Aluminum 7042: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

7042 is a 7xxx-series aluminum alloy within the Al-Zn-Mg family that leans to medium–high strength through precipitation hardening. Its principal alloying elements are zinc and magnesium with minor copper and trace elements to control grain structure and toughness.

The strengthening mechanism for 7042 is classic heat-treatable precipitation hardening: solution heat treatment followed by quench and artificial aging produce fine η (MgZn2)-type precipitates that impede dislocation motion. In practice this alloy can also be supplied in overaged or thermally stabilized tempers to trade some peak strength for improved fracture toughness and stress-corrosion resistance.

Key traits of 7042 include a high specific strength, moderate-to-good fatigue performance when properly aged, and reasonable machinability for a high-strength Al-Zn-Mg alloy. Corrosion resistance is generally better than some high-copper 7xxx alloys but inferior to 5xxx or 6xxx families unless protected by cladding or coatings.

Typical industries using 7042 are aerospace structures and fittings, high-performance automotive components, defense and ordnance hardware, and select marine structural elements. Engineers choose 7042 when a combination of elevated strength, toughness, and reasonable corrosion resistance is needed where weight saving is critical.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O	Low	High	Excellent	Excellent	Fully annealed; maximum ductility for forming
T4	Moderate	Moderate	Good	Poor to fair	Solution treated and naturally aged; starting point for artificial aging
T6	High	Low–Moderate	Limited	Poor	Solution treated and artificially peak-aged; highest practical strength
T651	High	Low–Moderate	Limited	Poor	T6 with stress relief by stretching after quench; commonly used for aerospace
T7	Moderate	Moderate	Fair	Poor	Stabilized/overaged to improve SCC resistance at expense of peak strength
H1x / H2x (strain hardened)	Variable	Lower	Variable	Good	Combined work hardening and partial heat treatment for specific applications

Temper has a strong influence on mechanical performance, fracture resistance and formability. Peak-aged tempers like T6 maximize tensile strength and hardness but reduce elongation and cold formability, creating large property gradients across welded or heat-affected zones.

Overaging or selecting T7/T651 tempers trades strength for improved stress-corrosion cracking resistance and more stable properties in service; this is a common design choice for structural aerospace parts where in-service environmental exposure is a concern.

Chemical Composition

Element	% Range	Notes
Si	≤ 0.25	Impurity control; too much reduces toughness
Fe	≤ 0.5	Intermetallic-forming impurity; reduces ductility if high
Mn	≤ 0.1	Grain structure control at low levels
Mg	1.0 – 2.0	Primary strengthening assistant with Zn; forms MgZn2 precipitates
Cu	0.05 – 0.30	Typically low in 7042 compared with 7075; less Cu improves SCC behavior
Zn	4.0 – 6.0	Main strength alloying element; higher Zn raises peak strength
Cr	≤ 0.25	Controls recrystallization and grain structure
Ti	≤ 0.15	Grain refiner when intentionally added
Others (each)	≤ 0.05	Trace elements and residuals; total others limited

The alloy balance is essentially aluminum with zinc and magnesium driving precipitation hardening; copper is intentionally kept relatively low versus some 7xxx variants to reduce susceptibility to stress-corrosion cracking. Minor elements such as chromium and titanium act as microalloying additions to stabilize grain size and prevent excessive recrystallization during thermomechanical processing.

Mechanical Properties

In tensile behavior 7042 exhibits a wide range depending on temper: annealed material is ductile with moderate tensile strength, whereas peak-aged tempers show marked increases in both yield and ultimate strength. Yield and ultimate values are typically temperature- and temper-dependent with T6/T651 tempers delivering the highest yield strength at the expense of elongation.

Hardness correlates closely with aging condition and precipitation state; hardness increases substantially from O to T6. Fatigue behavior benefits from fine, uniformly distributed precipitates and careful control of residual stresses; forgings and extrusions with optimized aging and heat treatment show improved crack-initiation resistance.

Thickness and section size affect quench uniformity and hence achievable properties; thicker sections are harder to solution-treat and quench without softening or overaging in the core. Designers must account for HAZ softening around welds and for reduced peak properties in thick forgings or plates where quench rate is low.

Property	O/Annealed	Key Temper (T6 / T651)	Notes
Tensile Strength	~200–260 MPa (typical)	~420–510 MPa (typical)	Wide range by temper and section thickness
Yield Strength	~90–160 MPa	~350–470 MPa	T651 often specified for improved residual stress control
Elongation	~15–25%	~6–12%	Elongation reduced in peak-aged conditions
Hardness (Brinell)	~40–70 HB	~120–160 HB	Values depend on aging parameters and section size

Physical Properties

Property	Value	Notes
Density	~2.78 g/cm³	Slightly higher than pure Al due to Zn/Mg additions
Melting Range	~500–640 °C (solidus–liquidus spread)	Alloy solidus lowered compared with pure Al; exact range depends on composition
Thermal Conductivity	~120–150 W/m·K	Lower than pure Al and some 6xxx alloys due to solute scattering
Electrical Conductivity	~28–40 % IACS	Reduced by alloying; varies with temper (solute in solid solution reduces conductivity)
Specific Heat	~0.88–0.90 J/g·K	Typical for Al alloys near room temperature
Thermal Expansion	~23–24 µm/m·K (20–100 °C)	Comparable to other high-strength Al alloys

7042's physical properties make it attractive where a favorable strength-to-weight ratio and reasonable thermal performance are required. Thermal and electrical conductivities are reduced compared with low-alloy or pure aluminum because of solute atoms and precipitates scattering electrons and phonons.

The melting/solidus range requires care during casting and welding operations; localized overheating promotes liquation of low-melting eutectics, increasing risk of hot cracking or soft zones if not controlled.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sheet	0.5 – 6 mm	Uniform in thin gauges if properly heat treated	O, T4, T6, T651	Common for aerospace skins and panels
Plate	6 – 150+ mm	Strength can vary through thickness due to quench	T6, T651, T7	Thick plates require optimized quenching or post-heat treatment
Extrusion	Profiles up to several hundred mm	Good directional strength; T6 achievable on limited cross-sections	T4, T6	Extrusion die design and quench capacity limit peak properties
Tube	1 – 50 mm wall	Behavior similar to extrusions; cold drawing used for precision	O, T6	Used for structural and hydraulic applications with suitable heat treatment
Bar/Rod	Diameters up to 200 mm	Properties depend on cooling; forgings provide superior toughness	O, T6	Forged bars often used for critical fittings and fasteners

Processing differences for sheet, plate and forgings primarily revolve around section thickness and achievable quench rates. Thin sheet and small extrusions quench rapidly and attain near-peak aged properties after standard aging, whereas thick plates and forgings require more elaborate quenching or post-quench stretch/age cycles to avoid soft cores and distortion.

Forming and machining also vary by product form; rolled sheet offers the best cold formability in annealed conditions while extrusions and forged bars provide directionally optimized properties for load-bearing components.

Equivalent Grades

Standard	Grade	Region	Notes
AA	7042	USA	American Aluminum Association designation; matches Al-Zn-Mg class
EN AW	7042	Europe	EN AW-7042 commonly cited; composition and delivery conditions may differ slightly
JIS	A7042 (approx)	Japan	Japanese standards may list similar Al–Zn–Mg compositions under related designations
GB/T	7042	China	Chinese standardized designation exists; chemical limits and tempers can vary

Differences between standards tend to be in allowable impurity levels, exact limits for copper and other minor elements, and the specified mechanical property baselines for particular tempers. When specifying 7042 internationally, engineers must call out the controlling standard and temper, and they should verify chemistry and mechanical tables to ensure interchangeability for mission-critical applications.

Corrosion Resistance

7042 provides moderate atmospheric corrosion resistance that is generally superior to the highest-strength, high-copper 7xxx alloys but poorer than 5xxx (Al-Mg) and most 6xxx (Al-Mg-Si) series alloys. Zinc-rich microstructures and heterogeneous precipitate distributions can create anodic/cathodic sites that promote pitting in aggressive environments unless surfaces are treated.

In marine and saline environments, untreated 7042 will exhibit more localized corrosion than 5xxx alloys and therefore typically requires anodizing, cladding, or protective coatings for long-term service. Cladding or protective conversion coatings restore acceptable performance for structural use near the coast.

Stress corrosion cracking (SCC) risk is a major design consideration for 7xxx alloys; 7042's relatively lower copper content reduces SCC susceptibility compared with 7075 but does not eliminate it. Galvanic interactions with dissimilar metals (steel, copper) can accelerate localized attack—designers should insulate joints and use compatible fasteners or protective barriers.

Fabrication Properties

Weldability

Fusion welding of 7042 is challenging; conventional TIG/MIG welding often results in significant loss of strength in the heat-affected zone and can produce hot-cracking due to low-melting eutectics. Preferred joining methods for structural applications include friction stir welding (FSW) and electron beam welding, which produce narrower HAZs and better property retention. When fusion welding is unavoidable, use low-susceptibility filler alloys and post-weld heat treatment strategies where feasible, though full restoration of original peak properties is uncommon.

Machinability

Machinability of 7042 is fair to good for a high-strength Al-Zn-Mg alloy; higher strength tempers increase cutting forces and tool wear compared with 6xxx alloys. Carbide tooling, rigid workholding, and high-speed machining with copious coolant produce the best results; expect short, discontinuous chips in many operations. Surface finish and dimensional control are typically excellent when stable cutting parameters are maintained.

Formability

Formability is excellent in the O annealed condition, allowing tight bends and deep draws in thin gauges. Peak-aged tempers (T6/T651) are restrictive for cold forming and are commonly formed in the softer condition and then solution treated and aged after forming. Typical minimum bend radii depend on temper and thickness; designers should use bend allowances based on O or T4 conditions to avoid cracking.

Heat Treatment Behavior

As a heat-treatable Al-Zn-Mg alloy, 7042 is processed through solution treatment, quench and aging to develop its characteristic high strength. Typical solution treatment temperatures are in the range of 470–490 °C with times dependent on section thickness to achieve dissolution of Zn/Mg-rich phases. Fast quenching is required to retain solute in solid solution and enable subsequent precipitation during aging.

Artificial aging (T6) is commonly performed at temperatures of approximately 120–160 °C for times tuned to reach the desired balance of tensile strength and toughness. Overaging (T7) uses higher temperatures or longer times to coarsen precipitates, improving stress-corrosion resistance at the cost of some peak strength. Transition between tempers (T4→T6→T7) is controlled through variations in aging schedule; natural aging can also occur at room temperature and affect final properties if not accounted for.

High-Temperature Performance

7042 is intended for ambient to moderately elevated temperature service; its mechanical properties decline with increasing temperature and significant strength degradation appears above roughly 100–150 °C. Creep resistance is limited compared with heat-resistant alloys, so designers should avoid sustained loads at elevated temperatures.

Oxidation is not a primary concern compared with ferrous alloys, but prolonged high-temperature exposure can promote precipitate coarsening and loss of strength; thermal stability is a limiting factor for long-term elevated-temperature applications. In welded structures, HAZ softening combined with thermal exposure can further reduce load-carrying capacity.

Applications

Industry	Example Component	Why 7042 Is Used
Aerospace	Structural fittings and forgings	High strength-to-weight and good fracture toughness when properly aged
Marine	Non-primary structural frames and brackets	Moderate corrosion resistance and favorable strength/weight
Automotive	Suspension components, mounting brackets	High strength with lower density for weight reduction
Defense	Bolsters, weapon mounts, connector bodies	Strength and fatigue performance under cyclic loads
Electronics	Structural housings and heat spreaders	Combination of stiffness and machinability for complex parts

7042 is commonly selected where a combination of elevated strength, reasonable corrosion resistance, and good machinability is required in a lightweight package. It bridges the gap between the highest-strength but more SCC-prone 7xxx alloys and more corrosion-tolerant but lower-strength 5xxx/6xxx families.

Selection Insights

Choose 7042 when you need higher strength than typical work-hardened alloys but desire better SCC resistance than copper-rich 7xxx alloys; it is a good compromise alloy for structural parts requiring high specific strength. Compared with commercially pure aluminum (1100), 7042 trades conductivity and forming ease for substantially increased strength and hardness, making it inappropriate where electrical or thermal conductivity is the primary concern.

Compared with work-hardened alloys like 3003 or 5052, 7042 offers markedly higher static strength and improved fatigue life while sacrificing some corrosion resistance and cold formability; use 5052/3003 where forming and seawater resistance are primary. Compared with heat-treatable Al-Mg-Si alloys such as 6061/6063, 7042 can deliver higher peak strength for heavily loaded structural fittings, but 6061 has better weldability and corrosion behavior in many service environments; select 7042 when strength-to-weight and stiffness are prioritized over ease of welding and universal availability.

Closing Summary

7042 remains a relevant engineering alloy where a high-strength, heat-treatable aluminum is needed with better SCC performance than the highest-strength 7xxx alloys. Its balanced chemistry and temper options allow engineers to tune strength, toughness and corrosion performance for demanding structural applications while preserving the light weight that makes aluminum attractive.