Superplastic forming process (SPF) is an advanced process conducted at high temperature using moderate strain rates, typically used for shaping TA6V sheets for aerospace applications. Thermomechanical stresses on the forming dies due to successive forming cycles may result in the early degradation and even fracture of SPF tools through fatigue crack propagation. To reduce cost and extend service life, dies are generally weld-repaired and subsequently re-used in the typical severe conditions of SPF. The implementation of robust, easy processing welding techniques resulting in high quality repair able to sustain cumulative thermomechanical stresses is of utmost concern to SPF parts manufacturers. The paper focuses on the development of an automated TIG technique to weld repair high nickel, high chromium heat resistant alloys based on a complementary approach including thermal instrumentation, numerical simulation using Sysweld TM and metallurgical investigation; this former being performed on either as-received, repaired and repaired plus damaged materials.
WeldingCracking Low-cycle fatigue a b s t r a c tThe multi-pass weld-repair of heat-resistant cast steels is carried out using an automated shielded metal arc welding (SMAW) process, with various filler materials and pre-heating at 400 • C. Specimens weld-repaired with a filler material more resistant than the heat-resistant cast steel (over-matching) generally crack within the base metal following the tenth filling pass, whereas specimens buttered with a soft alloy prior to welding remain free of cracks.The high temperature strain-controlled fatigue lifetime of material weld-repaired without buttering is lower than that of bulk initial material. This is due to an increase of the stress amplitude as a result of the so-called over-matching. In the case of material welded following a prior buttering, the fatigue lifetime is reduced because of the stress tri-axiality generated in the thin soft layer which prevents its plastic flow. As a consequence, it is concluded that even though buttering prevents cracking efficiently during welding, it is not acceptable as far as fatigue performance, especially lifetime, is concerned.
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