensively. During resistance spot welding (RSW) of these steels, which are generally preferred as galvanized coated, they pass into the liquid phase due to the low melting temperature of the zinc (Zn) element in the galvanized coating. This situation causes liquid metal embrittlement (LME) with the diffusion of molten Zn atoms into the main material after welding. LME sensitivity occurring in the welding area can cause micro cracks in the main material. In addition, LME negatively affects the mechanical properties of the weld area. This study includes a compilation of literature reviews on LME sensitivity that occurs after spot resistance welding. In the study, the effects of the mechanical properties and microstructure of the 3rd generation steel group on the LME sensitivity are explained, and the effects of different galvanized coating types on the LME formation are also presented. Finally, possible mechanisms of LME sensitivity during RSW are discussed and suitable methods to suppress its occurrence are suggested.

Keywords

• 3rd generation steels
• liquid-metal embrittlement
• galvanized coating
• resistance spot welding

Galvaniz Kaplı 3. Nesil Çeliklerde Nokta Direnç Kaynağı Sonrası Sıvı Metal Kırılganlığı Oluşumu

Abstract

3. nesil çelikler, ağırlık azaltma çalışmaları kapsamında son yıllarda otomotiv endüstrisinin uygulamalarında büyük bir öneme sahiptir. Özel üretim yöntemine sahip 3. nesil çeliklerin şekillendirme problemleri ile birlikte kaynaklanabilirliği de yoğun olarak çalışılmaktadır. Genellikle galvaniz kaplamalı olarak tercih edilen bu çeliklerin nokta direnç kaynağı (NDK) sırasında, galvaniz kaplamada yer alan çinko (Zn) elementinin ergime derecesinin düşük olması nedeniyle sıvı faza geçmektedir. Bu durum ergimiş Zn atomlarının kaynak sonrası ana malzemeye difüzyonu ile sıvı metal kırılganlığına (SMK) neden olmaktadır. Kaynak bölgesinde meydana gelen SMK hassasiyeti, ana malzemede mikro çatlaklara sebebiyet verebilmektedir. Ayrıca SMK kaynak bölgesinin mekanik özelliklerini de olumsuz yönde etkilemektedir. Bu çalışma, nokta direnç kaynağı sonrasında oluşan SMK hassasiyeti üzerine yapılan literatür incelemelerinin derlemesini içermektedir. Çalışmada 3. nesil çelik grubunun mekanik özelliklerinin ve mikroyapısının SMK hassasiyetine etkileri anlatılmış olup, aynı zamanda farklı galvaniz kaplama türlerinin SMK oluşumuna etkileri sunulmuştur. Son olarak, nokta direnç kaynağı sırasında SMK hassasiyetinin olası mekanizmaları tartışılmış ve oluşumunu bastırmak için uygun yöntemler önerilmiştir.

Keywords

• 3. nesil çelikler
• sıvı metal kırılganlığı
• galvaniz kaplama
• nokta direnç kaynağı

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