Tensile Properties and Fusion Zone Hardening for GMAW and MIEA Welds of a 7075-T651 Aluminum Alloy

doi:10.1007/s40195-014-0103-x

Abstract

Abstract:

Tensile and hardness values for 7075-T651 aluminum alloy in the as welded and post weld heat treated conditions (solubilization and artificial aging-T6), obtained using GMAW and modified indirect electric arc (MIEA) welding processes are presented. Results showed that the base material along rolling direction exhibited a tensile strength of around 600 MPa and elongation of 11%. For the as welded condition, tensile strength was 260 MPa and elongation percent of 3%. This behavior was attributed to brittleness induced by the microstructural characteristics of the welded alloys, as well as high porosity. Hardness profiles along the welds were obtained and different welded zones were identified. A soft zone (~100 HV_0.1) in the heat affected zone for GMAW and MIEA was observed, the minimum hardness corresponding to weld metal (~85 and ~96 HV_0.1 for GMAW and MIEA, respectively). The high dilution between filler and base metal during welding in MIEA allows to the Zn and Cu to flow from the base metal into the weld metal, inducing hardening by solution and subsequent artificial aging. In this regard, the hardness of the weld metal for MIEA increases by 56%, while the tensile strength reaches a value close to 400 MPa. For GMAW, non-favorable hardening effect was observed for the weld metal after solution and artificial aging.

Key words: Welding, 7075-T651, Dilution fraction, Tensile properties, Fusion zone hardening

N. Alatorre, R. R. Ambriz, B. Noureddine, A. Amrouche, A. Talha, D. Jaramillo. Tensile Properties and Fusion Zone Hardening for GMAW and MIEA Welds of a 7075-T651 Aluminum Alloy[J]. Acta Metallurgica Sinica (English Letters), 2014, 27(4): 694-704.

Figures/Tables 19

Table 1 Chemical composition of the materials employed (wt%)

Material	Al	Si	Fe	Cu	Mn	Mg	Cr	Zn	Ti	Others
7075-T651*	88.6	0.03	0.19	1.7	0.02	2.7	0.18	6.4	0.02	0.16
ER5356**	95.04	–	–	–	–	4.96	–	–	–	–

Fig. 1 Joint preparation and thermocouples localization: a GMAW; b MIEA

Table 2 Welding parameters of GMAW and MIEA processes

Process	Voltage (V)	Current (A)	Feeding speed (mm/s)	Welding beads	Thermal efficiency	Heat input (J/mm)
GMAW	23	142	110	3	0.80	725.7
MIEA	25.4	203	180	1	0.95	1,360.6

Fig. 2 Traction specimen dimensions a & schematic representation of the welded plate b

Fig. 3 Indentation mesh: a GMAW; b MIEA

Fig. 4 Grain structure of the 7075-T651 aluminum alloy showing second phase particles and representation of the grain structure to determine the grain size area: a three dimensional overview; b rolling (L) plane; c transverse to rolling (TL) plane; d short transverse (ST) plane

Fig. 5 Grain size area for base metal at L, ST and LT planes, and for the fusion zone in GMAW and MIEA welded joints

Fig. 6 Welding profiles and schematic representation of the welded joints: a GMAW; b MIEA

Table 3 Welded joint dimensions of GMAW and MIEA processes

Welding technique	Joint area (mm²)	Fusion zone (mm²)	Dilution zone (mm²)	HAZ area (mm²)	Dilution fraction (%)	Porosity area (mm²)	Porosity ratio
GMAW	55	102.0	20.5	445.4	20.0	1.68	0.0165
MIEA	32	160.0	83.6	440.1	52.2	2.35	0.0147

Fig. 7 EDS spectra in the center of the fusion zone: a GMAW; b MIEA

Fig. 8 Grain structures of the welded joints in as welded condition: a fusion interface in GMAW; b weld metal in GMAW; c fusion interface in MIEA; d weld metal in MIEA

Fig. 9 Grain structure of the welded joints after post weld heat treatment: a fusion interface in GMAW; b weld metal in GMAW; c fusion interface in MIEA; d weld metal in MIEA

Fig. 10 True stress–true strain curves for base metal, as welded and post weld heat treatment conditions

Table 4 Mean tensile mechanical properties of base metal and the weld joints

Material	E (GPa)	σ_0.2 (MPa)	σ_max (MPa)	σ_f (MPa)	ε_max (%)	ε_f (%)	H (MPa)	n
Base metal (transverse)	72.0	530	568	530	8.0	13.8	794	0.08
Base metal (longitudinal)	72.2	549	600	581	8.0	11.3	729	0.07
GMAW weld joint	68.0	163	262	262	3.4	3.4	647	0.25
MIEA weld joint	67.2	165	260	260	2.8	2.8	677	0.25
GMAW + PWHT weld joint	69.2	183	258	258	4.1	4.1	523	0.19
MIEA + PWHT weld joint	71.0	342	400	400	2.0	2.0	707	0.14

Fig. 11 Hardness representation in as welded and post weld heat treatment conditions: a GMAW; b MIEA

Fig. 12 Vickers hardness maps of the weld joints: a as welded for GMAW; b after post weld heat treatment for GMAW; c as welded for MIEA; d after post weld heat treatment for MIEA

Fig. 13 Weld thermal cycles of GMAW a & MIEA b processes, & continuous cooling transformation curve for 7075 aluminum alloy (starting from 475 °C) [19] c

Fig. 14 Load–depth curves of weld joints: a GMAW in as welded condition; b MIEA in as welded condition; c GMAW and MIEA after post weld heat treatment

Table 5 Elastic modulus (GPa) of the base metal and weld joints obtained by instrumented indentation

Process	Weld metal (as weld)	HAZ (soft zone)	Weld metal (PWHT)	Base metal
GMAW	81.4 ± 0.7	81.8 ± 0.4	82.4 ± 1.0	88.5 ± 1.6
MIEA	85.9 ± 1.8	84.9 ± 0.5	84.0 ± 0.8	88.5 ± 1.6

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