Experimental and numerical investigation of the weld repair of superplastic forming dies

Abstract

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.

Study and prevention of cracking during weld-repair of heat-resistant cast steels

“…Beyond 16 passes, cracks further propagate or new cracks initiate resulting in an increased total crack length reaching its maximum value as the entire filling of the V-groove is completed (40 passes). These preliminary experimental observations confirm the results related in a previous work (Branza et al, 2004a) showing an increase of the calculated maximal stress as welding passes cumulate in the frame of a multi-pass process. Fig.…”

“…These materials, used for manufacturing large SPF tools (Branza et al, 2004a), are investigated in the shape of bulk parallelepipedic specimen with dimensions representative of industrial dies. A standard weld-repair technique with two different filler metals as well as the results obtained in terms of structural integrity are first presented and discussed.…”

Analysis of weld-cracking and improvement of the weld-repair process of superplastic forming tools

“…In contrast, the samples weldrepaired with 17 and 40 passes show numerous cracks in the base metal, close to the welding zone ( Figure 13). This area corresponds to the zone where the stresses induced by thermal gradients are higher, as evidenced in previous studies [12,13]. The cracks number and size increase when the pass number increases from 17 to 40.…”

“…A strain-controlled fatigue test was selected in order to simulate the mechanical strain resulting from the thermal gradient due to multi-pass welding, each welding pass (up to 50 and more during weld-repair of SPF dies) generating a rapid thermal cycle. The strain amplitudes were selected to be representative of typical strain cycles suffered by the material during multipass welding, evaluated using numerical simulation [12,13]. Five strain amplitudes, 0.4, 0.5, 0.6, 0.7 and 0.8%, resulting in a slight plastic strain at each cycle, were chosen.…”

Analytical solutions of the thermal field induced by moving double-ellipsoidal and double-elliptical heat sources in a semi-infinite body

“…In welding numerical analyses, the moving double-ellipsoidal model, proposed by Goldak et al [1], is one of the most widely used heat source models. The model is unique due to its ability to accommodate different fusion welding techniques, which include gas metal arc welding (GMAW) [2][3][4][5][6], gas tungsten arc welding [5,[7][8][9][10][11][12][13][14][15][16], shielded metal arc welding [7], submerged arc welding [1], laser spot welding [17], electron beam welding [1,18], and laser-GMA hybrid welding [19].…”